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The uneven (flow) resistance of branched pipes commonly causes local overheating and freezing in solar collector fields. In this paper, the resistance characteristics of the direct return flat plate solar collector field are studied using a numerical model. The accuracy of the model is verified with experimental data, and the maximum relative error is below 10%. A new evaluation index, the ratio of resistance imbalance rate to efficiency (RRIRE), is introduced. In this way, it is possible to comprehensively evaluate the effect of different factors (configuration, flow rate per unit heat collection area, inlet temperature, working medium) on both the resistance characteristics and efficiency of the solar collector field. The results show that, when the number of rows (Nrow) exceeds the number of collectors in each row (Ncol), the inlet temperature is below 40 °C. In addition, the flow rate per unit heat collection area is below 0.06 m3/(h m2). Hence, the RRIRE is smaller and the system has better resistance efficiency. Considering the viscosity change due to the temperature change in the fluid of the solar collector field, a new temperature difference correction method is proposed. This makes it possible to calculate the frictional drag for a fluid in the collector field, which allows a more realistic calculation of flow resistance. This study lays the foundation for improved layout design and parameter optimization in solar collector fields.

The paper summarizes the implementation experiences and lessons learned from the solar water heating (SWH) system mandatory installation policy in China. In-depth interviews, questionnaire surveys and field investigation were conducted to understand the prevailing attitudes of the stakeholders associated with the application of SWH in buildings. Scenario analysis was used to analyze the effect of expanding the coverage of mandatory installation policies. The results show that due to the strong market externalities of SWH building applications, the government must provide inducements to stakeholders through mandatory policies or fiscal incentives to create an impetus for them to use SWH extensively. For China, it is necessary to strengthen and support the utilization of renewable energy policies and extend the mandatory installation policy of SWH systems to taller buildings to fulfill the national renewable energy application goals. The most reasonable technical route, full process control, and attention to system operation and maintenance are the three key factors for the successful implementation of a mandatory installation policy for SWH systems. The policy recommendations presented in this paper can not only be applied to China but also other developing countries as well.

The photovoltaic (PV) roofs have two main energy-saving effects, which are shading and power supply. Considering the shading and power generation gain jointly, a roof is changed from the building energy end to the building energy supply end, thus changing its energy use system greatly. Therefore, this paper carries out research on the comprehensive energy-saving effect integrating the shading and the power supply gain. Three types of PV rooftops, namely, horizontally-mounted overhead PV rooftop, tilted overhead PV rooftop, and attached PV rooftop are studied to explore their impacts on the heat gain and heat loss of the roof and building’s heating and cooling load. In order to estimate the overall energy-saving in different climatic regions in China, an overall energy-saving evaluation method that considers the power generation and shading benefit effects of the PV rooftop is proposed. Based on the climate and solar radiation zones in China, 13 respective cities are selected to be included in the research. The results show that, by considering only the shading effect of PV panels, the tilted PV is more suitable in summer, reducing the heat input, whereas the horizontally-mounted PV is more effective in winter to prevent more heat loss. Regarding the overall energy-saving that considers both the shading and power generation effects of PV panels, building with horizontally-mounted PV rooftop has the highest efficiency in the summer season, while the building with tilted PV rooftop has the highest efficiency in the winter season. The model and analysis of the overall energy-saving presented in this work can provide a guide for the application of rooftop solar PV panels in different climate zones in China.

The ground thermal properties and borehole thermal resistance are the essential parameters for the design of borehole heat exchanger (BHE) field, and they are usually estimated using the experimental inlet/outlet fluid temperatures of BHE in thermal response test (TRT). This paper proposes the coupled two-step parameter estimation procedure (TSPEP) for estimating ground thermal conductivity and borehole thermal resistance of BHE by evaluating the actual averaged–over-the –depth mean fluid temperature (MFT) using the quasi-three-dimensional model inside the borehole. The simulated annealing algorithm (SAA) is used to iteratively find the minimum values of the two objective functions to obtain the optimal estimated results. In TSPEP, the estimated ground volumetric heat capacity and weighted factor f in the 1st step are transferred to calculate MFT using the experimental data in the 2nd step, which guarantees the direct approach based on the infinite line source model (ILSM)applied to improve the accuracy of the estimated borehole thermal resistance. For 50 m depth BHE, the estimated borehole thermal resistance is increased by 12.1% using TSPEP than the effective borehole thermal resistance evaluated by the arithmetic average fluid temperature (AFT). The estimated ground thermal conductivity in TSPEP is almost same with that from the direct approach based on ILSM, and the maximum relative error between them is only 0.91% even though borehole depth of BHE changes from 50 m to 200 m.

Water pit heat storage is an important part of smart district heating systems that integrate various renewable energy sources. This project studied the storage capacity and thermal stratification in a 3000 m3 underground water pit in Huangdicheng, China using a finite difference model of the water pit that was validated by experimental data. The total heat loss from the water pit in the first year was measured to be 98 MWh and the storage efficiency was 62%. Further investigations using the validated model show that approximately 57% of the total heat loss took place through the side wall, 30% through the top and the rest through the bottom of the pit. The heat loss coefficient was largest along the side wall at 0.702 W m−2∙oC−1. Higher charging temperatures create higher temperature differences between the top and bottom of the water pit, i.e. greater thermal stratification. The MIX number increases during most of the charging period and cannot represent the thermal stratification in the water pit during charging while the stratification number more accurately represents the stratification. Therefore, the stratification number is recommended for characterizing stratified water pits.

The aim of the paper was to investigate thermal performance of two newly developed 15.4 m2 compound parabolic concentrator (CPC) solar collectors both theoretically and experimentally. The two collectors are identical except that one collector is equipped with a transparent Ethylene tetrafluoroethylene (ETFE) foil and the other collector is not. Experimentally the solar collectors were tested in an outdoor test facility at different temperatures. Collector efficiencies were determined. Theoretically simplified CFD models of the CPC solar collectors were developed and validated against the measurements. A ray tracing tool Tonatiuh was used to calculate distribution of solar irradiance on the receiver. The influence of the ETFE foil on collector heat loss coefficient and efficiency was investigated. The results show that the CFD model predicts satisfactorily temperature rises of the collectors in the temperature range between 20 °C and 81 °C. The deviations between the measured and the calculated temperature rises were limited to ±0.3 K with a relative error less than 3.7%. By installation of the ETFE foil, the first order collector heat loss coefficient decreased from 1.42 to 0.82 W/m2/K. Efficiency of the collector with the ETFE foil was measured to be 63%, 55% and 54% for a mean collector fluid temperature of 27 °C, 63 °C and 81 °C respectively. For a solar irradiance of 895 W/m2 and a mean collector temperature of 133 °C, collector efficiency is predicted to increase from 36% to 41% by installation of the ETFE foil. Another 5% increase of collector efficiency is expected if installation of the foil is made airtight.

Sodium acetate trihydrate (SAT) can be used for long-term heat storage in buildings by utilizing its ability to supercool stably to ambient temperatures while preserving its heat of fusion. Additives are used to stabilize the SAT and avoid phase separation. A reliable method for initializing the solidification of supercooled SAT composites is needed to operate a heat storage unit based on supercooled SAT. The crystallization temperatures of SAT composites during cooling were therefore investigated and experiments carried out using methods applying local cooling in a small part of prototype heat storage units to initialize crystallization. To find the crystallization temperatures of SAT composites, supercooled samples were cooled down in a freezer. The influence of rusty metal parts submerged in melted SAT composite samples and various periods at rest in a supercooled state were investigated with regard to supercooling stability and crystallization temperature. Carboxymethyl cellulose, extra water, liquid polymer HD 310, metal-based graphite flakes, and silicone oil were applied as additives to form the different SAT compositions. Samples with 60 g SAT in glass jars were subject to repeated heating and cooling cycles. It was found that samples containing steel profiles crystallized in the range of − 9 to − 15 °C, while SA-water mixtures without steel profiles cooled down to − 24 °C before crystallizing. Furthermore, samples with carboxymethyl cellulose and liquid polymer HD 310 showed a greater temperature rise during crystallization, which is in accordance with previous findings on heat contents. SA-water mixtures showed a second minor temperature peak at temperatures below − 20 °C, when the sample was cooled down again after the first temperature rise. Devices were developed to initialize the crystallization of supercooled SAT composites in prototype heat storage units using rapid local cooling of the SAT composite. We successfully initialized crystallization by evaporating pressurized liquid carbon dioxide in a small chamber on one side of the PCM container and by using Peltier elements. Our experiments showed that the controlled initialization of crystallization by cooling is feasible for all the SAT composites investigated.

Future energy systems with a large share of fluctuating renewable energies demand thermal energy storages that are flexible and reliable. Sodium acetate trihydrate (SAT) has been investigated for many years as heat storage materials but the focus of the investigations were mostly on short-term applications. SAT has a high energy storage density and a large supercooling degree which make it an ideal flexible heat storage material. Heat storages utilizing stable supercooling of SAT can store heat almost heat loss free in both short-term and long-term, which offers great benefits for the energy system. A review of SAT heat storages is presented in this article. The investigations show that flexible heat storages using SAT have clear advantages over a traditional water based heat storage in both short-term and long-term heat storage. Significant decrease of the storage size (at least 60%) is considered a merit for applications in single family houses where there is a spatial constrain. Moreover, the SAT heat storages offers flexibility in system operation, which is considered beneficial for renewable energy such as solar heating systems. The main challenges for the heat storages, for instance, low heat exchange capacity rate, phase separation and spontaneous crystallization, are addressed. Remedies to overcome the challenges are discussed. The paper will also investigate stability of supercooling and reliability of triggering methods for controllable release of the latent heat stored in supercooled sodium acetate trihydrate. Recommendations for further investigations are proposed.

Water pit heat storage is an effective long-term heat storage. Experimental and theoretical investigations of a 60,000 m3 water pit heat storage in Dronninglund, Denmark was carried out with an aim to study its long-term thermal performance. Experimentally, detailed measurements were analyzed to monitor thermal behavior of the store, including flow rates, temperatures of the inlet and outlet flow, thermal stratification in the store and temperatures of the surrounding soil at four different depths. Theoretically, a simulation model of the water pit heat storage was developed based on the Type 343 model (ICEPIT) in TRNSYS. The calculated temperatures were compared to the measurements. The developed model was used to investigate the influence of soil properties and inlet arrangement on the thermal performance of the store. The result show that the simulation model predicts well the thermal behavior of the store. Over the whole year, the calculated average temperature of the store differed within ±4.7 K from the measured values. The measurements show that the annual charged and discharged heat are 12,787 MWh and 11,957 MWh respectively. The deviation of the charged and discharged energy between the simulation and the experiment did not exceed ±4%. The annual storage efficiency reached 90% in the second year of operation. The calculated ground temperatures were consistent with the measured ones at different depths. Measured heat loss of the store is 1392 MWh. As elucidated by the model, the heat loss from the cover contributed to 60% of the annual heat loss, while the heat loss from the sidewall and the bottom contributed 38% and 2% respectively. The influence of soil types on thermal performance of the store is significant. For a PTES surrounded by dry soil, the storage efficiency can reach 90%, while for a PTES surrounded by wet sandy gravel, the storage efficiency decreases by 7%. Change of the height of the middle inlet/outlet diffuser could slightly improve the storage efficiency by 1%. The restrains of the model were analyzed and the focus areas of further investigations were pointed out. The findings of the paper give good reference for researchers, designers and consultants in planning of a solar heating plant with a large water pit heat storage.

The pollutant emissions from ships in harbor are a pressing concern due to their direct impact on the health of the population. The use of electric-battery propulsion is a viable solution to reduce the emissions in coastal areas, but it is only applicable to small ferries. Large cruise ships commonly utilize shore power connection to provide onboard electricity, avoiding pollution during harbor stays. However, this solution is not applicable during short stays. This paper presents a novel and energy-efficient way to supply zero-emission power during harbor stays of marine vessels. The proposed system combines the use of a thermal energy storage and a waste heat recovery system based on the organic Rankine cycle technology. The objective of this work is to investigate the technical feasibility of the proposed system and to compare its cost-effectiveness with the alternative solution of using batteries during harbor stays. The study is based on a case study of a hypothetical ferry requiring 1 MW of auxiliary power during harbor stays. The results suggest that the proposed system would require the installation of a storage tank of around 82 m3, and that it could be economically competitive with the battery-based solution, especially when considering its installation on newly built ships. Lastly, it is estimated that the installation of the proposed concept would lead to reduction of the ferry's carbon dioxide emissions by 8%.

Experimental and theoretical investigations have been carried out on the first large scale solar assisted ground source heat pump (SAGSHP) for heating and cooling of a village in the outskirts of Beijing. Long term performances of the system without and with solar heating were monitored. A TRNSYS model of the SAGSHP system was developed and validated against the measured data. Based on a parametrical investigation using the model, an optimized solution was proposed. It is shown that adding a domestic hot water system to the original SAGSHP system will not only help to secure thermal balance of the ground but also reduces primary energy consumption for hot water by 70%. The system COP increases by 9.4% from 2.42 to 2.65 and the annual overall operating cost decreases from 892,000 CNY to 794,000 CNY with only a small increase in equipment cost. Under the premise of ensuring thermal balance of the ground, there is an optimal match between the size of the ground heat storage, the collector area and the tank volume. Experience and lessons learned from the project are conducive to successful implementation of similar projects in the future.

To develop an appropriate solar DHW (Domestic Hot Water) tank for residential dwellings and put it into the European solar thermal market for promotion, thermal performance tests of PCM (Phase Change Material) hot water storage tanks of both a prototype and an improved version with a water volume of 148 l and 35 kg PCM in the mantle has been carried out. The tank was designed to provide DHW for residential dwellings through a combination of solar and auxiliary heating, concurrently using PCM on the basis of cheap SAT (Sodium Acetate Trihydrate) as a thermal battery to shave off peak auxiliary power or to work under power outage. Heat transfer matching properties of the bottom and the top spirals separately for solar charge and auxiliary charge of the prototype DHW tank were ascertained in terms of heat exchanger capacity rate (HXCR) and the rule of thumb of boiler powers, respectively. Moreover, heat content of the PCM was estimated via a series of test cycles in order to infer its capacity and stability. It was found that there were some technical problems for the prototype tank module, such as mismatching property of the heat exchanger spirals, heat mixing phenomena during hot water draw-off. Thus, an improved tank was manufactured based on the test results of the prototype. Further tests indicated that the matching property of the top heat exchanger spiral was ameliorated for auxiliary charge and the heat mixing between hot water supply pipe and water tank was restrained during discharge, except that the length of the bottom spiral should be further reduced. Regarding the PCM in the mantle, it was inferred that the PCM heat content was somewhat lower than that of ideally working SAT. The PCM tended to perform stably under 16 test cycles with more than 3-month consecutive tests, implying no phase segregation occurred as that would degraded its performance.

A solar heating system with 22.4 m2 of solar collectors, a heat storage prototype consisting of four 200 kg phase-change material (PCM) storage units, and a 735 L water tank was designed to improve solar heat supply in single-family houses. The PCM storage utilized stable supercooling of sodium acetate trihydrate composites to conserve the latent heat of fusion for long-term heat storage. A control strategy directed heat from a solar collector array to either the PCM storage or a water buffer storage. Several PCM units had to be charged in parallel when the solar collector output peaked at 16 kW. A single unit was charged with 27.4 kWh of heat within four hours on a sunny day, and the PCM temperature increased from 20 °C to 80 °C. The sensible heat from a single PCM unit was transferred to the water tank starting with about 32 kW of thermal power after it had fully melted at 80 °C. A mechanical seed crystal injection device was used to initialize the crystallisation of the sodium acetate trihydrate after it had supercooled to room temperature. The unit discharge during solidification peaked at 8 kW. Reliable supercooling was achieved in three of the four units. About 80% of latent heat of fusion was transferred from PCM units after solidification of supercooled sodium acetate trihydrate to the water tank within 5 h. Functionality tests with practical operation conditions on the novel, modular heat-storage configuration showed its applicability for domestic hot water supply and space heating.

In this study, the repeated mutagenic effects of 60Co-γ irradiation combined with an optimized high-throughput screening method was applied to Chlorella pyrenoidosa to improve lipid productivity and characteristics for biodiesel. Two mutants of Chlorella pyrenoidosa, CP-232 and CP-269, were obtained. The lipid contents obtained from CP-232 and CP-269 were respectively increased by 20.8% and 20% under heterotrophic seed-photoautotrophic culture (HS-PC) model, and by 15% and 20.8% under photoautotrophic seed-photoautotrophic culture (PS-PC) model compared with the wild type strain in 1 L bubble column reactor. In addition, 60Co-γ irradiation mutagenesis increased the proportion of saturated fatty acids in lipids accumulated in CP-232 and CP-269 compared to the wild type strain. In 3 L and 15 L bioreactor scale-up, lipid productivity was equally significantly increased in both CP-232 and CP-269 mutant strains. Our findings indicated that 60Co-γ irradiation mutagenesis improves the accumulation of lipids with potential for industrial biodiesel production in mutant C. pyrenoidosa strains.

This paper presents experimental investigations on a combined solar and ground source heat pump (SGHP) system for a single-family house. The SGHP system was installed at the Lyngby campus of Technical University of Denmark with horizontal ground heat exchanger (HGHE). Detailed experiments were carried out on the system in 2019 under real weather conditions and the heating demand for domestic hot water (DHW) and space heating (SH). The focuses of the investigation are energy flow analysis and performance assessment on the SGHP system. The results showed that the yearly system seasonal performance factor of the SGHP system (SPFSHP) was 2.8, and the maximum monthly SPFSHP was up to 5.7 in April with the highest solar radiation. Directly utilized solar energy for the combi-storage was 500 kWh lower than the annual solar heat for charging the ground, and it accounted for 36.6% of annual heat input of the combi-storage, the rest of the heat was supplied by the heat pump system that consumed 84% of total electricity energy. Different from the large fluctuations of monthly solar fraction due to solar radiation, the monthly renewable heat fraction was within the range from 57% to 83%, and the yearly value was 65%.

Flat plate collectors have relatively low efficiency at the typical supply temperatures of district heating networks (70–95 °C). Parabolic trough collectors retain their high efficiency at these temperatures. To maximize the advantages of flat plate collectors and parabolic trough collectors in large solar heating plants for a district heating network, a hybrid solar collector field with 5960 m2 flat plate collectors and 4039 m2 parabolic trough collectors in series was constructed in Taars, Denmark. The design principle is that the flat plate collectors preheat the return water from the district heating network to about 70 °C and then the parabolic trough collectors would heat the preheated water to the required supply temperature of the district heating network. Annual measured and simulated thermal performances of both the parabolic trough collector field and the flat plate collector field are presented in this paper. The thermal performance of both collector fields with weather data of a Design Reference Year was simulated to have a whole understanding of the application of both collectors under Danish climate conditions as well. These results not only can provide a design basis for this type of hybrid solar district heating plants with flat plate collectors and parabolic trough collectors in the Nordic region, but also introduce a novel design concept of solar district heating plants to other high solar radiation areas.

Densely occupied spaces such as classrooms can very often have problems with overheating. It can be difficult to cool such spaces by means of a ventilation system without creating draughts and causing discomfort for occupants. The use of a wall radiant cooling system is a suitable option for spaces with a high occupant density. Radiant systems can remove most sensible heat loads resulting in a relatively small requirement for supply air for ventilation.

High-rise buildings have a significant impact on the surrounding environment. Building-integrated solar water heating (SWH) systems are effective ways to use renewable energy in buildings. Impediments, such as security concerns, aesthetics and functionality, make it difficult to apply SWH systems in high-rise buildings. At present, only China uses SWH systems on a large scale in such buildings. What are China’s experiences and lessons learned in applying SWH systems in high-rises? Are these experiences scalable to other countries? This study used a combination of field investigation, literature review and case study to summarize 36 systems that had been in operation for 1−14 years. System types, collector types, installation methods, types of auxiliary heat sources, economic performance and various basic principles were summarized. The economic performance of SWH systems in high-rise buildings was analyzed and verified by a case study in Shanghai. The results show that the installation of SWH systems in high-rise buildings is feasible and reliable. Individual household systems (61%) were more popular than centralized systems (25%) and hybrid systems account (14%). The average area of solar collectors per household was 2.17 m2/household, the average design solar fraction was 52%. Flat plate solar collectors (53%) was the most commonly used collector, while electric heating elements (89%) were the most common auxiliary heat sources for SWH systems, followed by gas water heaters and air source heat pumps. The cost of SWH systems per m2 of a building area was between 22 CNY/m2 to 75 CNY/m2. China’s unique practical experience gives a reference for other countries in their efforts to make high-rise buildings more sustainable.

Phase separation is a key problem when using sodium acetate trihydrate (SAT) as phase change heat storage material. The formation of phase separation is highly correlated to the composition of SAT based heat storage material, the material height, operation method and environmental condition. This study focus on the phase separation investigation by means of measuring exact water proportions in different layers of SAT samples as well as phase separation mitigation agents. The SAT samples were made with or without different amount of excess water or thickening agents at different heights. Then the samples were tested in supercooled state under different environmental conditions such as short or long term supercooling periods, standing still or shaking from time to time during supercooling, repeated heating and activating and low ambient temperature. The solidification of supercooled samples was activated from top or bottom in order to verify the influence of activation methods. The water proportions in different layers of SAT samples were measured and summarized for comparison. It can be concluded from the experiments that suitable amount of excess water or thickening agents and proper shaking are beneficial to reduce or avoid phase separation. On the other hand, SAT without any additives, repeated heating and activating and low ambient temperature are favourable to forming phase separation for supercooled SAT materials.

China is the second-largest economy country, and the largest country worldwide in total solar energy system capacity installed. More than 70% of total solar collector capacity worldwide is installed in China. The production of solar collector in China had decreased significantly from 64 million m2 in 2012 to 37 million m2 in 2017, corresponding to a reduction of around 40% of the market in 2012. There are very limited comprehensive studies in existing literature, which have investigated the reasons on the decline of Chinese solar thermal market. This study aims to identify the reasons from both the demand and the supply sides in details. Furthermore, with the rise of emerging markets, such as the rapid development of the “Clean Heating Initiative” for space heating in North China, solar heat industrial process, and the mandatory installation of solar domestic hot water system for residential buildings below 100 m in urban area, the Chinese solar thermal market is expected to recover soon, called “U-shaped curve” in this study. The market share of flat plate collector and evacuated tube collector is predicted to change in the near future. The research and development of flat plate solar collectors will increase continually in the market other than evacuated tube solar collectors like the past 10 years. The main driver for the solar thermal market development in China in the near future will be the demand from the urban area.

A test method for evaluating and predicting the thermal performance of thermosyphon Solar Domestic Hot Water (SDHW) system was proposed. The evaluating mathematical model of SDHW system was developed based on the two-node theory – a SDHW system was divided into the solid part and the fluid part. By combining the dynamic energy conservation equations of the solid part and the fluid part, the evaluating mathematical model of SDHW system was derived. The model parameters have clear physical meaning which can be used to evaluate the SDHW systems. The evaluating mathematical model was further processed by using the Laplace transformation technique and the predicting mathematical model was then derived. The predicting model can be used to predict the thermal performance of SDHW system for short and long term period with flexible draw off load conditions. The experimental method was designed and experiments were carried out to validate the test method. The measured mean fluid temperature in the storage tank was compared to the predicted mean fluid temperature. The annual thermal performance prediction of the system with two draw off load conditions at different daily hot water consumptions was also carried out.

Solar thermal has contributed little for space heating in China. In 2014, although China shared 75.8% of the total solar collector installations in the world, only less than 0.3% of the solar collectors were used for space heating. To promote solar district heating (SDH) in China, based on Danish experiences and Chinese clean heating transformation practices, a PEST (policy, economics, social, and technology) analysis and a SWOT (strengths, weaknesses, opportunities, and threats) analysis on SDH development in China were conducted. An extensive survey and on-site investigation were carried out to identify the applicability of SDH in rural areas. SDH development strategies, roadmap, and decision-making process for a SDH project are summarized. SDH has a broad application prospect in China with abundant solar resources and favorable policies. The solar heated floor area can achieve 756 million m2 with an assumption of 3% coverage of the total heat demand of buildings. Particular areas with low population density, scarce resources, and strict environmental requirements, e.g., Tibet, should be given a high priority for SDH. Rural villages and small towns with better infrastructure, e.g., district heating networks, are the best target market for SDH in the next five years. With the development of seasonal heat storage technologies and the accumulation of practical experience, SDH can be expanded to industrial parks, large residential communities in sparsely populated northwest China. Integration of solar heat with existing heating networks in big cities with central heating will be challenging in the long run.

To reduce the energy consumption of buildings significantly, a novel solar combi-system with short and longterm heat storage has been developed. A system prototype with 22.4m2 (aperture) evacuated tubular collectors, a 735 L water tank and 4 phase change material (PCM) units each containing 150 L sodium acetate trihydrate composite has been built. Experimental investigation has shown advantages of utilization of stable supercooling of sodium acetate trihydrate in spring and autumn. In this paper, a newly developed numerical model was used to investigate the performance potential of the system with combined utilization of the water tank and the PCM units, including on-demand crystallization of supercooled sodium acetate trihydrate composites. PCM units, the water tank and the collector circuit models were validated with measurement data from system demonstration. Space heating and hot water demand patterns of a Danish single-family Passive House with a yearly heat demand of 3723 kWh were applied. Results showed that a 56% annual solar fraction of heat supply was achieved with the prototype specifications. A 69% solar fraction could be achieved with an optimized scenario including a 15% increased hot water demand. Sensitivity analysis of component sizing showed that PCM units of 200 L can be more efficiently used with a 0.6m3 water tank. Optimal solar collector array tilt was 70°. Aperture areas between 12.8 and 22.4m2 were found adequate for frequent utilization of a PCM volume up to 1m3. Thus, the PCM heat storage capacity could be utilized at least 5.5 times a year. With a 22.4m2 collector area and 5 PCM units of 200 L each, a solar fraction of 71% was calculated for the annual heat supply. Assuming full charge of a 0.6m3 water tank and 2.8m3 of sodium acetate trihydrate composite by electricity at the beginning of the year, the system could run 18 days without need for auxiliary heating. Thus, in periods without solar collector power available, generation maxima of wind power could be utilized. In conclusion, building heat demand could be covered close to 100% by renewable energy resources.

Large solar collector fields are very popular in district heating system in Denmark, even though the solar radiation source is not favorable at high latitudes compared to many other regions. Business models for large solar heating plants in Denmark has attracted much attention worldwide. Denmark is not only the biggest country in both total installed capacities and numbers of large solar district heating plants, but also is the first and only country with commercial market-driven solar district heating plants. By the end of 2017, more than 1.3 million m2 solar district heating plants are in operation in Denmark. Furthermore, more than 70% of the large solar district heating plants worldwide are constructed in Denmark. Based on the case of Denmark, this study reviews the development of large solar district heating plants in Denmark since 2006. Success factors for Danish experiences was summarized and discussed. Novel design concepts of large solar district heating plants are also addressed to clarify the future development trend. Potential integration of large solar district heating plants with other renewable energy technologies are discussed. This paper can provide references to potential countries that want to exploit the market for solar district heating plants. Policy-makers can evaluate the advantages and disadvantages of solar district heating systems in the national energy planning level based on the know-how and experiences from Denmark.

Energy storage is a key technology to provide stable renewable energy supply to buildings. Latent heat energy storage system is a promising technique for domestic thermal energy storage systems due to its high thermal energy storage density and the ability to store heat for both short and long periods. The common domestic heat storage accumulators are designed for short-term storage, where heat can’t be stored for a long time. In this experiment investigation, a latent heat storage with 75 L water and 137.8 kg phase change material is tested. Sodium acetate trihydrate based composite with stable supercooling ability is used as the phase change material. The sensible heat of the storage is used for short-term heat storage, the latent heat is for long-term heat storage. The combination of short-term heat storage and long-term heat storage is attractive for heating systems with variable heat supply. In this experiment, thermal power and heat exchange capacity rates of the heat storage were analyzed. The stable supercooling success rate and hence the long-term heat storage was also investigated. The results showed that, under the test conditions, 21.7 kWh heat was stored in the tank during charging, 14.0 kWh heat was released during discharging of the sensible heat (short-term heat storage) and 7.6 kWh heat was discharged during discharging of the latent heat (long-term heat storage). The sodium acetate trihydrate remained stable supercooled in 66% of the test cycles. The storage had a relatively high heat exchange capacity rate of at least 340 W/K during charging.

A pilot project of the solar water heating system combined with a low temperature air source heat pump (ASHP) unit was established in 2014 in a detached residential house in the rural region of Beijing, in order to investigate the system application prospect for single family houses via system optimization design and economic analysis. The established system was comprised of the glass heat-pipe based evacuated tube solar collectors with a gross area of 18.8 m 2 and an ASHP with a stated heating power of 8 kW for the space heating of a single family rural house of 81.4 m 2 . The dynamic thermal performance of the pilot system was measured for continuous 20 days under typical cold climate conditions and the test data was used to validate the TRNSYS simulation model established. On the basis of model validation, system optimizations of both the existing pilot household and the typical rural house with good building insulation were undertaken to figure out the system economical efficiency in the rural regions of Beijing. The results show that the payback periods of the solar space heating system combined with the ASHP with the collector areas 15.04–22.56 m 2 are 17.3–22.4 years for the established pilot household on the current electricity price level of 0.5 RMB/kWh, comparing with the reference condition of the fully ASHP space heating. It is further found that the equivalent solar heat price per kWh is too high under the current solar market cost price and collector technology. To put forward the integrated solar space heating for reducing carbon emission, it is suggested that the Beijing municipal government should offer some financial subsidy to compensate the equivalent solar heat price per kWh.

To enable the transition from fossil fuels as a primary heat source for domestic hot water preparation and space heating solar thermal energy has great potential. The heat from the sun has the disadvantage that it is not always available when there is a demand. To solve this mismatch a thermal seasonal storage can be used to store excess heat from the summer to the winter when the demand is higher than the supply. Installing a long term thermal storage in a one family house it needs to be compact and sensible heat storages are not suitable. A latent heat storage with a phase change material (PCM) can provide a more compact way of storing heat. Sodium acetate trihydrate (SAT) is a good candidate material as it has a relatively high heat of fusion and in addition it has the ability to supercool to room temperature without solidifying. In this paper results from the test of a solar combi system with a latent heat storage with SAT is presented. The SAT heat storage modules were heated to 80 °C by the solar collectors 53 times in the test period from June to November 2015 and this enabled the modules to supercool. Supercooling was achieved for 39 days for a SAT module after which 11 kWh of heat were discharged.

Laboratory tests of two heat storage units based on the principle of stable supercooling of sodium acetate trihydrate (SAT) mixtures were carried out. One unit was filled with 199.5 kg of SAT with 9% extra water to avoid phase separation of the incongruently melting salt hydrate. The other unit was filled with 220 kg SAT mixture thickened with 1% carboxymethyl cellulose. The heat exchange capacity rate during the charging of the unit with the extra water was significantly higher than for the unit with the thickening agent due to the different levels of convection. The SAT mixtures in the units were stable and supercooled at indoor ambient temperatures for up to two months, after which the units were discharged. The energy discharged after solidification of the supercooled SAT and water mixture was 194 kJ/kg in the first test cycle, dropping to 179 kJ/kg after 20 test cycles. The energy discharged from the unit with SAT and the thickening agent after solidification was stable at 205 kJ/kg over 6 test cycles.

Large-scale solar thermal systems are a cost-efficient technology to provide renewable heat. The rapid market growth in the last decade has been concentrated on a small number of countries, with the outstanding position of Denmark followed by China, Germany and Austria. This paper provides a comprehensive overview of the market and common technological solutions for large-scale solar thermal systems in these countries. Country-specific factors, including solar resources, heat supply systems, competing technologies, promotion schemes and business models, which put these countries in a leading role, are analyzed in detail using an integrated assessment framework. For each country, a best practice solar heating system is introduced. The analysis shows that heat supply by large-scale solar thermal systems is a mature technology with a broad field of applications and that mainly country-specific boundary conditions regarding the heat supply system and incentive policy are responsible for the preeminent role of these four countries. The unique role of district heating utilities in Denmark, which are subject to a national emissions trading system and often act both as investors and operators of large-scale solar thermal systems, can serve as a role model for other countries.

A cylindrical heat storage prototype was designed to utilize sodium acetate trihydrate (SAT) composite with 2%wt. extra water and 3%wt. of liquid polymeric solution for combined short and long-term heat storage. It was manufactured with inexpensive standard components of water stores. It contained 150 l of SAT composite in the inner tank and 59 l of water in the mantle surrounding the inner tank and in a spiral heat exchanger going through the inner tank. The concept of stable supercooling of SAT and the heat transfer properties of the store filled with water or the SAT composite were studied. Results showed that 27 kWh of heat was stored between 25 °C and 90 °C, where the energy storage capacity of the composite was determined to be 21.3 kWh. This was 76% higher than for a water heat store of the same volume. After a storage period in supercooled state at ambient temperature, 11.5 kWh (long-term capacity) of heat was discharged when the SAT composite solidified. This value corresponds to a heat of fusion of 207 kJ/kg. During charge and discharge in periods with solidification, the heat exchange capacity rates did not change with increase of flow rates. With discharge flow rates of 2 l/min applied in the mantle surrounding, thermal stratification was utilized. Thus, flow temperatures higher than the average SAT composite temperature resulted in liquid state. By additional use of the spiral, the discharge power reached 15 kW. During solidification the heat transfer was constantly decreasing, which resulted in a rather low discharge power. In building applications, heat transfer limitation could be overcome by discontinuous discharge via the mantle with intervals of 2–24 h. Thus, thermal power of up to 4 kW was achieved and the outlet temperature was close to the average temperature of the SAT composite.

Achieving a high fraction of solar heat in heat supply for domestic buildings would reduce the use of fossil fuels for heat generation and has been a goal for a long time. Combined short and long-term heat storage has been identified as one way of achieving solar fractions higher than 50 percent in heat supply for domestic buildings. To this end, a laboratory solar heating system was built with heat-pipe tubular collectors 22.4 m2 in aperture and a heat-storage prototype consisting of a 735 L water tank and four PCM units each containing 200 kg sodium acetate trihydrate (SAT) composite. The SAT composite was utilized as sensible heat storage with the additional ability to release heat of fusion on demand. Operation was demonstrated with the space heating and hot water demand patterns of a standard-size Passive House in the Danish climate. A strategy was developed to control the system. Seven operation modes enabled combined charging of water tank and PCM units, heat transfer from PCM units to the water tank when heat was in demand, and the right timing of auxiliary heating. We present the controller settings identified and the heat transfer fluid flow rates applied. Sequences of water tank charging, and single and parallel PCM unit charging were used to match the collector power available and the heat transfer limitations of the stores. During the charging of PCM units, the flow temperature was kept between 70 and 95 °C to allow continuous heat transfer rates of up to 16 kW. Peaks of up to 36 kW occurred when PCM units were added to the charging circuit. During heat transfer from PCM units to the water tank, flow temperatures were close to the SAT composite temperature and thermal power of up to 6 kW was measured. The heat stores were efficiently utilized in spring and autumn. The developed control strategy and measurement data from system demonstration will form the basis for numerical performance investigations.

To find an optimal economic solution for single-family solar heating systems, an evaluation model based on the levelized cost of heat (LCoH) is developed. The initial investment and operating costs of typical single-family solar heating systems are determined using case collection and literature study. These data are used to validate the evaluation model. A Python program is developed and used to calculate the LCoH of differently designed solar heating systems. Based on these calculations, the dependence of LCoH on the solar collector area under different heating load intensities, heating terminal units and heated areas are investigated. The optimal solar collector area and the solar fraction are determined for combinations of solar thermal (ST) with four different types of auxiliary heat source was discussed. The results show that LCoH does not necessarily decrease in a solar-assisted household heating system with an increase of solar fraction (SF) compared to a single conventional heat source system. The findings of this study are useful for designers, professionals and government officials to economically optimize the solar heating system for single-family houses.

A quasi-dynamic TRNSYS simulation model for a solar collector field with flat plate collectors and parabolic trough collectors in series was described and validated. A simplified method was implemented in TRNSYS in order to carry out long-term energy production analyses of the whole solar heating plant. The advantages of the model include faster computation with fewer resources, flexibility of different collector types in solar heating plant configuration and satisfactory accuracy in both dynamic and long-term analyses. In situ measurements were taken from a pilot solar heating plant with 5960 m2 flat plate collectors and 4039 m2 parabolic trough collectors in series in Taars, Denmark from Sep.2015 to Aug.2016. The simulated thermal performances of both the parabolic trough collector field and the flat plate collector field have a good agreement with the measured performances. The thermal performance of the hybrid solar district heating plants is also presented. The measured and simulated results show that the integration of parabolic trough collectors in solar district heating plants can guarantee that the system produces hot water with relatively constant outlet temperature. The daily energy output of the parabolic trough collector field can be more than 5 kWh/m2, while the daily energy output of the flat plate collector field is less than 5 kWh/m2 under Danish climate conditions. The simplified and validated TRNSYS model can be a useful tool to simulate and optimize thermal performance of solar heating plants with both flat plate and parabolic trough collectors.

Thermally Activated Building Systems (TABS) have proven to be an energy-efficient solution to achieve optimal indoor thermal environment in buildings. This solution uses the building mass to store heat and by means of water pipes embedded in the concrete slabs adjust the temperature in the premises. The active surfaces of TABS need to be as exposed as possible, but exposing bareconcrete surfaces has a negative impact on the acoustic quality in the premises. Acoustic solutions capable of providing optimal acoustic comfort while allowing the heat exchange between the TABS and the room are desirable. This study focuses on the influence of two types of free-hanging ceiling absorbers (horizontal and vertical) on the cooling performance of the TABS. Different scenarios are investigated for each type of sound absorber. Computational Fluid Dynamics (CFD) simulations are used to illuminate the nature of the heat exchange between the TABS and the room and the occupants. The simulations are validated by comparison with full scale measurements in laboratory conditions. The study shows that for equivalent sound absorption levels, free-hanging vertical sound absorbers have a lower impact on the heat exchange between the room and the TABS compared to free-hanging horizontal sound absorbers. Cold air stagnation between the sound absorber units and the TABS has been identified as the major cause of the cooling performance decrease of the TABS.

Large-scale solar heating plants for district heating networks have gained great success in Europe, particularly in Denmark. A hybrid solar district heating plant with 5960 m2 flat plate collectors and 4039 m2 parabolic trough collectors in series was built in Taars, Denmark in 2015. The solar heating plant was used as a reference case in this study. A validated TRNSYS-GenOpt model was set up to optimize the key design parameters of the plant, including areas of both collector types, storage size, orientation of the parabolic trough collectors and so on. This study introduces a generic method to optimize the hybrid solar district heating systems based on levelized cost of heat. It is found that the lowest net levelized cost of heat of hybrid solar heating plants could reach about 0.36 DKK/kWh. The system levelized cost of heat can be reduced by 5–9% by use of solar collectors in the district heating network in this study. The results also show that parabolic trough collectors are economically feasible for district heating networks in Denmark. The generic and multivariable levelized cost of heat method can guide engineers and designers on the design, construction and control of large-scale solar heating plants.

To find an optimal economic solution for solar district heating (SDH) in China, an evaluation model based on the levelized cost of heat (LCoH) is developed. A Python program is developed to calculate the LCoH of SDH systems using the quasi-dynamic test method. Based on these calculations, the trend of LCoH with solar collector area under different heating load intensities, heating terminal units, heated areas and land rents is discussed. The optimal solar collector area and the solar fraction are determined for combinations of solar thermal with four types of auxiliary heat sources, including air source heat pumps, ground source heat pumps, gas boilers and gas boilers with seasonal heat storage. The calculations show an economic optimal solar fraction of 11%–33% for a SDH system with heat pumps. High dependency of LCoH on network temperature is found for a SDH system with gas boilers. Seasonal heat storage minimizes LCoH of a SDH system with gas boilers at 100% solar fraction. The findings can be used as a reference for local authorities, consultants and engineers in the early energy planning to determine the optimal proportion of solar energy in a district heating system with the lowest operating cost.

The performance of a heating system consisting of a heat pump with an integrated storage tank was evaluated by means of Computational Fluid Dynamics (CFD) calculations. The aim of the investigations was to elucidate how thermal stratification in the tank would influence the COP of the system. Differently designed storage tanks were investigated under typical operation conditions. CFD models of heat storage tanks were developed and validated by experiments on a test tank. Calculations with the validated models were carried out in order to find the optimal tank design and system settings for achieving the highest possible COP. The investigated parameters were volume flow rate, tank geometry, diffuser design and tank wall material. Two different system COPs were defined: one based only on charge operation and the other on both charge and discharge operations. It was found that there is a direct connection between the thermal stratification inside the tank and the COP of the heating system. Higher degree of stratification leads to higher COP. For the ideal case where there is no mixing, no vertical thermal conduction in the tank and no heat loss from the tank, the system COP after a charge-discharge cycle is 3.69. When the effect of mixing is taken into account, the obtained COP of the system is decreased by approximately 9%. If heat transfer to the tank wall and vertical thermal conduction are also taken into account, the obtained system COP is lowered by another 5%. Moreover, the COP of a heating system with high flow rates can be significantly increased by a diffuser plate installed in the tank with a small distance to the inlet/outlet. The best performing system studied consisted of an insulated tall and slim tank with a h/d ratio of 3.64 and a perforated plate diffuser, giving a COP of 3.23 which was 32% higher compared to a case without a diffuser. The performance of the suggested system was not affected by variations of flow rates in the range from 0.12 to 0.24 kg/s.

A simple dynamic characterization model of flat-plate solar collectors based on the piston flow concept is used both to identify the collector characteristic parameters and to predict the dynamic thermal performance. The heat transport time originally defined as (1 − e−1)−1τC by Amrizal et al. (2012) for the model turns out to be the collector static response time constant τC by analytical derivation. The nonlinear least squares method is applied to determine the characteristic parameters of a flat-plate solar air collector previously tested by the authors. Then the obtained parameters are used to predict the dynamic behavior of the collector outlet temperature. The model coefficients particularly c3 in the simple dynamic characterization model are examined by the collector dynamic prediction under variable meteorological conditions. Meanwhile, the prediction accuracy of the simple dynamic model based on the first-order difference method is compared to that of the numerical solution of the collector ordinary differential equation (ODE) model using the fourth-order Runge-Kutta method. The improved thermal inertia model (TIM) on the basis of closed-form solution presented by Deng et al. (2016a) is also considered. The results show that the prediction performance of the simple dynamic model is nearly as accurate as the ODE numerical solution and the TIM by Deng et al. (2016a) except some special conditions such as sharply changed solar irradiance and collector inlet temperature.

Sodium acetate trihydrate is a phase change material that can be used for long term heat storage in solar heating systems because of its relatively high heat of fusion, a melting temperature of 58 °C and its ability to supercool stable. In practical applications sodium acetate trihydrate tend to suffer from phase separation which is the phenomenon where anhydrous salt settles to the bottom over time. This happens especially in supercooled state. The heat released from the crystallization of supercooled sodium acetate trihydrate with phase separation will be lower than the heat released from sodium acetate trihydrate without phase separation. Possible ways of avoiding or reducing the problem of phase separation were investigated. A wide variety of composites of sodium acetate trihydrate with additives including extra water, thickening agents, solid and liquid polymers have been experimentally investigated by a simple heat loss method. The aim was to find compositions of maximum heat released from the crystallization of supercooled sodium acetate trihydrate samples at ambient temperature. It was found that samples of sodium acetate trihydrate with 0.5–2% (wt.%) Carboxy-Methyl Cellulose, 0.3–0.5 % (wt.%) Xanthan Gum or 1–2% (wt.%) of some solid or liquid polymers as additives had significantly higher heat contents compared to samples of sodium acetate trihydrate suffering from phase separation.

A novel combined solar heating plant with flat plate collectors (FPC) and parabolic trough collectors (PTC) was constructed and put into operation in Taars, 30 km north of Aalborg, Denmark in August 2015. To assess the thermal performance of the solar heating plant, global radiation, direct normal irradiance (DNI) and total radiation on the tilted collector plane of the flat plate collector field were measured. To determine the accuracy of the measurements, the calculated solar radiations, including horizontal diffuse radiation, DNI and total tilted solar radiation with seven empirical models, were compared each month based on an hourly time step. In addition, the split of measured global radiation into diffuse and beam radiation based on a model developed by DTU (Technical University of Denmark) and the Reduced Reindl correlation model was investigated. A new method of combining empirical models, only based on measured global radiation, was proposed for estimating hourly total radiation on tilted surfaces. The results showed that the DTU model could be used to calculate diffuse radiation on the horizontal surface, and that the anisotropic models (Perez I and Perez II) were the most accurate for calculation of total radiation on tilted collector surfaces based only on global radiation under Danish climate conditions. The proposed method was used to determine reliable horizontal diffuse radiation, DNI and total tilted radiation with only the measurement of global radiation. Only a small difference compared to measured data, was found. The proposed method was cost-effective and needed fewer measurements to obtain reliable DNI and total radiation on the tilted plane. This method may be extended to other Nordic areas that have similar weather.

A novel solar heating plant with Concentrating Solar Power (CSP) collectors and Flat Plate (FP) collectors has been put into operation in Tars since July 2015. To investigate economic performance of the plant, a TRNSYS-Genopt model, including a solar collector field and thermal storage tank, was established. The optimization showed that there was a synergy in combining CSP and FP collectors. Even though the present cost per m 2 of the CSP collectors is high, the total energy cost is minimized by installing a combination of collectors in such solar heating plant. It was also found that the CSP collectors could raise flexibility in the control strategy of the plant. The TRNSYS-Genopt model is based on individually validated component models and collector parameters from experiments. Optimization of the cost performance of the plant has been conducted in this paper. The simulation model remains to be validated with annual measured data from the plant.

A latent heat storage based on the phase change material Sodium Acetate Trihydrate (SAT) has been tested as part of a demonstration system. The full heat storage consisted of 4 individual modules each containing about 200 kg of sodium acetate trihydrate with different additives. The aim was to actively utilize the ability of the material to supercool to obtain long storage periods. The modules were charged with solar heat supplied by 22.4 m2 evacuated tubular collectors. The investigation showed that it was possible to fully charge one module within a period of 270 minutes with clear skies. In long periods with high level of irradiance several modules were charged in parallel due to the limited heat exchange capacity of the integrated heat exchanger of the modules. After the modules were heated to more than 80° C they were set to passively cool down. Modules reached 30 °C in a period of parallel cool down without the sodium acetate trihydrate solidified in 3 of the 4 modules. Further tests showed that stable supercooling at ambient temperature is possible.