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A novel Compound Parabolic Collector (CPC) was developed and specially designed for operating in solar heating plants at the temperature range of 60 °C - 120 ºC. The prototypes of the collector were tested at Technical University of Denmark (DTU) in 2016. Four of the CPC collectors were then installed in series and connected to the flat plate solar collector field in Sæby solar heating plant in Denmark. The monitoring system was implemented for both the CPC collector array and a four flat plate collectors array in front. The thermal performance model was developed for both the CPC collector array and the flat plate collector array by using the QDT model and the tested collector parameters. The predicted annual thermal performance of both the CPC collector and the flat plate collector was calculated at different operating fluid temperatures under Denmark’s weather condition. The conclusion is that the annual thermal performance of the new CPC collector will be higher than the annual thermal performance of the flat plate collector if the operating mean fluid temperature is higher than 78 °C in theory, in practice if the temperature is higher than 55 °C.

The performance of two unglazed aluminum PVT collectors integrated into a concrete wall was investigated in a laboratory test facility. One PVT collector was installed in direct contact with the concrete wall and the other was installed with 10 mm of Styrofoam between the PVT collector and the wall. It was found that the temperatures of the walls behind the PVT collectors had big impacts on the thermal performance of the collectors. For this reason, an extended version of the traditional QDT formula was suggested for façade integrated collectors, where the effect of the temperature of the wall was also taken into account.

The parabolic-trough collectors (PTC) are the technology most widely used worldwide in concentrating solar power (CSP) and solar thermal electrical (STE) plants. This kind of collectors have not been certified during the first decades of the CSP plant installation because of the absence of a standard for testing these large-size collectors. The existing standards for testing solar collectors were not adapted to its peculiarity. In 2018 a standard IEC 62862-3-2 has been published especially for large-size PTC testing. In the present work, this methodology has been validated in-situ in a solar heating plant in Denmark on the solar field of six PTC rows, using the monitoring data of 15 testing days selected within more than one year. Furthermore, different possible improvements are proposed.

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 inbuildings. Impediments, such as security concerns, aesthetics and functionality, make it difficult toapply SWH systems in high-rise buildings. At present, only China uses SWH systems on a large scalein such buildings. What are China’s experiences and lessons learned in applying SWH systems inhigh-rises? Are these experiences scalable to other countries? This study used a combination of fieldinvestigation, literature review and case study to summarize 36 systems that had been in operationfor 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 ofSWH systems in high-rise buildings was analyzed and verified by a case study in Shanghai. Theresults 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 hybridsystems 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 commonlyused collector, while electric heating elements (89%) were the most common auxiliary heat sources forSWH systems, followed by gas water heaters and air source heat pumps. The cost of SWH systemsper m2 of a building area was between 22 CNY/m2to 75 CNY/m2. China’s unique practical experiencegives a reference for other countries in their efforts to make high-rise buildings more sustainable.

Both district heating and solar collector systems have been known and implemented for many years. However, the combination of the two, with solar collectors supplying heat to the district heating network, is relatively new, and no comprehensive review of scientific publications on this topic could be found. Thus, this paper summarizes the literature available on solar district heating and presents the state of the art and real experiences in this field. Given the lack of a generally accepted convention on the classification of solar district heating systems, this paper distinguishes centralized and decentralized solar district heating as well as block heating. For the different technologies, the paper describes commonly adopted control strategies, system configurations, types of installation, and integration. Real-world examples are also given to provide a more detailed insight into how solar thermal technology can be integrated with district heating. Solar thermal technology combined with thermally driven chillers to provide cooling for cooling networks is also included in this paper. In order for a technology to spread successfully, not only technical but also economic issues need to be tackled. Hence, the paper identifies and describes different types of ownership and financing schemes currently used in this field.

The solar tracking system is a device which orientates solar concentrating systems in order to allow the focusing of the solar radiation on the receiver along the day. The accuracy of the solar tracker is a key parameter when compared to the acceptance angle of the concentrator in order to maximize the optical efficiency. The Taars solar heating plant was put into operation in Denmark in summer 2015 with a PTC solar field. The accuracy of the tracking system of the 6 PTC rows has been studied.

The thermal performance of solar collector fields depends mainly on the mean solar collector fluid temperature of the collector field and on the solar radiation. For Danish solar collector fields for district heating the measured yearly thermal performances per collector area varied in the period 2012–2016 between 313 kWh/m2 and 577 kWh/m2, with averages between 411 kWh/m2 and 463 kWh/m2. The percentage difference between the highest and lowest measured yearly thermal performance is about 84%. Calculated yearly thermal performances of typically designed large solar collector fields at six different locations in Denmark with measured weather data for the years 2002–2010 vary between 405 kWh/m2 collector and 566 kWh/m2 collector, if a mean solar collector fluid temperature of 60 °C is assumed. This corresponds to a percentage difference between the highest and lowest calculated yearly thermal performance of about 40%. This variation is caused by different weather conditions from year to year and from location to location. Approximately half of the variations of yearly thermal performances can be related to variable weather conditions.

The paper describes the impact of different measures to improve the thermal performance of a solar heating plant for district heating applications. The impact of the different measures was evaluated through a validated TRNSYS-Matlab model. The model included details such as effect of the flow regime in the absorber pipes on the collector efficiency, flow distribution in the collector field, thermal capacity of the pipes and shadows from row to row. The improvement measures included variation of the operating temperatures, accurate input to the control strategy, feedback control on the outlet temperature of the collector field, control strategy based on weather forecast and use of different heat transfer fluids. The results showed that accurate input to the control strategy improved the yearly energy output of the plant by about 3%. If accurate input is not technically or economically feasible, a feedback control on the field outlet temperature seemed to be a valid alternative. The integration of weather forecast in the control strategy did not give relevant improvements. Higher glycol concentrations in the solar collector fluid gave better results than lower concentrations, as the higher frost protection guaranteed by the former outweighed the better thermophysical properties of the latter.

Sodium acetate trihydrate (SAT) can be used as phase change material in latent heat storage with or without utilizing supercooling. The change of density between liquid to solid state leads to formation of cavities inside the bulk SAT during solidification. Samples of SAT which had solidified from supercooled state at ambient temperature and samples which had solidified with a minimal degree supercooled were investigated. The temperature dependent densities of liquid and the two types of solid SAT were measured with a density meter and a thermomechanical analyzer. The cavities formed inside samples of solid SAT, which had solidified after a high or minimal degree of supercooling, were investigated by X-ray scanning and computer tomography. The apparent density of solid SAT depended on whether it solidified from a supercooled state or not. A sample which solidified from a supercooled liquid contained 15% cavities and had a density of 1.26 g/cm3 at 25 °C. SAT which had solidified with minimal supercooling contained 9% cavities and had a density of 1.34 g/cm3 at 25 °C. The apparent densities of the solid SAT samples were significant lower than the value of solid SAT reported in literature of 1.45 g/cm3. The density of liquid and supercooled SAT with extra water was also determined at different temperatures.

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.

Large scale solar heating plants develop fast in Europe, especially in Denmark. Most solar collectors used in previous solar heating plants are flat plate collectors. Flat plate collectors have relatively low efficiency at high temperature levels, such as 70 - 95 ℃, which is the supply temperature of district heating networks. Parabolic trough collectors keep a high efficiency at the high temperature level. To maximize the advantages of flat plate collectors and parabolic trough collectors, a novel combined solar heating plant with flat plate collectors and parabolic trough collectors in series has been constructed in Taars, Denmark. The flat plate collectors preheat the return water from the district heating networks to about 75℃, then the parabolic trough collectors heat the preheated water from the flat plate collector field to the required supply temperature of the district heating network. The thermal performance of the combined plant was measured from September, 2015. More than 2 years` thermal performance will be presented in this paper.

Because of its ability to preserve latent heat of fusion at room temperature in supercooled state, the phase change material (PCM) sodium acetate trihydrate (SAT) can be utilized for combined short and long-term heat storage in buildings. This was experimentally proved with a novel solar combisystem demonstrator including a segmented heat storage prototype, consisting of 4 flat units containing 200 kg of SAT composites each. A numerical model of the demonstrator was developed in TRNSYS environment. Ongoing system simulation indicates potential for more efficient domestic hot water and space heating supply. Further, based on experience with flat units, an inexpensive, cylindrical heat storage unit of the same PCM volume as one flat unit was designed and tested. Results showed that 27 kWh of heat can be stored. After cooling to room temperature, 11 kWh (long-term storage potential) were discharged during SAT composite solidification.

A cylindrical heat storage prototype was designed to utilize Sodium Acetate Trihydrate (SAT) for combined short and long term heat storage. It was manufactured with inexpensive standard components of water stores. It contained 150 liter of SAT composite and 55 l of water. A heat storage test facility was used to investigate if the concept of stable supercooling of SAT can be applied and if thermal power during discharge is sufficient for domestic heat supply. Preliminary results showed that 27 kWh of heat can be stored during heat up to 90°C, which is significantly higher than water heat stores of the same volume. After a heat loss free storage period, 11 kWh (long term storage potential) were discharged during SAT composite solidification. Parallel application of heat storage units should be investigated to overcome heat transfer limitations.

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. Anew 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.

Both district heating and solar collector systems have been known and imple- mented for many years. However, the combination of the two, with solar collec- tors supplying heat to the district heating network, is relatively new, and no comprehensive review of scientific publications on this topic could be found. Thus, this paper summarizes the literature available on solar district heating and presents the state of the art and real experiences in this field. Given the lack of a generally accepted convention on the classification of solar district heating systems, this paper distinguishes centralized and decentralized solar district heating as well as block heating. For the different technologies, the paper describes commonly adopted control strategies, system configurations, types of installation, and integration. Real‐world examples are also given to provide a more detailed insight into how solar thermal technology can be integrated with district heating. Solar thermal technology combined with thermally driven chillers to provide cooling for cooling networks is also included in this paper. In order for a technology to spread successfully, not only technical but also eco- nomic issues need to be tackled. Hence, the paper identifies and describes dif- ferent, SHINE - SolNet

This study describes the development of a detailed TRNSYS-Matlab model to simulate the behavior of a large solar collector field for district heating application. The model includes and investigates aspects which are not always considered by simpler models, such as flow distribution in the different rows, effect of the flow regime on the collector efficiency, thermal capacity of the components and effect of shadows from row to row. The model was compared with measurements from a solar collector field and the impact of each aspect was evaluated. A good agreement between model and measurements was found. The results showed that a better agreement was achieved, when a flow regime-dependent efficiency of the collector was used. Also the precise flow distribution in the collector field improved the model accuracy, but it must be assessed if the aimed level of accuracy justifies the much longer programming and computing time. Thermal capacity was worth being considered only for the bulkier components, such as the longer distribution and transmission pipes. The actual control strategy, which regulates the flow rates in the solar heating plant, was accurately reproduced in the model, as proved by the good agreement with the measurements.

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.

Thermal stratification in a water storage tank can strongly increase the thermal performance of solar heating systems. Thermal stratification can be built up in a storage tank during charge, if the heated water enters through an inlet stratifier. Experiments with a test tank have been carried out in order to elucidate how well thermal stratification is established in the tank with differently designed inlet stratifiers under different controlled laboratory conditions. The investigated inlet stratifiers are from Solvis GmbH & Co KG and EyeCular Technologies ApS. The inlet stratifier from Solvis GmbH is a rigid plastic pipe with holes for each 30 cm. The holes are designed with flaps preventing counter flow into the pipe. The inlet stratifier from EyeCular Technologies ApS is made of a flexible polymer with openings all along the side and in the full length of the stratifier. The flexibility of the stratifier prevents counterflow. The tests have shown that both types of inlet stratifiers had an ability to create stratification in the test tank under the different test conditions. The stratifier from EyeCular Technologies ApS had a better performance at low flows of 1-2 l/min and the stratifier for Solvis GmbH & Co KG had a better performance at 4 l/min. In the intermediate charge test the stratifier from EyeCular Technologies ApS had a better performance in terms of maintaining the thermal stratification in the storage tank while charging with a relative low temperature. [All rights reserved Elsevier].

This study presents a numerical model to evaluate the flow distribution in a large solar collector field, with solar collectors connected both in series and in parallel. The boundary conditions of the systems, such as flow rate, temperature, fluid type and layout of the collector field can be easily changed in the model. The model was developed in Matlab and the calculated pressure drop and flow distribution were compared with measurements from a solar collector field. A good agreement between model and measurements was found. The model was then used to study the flow distribution in different conditions. Balancing valves proved to be an effective way to achieve uniform flow distribution also in conditions different from those for which the valves were regulated. For small solar collector fields with limited number of collector rows connected in parallel, balancing valves are not strictly necessary if the pressure drop across the collector rows is much higher than the pressure drop along the longest distribution pipe.

Seasonal heat storages are considered essential for district heating systems because they offer flexibility for the system to integrate different fluctuating renewable energy sources. Water pit thermal storages (PTES) have been successfully implemented in solar district heating plants in Denmark. Thermal behavior of a 75,000 m3 water pit heat storage in Marstal solar heating plant was investigated experimentally and numerically. Temperatures at different levels of the water pit storage and temperatures at different depths of the ground around the storage were monitored and analyzed. A simulation model of the water pit storage is built to investigate development of temperatures in and around the storage. The calculated temperatures are compared to the monitored temperatures with an aim to validate the simulation model. Thermal stratification in the water pit heat storage and its interaction with the ground are elucidated by calculations using the validated CFD model.

Detailed measurements were carried out on a large scale solar heating plant located in southern Denmark in order to evaluate thermal performances of the plant. Based on the measurements, energy flows of the plant were evaluated. A modified Trnsys model of the Marstal solar heating plant was developed to calculate thermal performances of the plant. In the Trnsys model, three solar collector fields with a total solar collector area of 33,300 m2, a seasonal water pit heat storage of 75,000 m3, a simplified CO2 HP, a simplified ORC unit and a simplified wood chip boiler were included. The energy consumption of the district heating net was modeled by volume flow rate and given forward and return temperatures of the district heating net. Weather data from a weather station at the site of the plant were used in the calculations. The Trnsys calculated yearly thermal performance of the solar heating plant was compared to the measurement results. Validity of the Trnsys model was analyzed. Recommendations are given with aim to develop a Trnsys model that can be used to optimize design of a solar heating plant under different scenarios.