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This paper presents the analysis, simulation and performance study of heat transfer in a single U-tube borehole heat exchanger (BHE). Unsteady heat transfer method was used to analyze the heat transfer process of both inside and outside the borehole. Implicit numerical method applied on heat transfer equations obtained from energy balance (accompanied with thermal resistance model) was used to obtain the solution. The variation of mean fluid temperature, average borehole wall, grout and nearby ground temperature as well as borehole loading with borehole depth and time were investigated. Dynamic simulation was also performed to assess the influence of important parameters. The effect of two parameters, fluid mass flowrate and thermal conductivity of grout, on mean fluid temperature, borehole wall, grout and ground temperature as well as on borehole loading and borehole thermal effectiveness were investigated. The outcome of this study can substantially reduce the time devoted for research and to quickly determine the impact of various parameters on performance of vertical single U-tube borehole heat exchanger. Furthermore, the obtained result can be utilized as a reference for the design and optimization of heating and cooling system integrated with ground coupled heat pump system.

Borehole heat exchanger (BHE) is a very suitable and cost-effective technology for heat storage and extraction of heat from the ground. In this paper, comprehensive analysis of major influencing parameters on thermal performance of single U-tube BHE (sBHE) and double U-tube BHE (dBHE) was performed by using validated numerical heat transfer model. Combined effect of increasing borehole size and soil or grout thermal conductivity (for different cases of shank spacing) on performance of sBHE and dBHE was studied. Detailed comparative analysis of thermal performance was made among sBHE and dBHE with small and large borehole sizes. The impact of borehole depth and diameter, fluid inlet temperature, mass flow rate, U-tube pipe material type and diameter on total heat transfer per unit borehole depth, borehole thermal resistance and thermal effectiveness was also investigated. The simulation result revealed that when the shank spacing is kept at high or low value in sBHE and dBHE (both with small and large borehole size), dBHE with larger borehole size has the highest thermal performance while sBHE with smaller borehole size has the lowest performance; and the highest and the lowest thermal resistance is obtained for sBHE with smaller borehole size and dBHE with larger borehole size, respectively. Grout with high thermal conductivity is preferred in terms of improving thermal effectiveness and reducing thermal resistance of the BHE when the soil thermal conductivity is sufficiently high; while low grout thermal conductivity is preferred with large borehole size and high shank spacing. For high soil thermal conductivity (4W/m.K), the heat transfer rate per unit borehole depth increases with grout thermal conductivity to a maximum value; and then further increasing of the grout conductivity results in smooth declining of the heat transfer in the BHE. Thus, ground and grout thermal conductivity have an optimal range to achieve a lower value of the BHE thermal resistance, and consequently for better BHE sizing. With increasing fluid inlet temperature (15oC to 42oC), the total heat transfer is improved by 51.5 W/m and 36.2 W/m for dBHE and sBHE respectively; and more heat is injected into the borehole when dBHE is used than sBHE, particularly at higher inlet temperature showing that dBHE is preferable than sBHE for system that works with higher fluid inlet temperature. The result obtained can be used as a quick reference for the design, operation optimization and performance study of ground coupled heat pump system where BHE is used as aborehole thermal energy storage.

In the ground coupled heat pump system that delivers space heating and cooling to the buildings, often questions are raised as to which borehole heat exchanger configuration (single or double U-tube) provides higher thermal performance. To answer this question, in this paper, detailed comparative transient heat transfer analysis between single and double U-tube borehole heat exchanger (BHE) with two independent circuits (that can operate with the same or different mass flowrate and inlet fluid temperature) is conducted. Validated numerical heat transfer model was applied to perform the dynamic simulation and analysis. The two borehole heat exchangers are compared from the perspectives of their thermal performance in the heating and cooling mode of operation. Detailed sensitivity analysis of the BHE has also been performed by investigating the impact of the important borehole parameters, thermal properties and working fluids on the thermal performance of the double U-tube BHE. The simulation result revealed that the heat injected and extracted by the double U-tube BHE at steady state is 21.8 (77%) and 10.2 W/m (71.8%) more than that of the single U-tube BHE respectively. The simulation results also indicate that the thermal effectiveness of the single and double U-tube BHE at steady state are 0.24 and 0.31 respectively. The single U-tube BHE showed higher borehole thermal resistance than that of the double U-tube BHE with values of 0.47 and 0.31 m.K/W, respectively. The results of the study also revealed that among the studied working fluids, CaCl2 (25%) solution showed better thermal performance than other anti-freeze solutions with thermal effectiveness of 35.6%. The simulation result of this work is crucial as an input for the optimization studies made to obtain a single set of optimal borehole paramaters and thermal properties that results in the best performance of BHE coupled to ground source heat pump system designed to provide space heating and cooling of buildings.

This paper presents a comprehensive thermodynamic modeling of the solar-powered lithium bromide -water (LiBr-H2O) absorption chiller system. The study examined the influence of the solar collector types on the collector efficiency and the useful heat gain by the collector for the best performance. The study also analyzed the effects of generator inlet temperature, effectiveness of the solution heat exchanger and the pump mass flow rate on the energetic and exergetic performance of the chiller system. The examined performance parameters were coefficient of performance, exergetic efficiency, exergy destruction, fuel depletion ratio and improvement potential. The study revealed that ‘evacuated selective surface’ collector type has resulted in higher efficiency and more useful heat gain than the single and double glazed collector types. Additionally, the increase in the heat exchanger effectiveness improved the system performance, while the increase in the pump mass flow rate reduced the chiller performance. Furthermore, the result also indicated that the main source of the exergy destruction is the solar collector. In the solar collector, 71.9% of the input exergy was destroyed which accounted for 84% of the total exergy loss. Additionally, 7.1% of the inlet exergy was lost in the generator which was equivalent to 8.3% of the total exergy loss. The overall exergetic improvement potential of the system was approximately 84.7%.

This work presents a thermodynamic analysis of a poly-generation system powered by solar energy using parabolic trough solar collectors. The system is composed of an organic Rankine cycle (ORC), a multiple effect distillation and an absorption cooling unit. The analysis is based on the solution of mass, energy and exergy balances of the set of equations of all the components of the system. It is also based on the technical specifications of sub-systems and working fluid properties. The validation of the computer program is achieved systematically. The performance of the poly-generation plant is investigated under Riyadh weather conditions and for several conditions of the operating parameters. The variation of the energy rates required for desalination, cooling and electricity generation has been obtained for two representative days in summer and winter of 2013 in Riyadh. Results expressing the plant performance using the energy and exergy efficiencies are presented and discussed. Specifically, energy utilization factor, artificial thermal efficiency, fuel energy saving factor and exergy efficiency were introduced and used as plant performance indicators. The fresh water production rate and the power to water ratio were also evaluated for two representative days in June and January of 2013. The main results of the study show that although there is a clear dispersion of the obtained values for the performance of the solar poly-generation plant, the exergy efficiency and the fuel energy saving factor give close values.

This paper addresses the development of portable and hybrid solar-powered membrane distillation (SPMD) system for producing freshwater in remote arid and/or coastal areas like in Saudi Arabia. It is a stand-alone system that only uses renewable solar energy to operate. Thus, the system is highly suitable to be implemented in remote arid and coastal areas without infrastructures or connection to the grid (water and power), but blessed with abundant solar irradiation. Because the system is portable, it can also be used for emergency case like a natural disaster (e.g. tsunami). The system uses Vacuum Multi-Effect Membrane Distillation unit from Memsys to desalinate the seawater or brackish water. The SPMD system is a hybrid system that combines thermal collectors and photovoltaic (PV) panel. The thermal collectors convert solar irradiation into thermal energy to drive the Memsys process, whereas PV panel converts solar irradiation into electrical to power electrical equipment. The overall operations of the system were governed by Programmable Logic Controller. The innovation of this system is the design of a hybrid solar desalination system that is portable and reliable. The system can also be considered as a “green” and sustainable desalination technology. To study the performance of the system, small-scale tests have been conducted at the Engineering College – King Saud University, Saudi Arabia. The system ran successfully by only utilizing solar energy. The total volume distillate output during the test was approximately 70 L with an approximate conductivity of 4.7 μS/cm. The average distillate output rate was 11.53 L/h with the maximum of 15.94 L/h at noon time, whereas the distillate flux was in the range of 1.5–2.6 L/m 2 h.

This paper proposes numerical unsteady heat transfer model that predicts the transient heat transfer in the double U-tube borehole heat exchanger (BHE) where the U-tubes can operate with either different or equal mass flow rate and inlet fluid temperature. The implicit numerical method applied on heat transfer equations, obtained from energy balance on fluid, grout and ground nodes, was utilized to get the solution. The profile of the borehole loading as well as the net heat exchange on each leg of the U-tube along the borehole depth were obtained for simultaneous charging, discharging, and charging-discharging conditions. Axial variation of the mean fluid temperature, grout and borehole wall average temperature for charging and discharging cases were investigated. The model is extended to consider the axial and radial variation of ground temperature outside the borehole. Dynamic simulation was performed for different operating conditions to investigate various temperature response, borehole thermal resistance as well as thermal performance of the BHE. The proposed model was verified with the previously published experimental results. The performed modeling, simulation and analysis are important for the vertical BHE system optimization, efficiency testing as well as for the quick investigation of the impact of the borehole parameters and thermal properties on the overall performance of the BHE. The model is crucial in the design, optimization and performance study of ground source heat pump and borehole thermal energy storage systems.

This paper presents a thermodynamic analysis of a poly-generation system powered by solar thermal energy using parabolic trough collectors. The proposed system consists of an organic Rankine cycle, a multiple effect distillation and an absorption cooling unit. The performance analysis of the solar system is conducted for different configurations: power generation only, cogeneration power and cooling, cogeneration power and desalination, and poly-generation. The effects of turbine inlet temperature and pump inlet temperature on the energetic and exergetic system performance as well as the net power output and total exergy loss of the system are examined. In addition, exergetic parameters, including system total exergy loss, fuel depletion ratio and improvement potential were analyzed. The study reveals that increasing the turbine inlet temperature increases the performance while it reduces the total exergy destruction rate of the system. The result of the study also shows that the two main sources of exergy destruction are the solar thermal collector and desalination unit; with 49.3% of the input exergy (76% of the total exergy loss) destructed in the collector while 9.6% of the inlet exergy (14.9% of the total exergy loss) is destroyed in the desalination system. The overall improvement potential of the system was found to be 64.8%.

This research focused on the development of an innovative design of solar-powered desalination (SPD) system which was expected to solve the water and energy problem simultaneously. We have developed a portable and hybrid solar-powered desalination (SPD) system for producing potable water from saline water. It is a self-contained and integrated system which combines solar-thermal collector and solar-photovoltaic for its operation, and thus the system can operate to produce water by only using solar energy. Therefore, the system is highly suitable to be implemented in remote arid and coastal areas without infrastructures or connection to the grid (water and power), but blessed with abundant solar irradiation, like in Saudi Arabia. A Memsys Vacuum Multi-Effect Membrane Distillation (V-MEMD) unit was used as the core of the SPD system. A heat pump was also integrated into the SPD system for energy recovery and to improve the performance of the system. The system could be considered as sustainable and “green” desalination technology, which will be very useful for the Kingdom of Saudi Arabia. To study the performance of the system, small-scale tests have been carried out at the Engineering College - King Saud University, Saudi Arabia. Based on the experimental results, the system has run successfully by only utilizing solar energy.

In this paper thermodynamic modeling of organic Rankine cycle (ORC) driven by parabolic trough solar collectors is presented. Eight working fluids for the ORC were examined. The effect of turbine inlet temperature on main energetic and exergetic performance parameters were studied. The influences of turbine inlet temperature on turbine size parameter, turbine outlet volume flow rate and expansion ratio were also investigated. Important exergetic parameters including irreversibility ratio and total exergy destruction rate were also included in the analysis and evaluated. The study reveals that increasing the turbine inlet temperature results in increasing the net electric efficiency, net power output, exergy efficiency and expansion ratio while the total exergy destruction rate and turbine size parameter are reduced. From the considered working fluids, o-xylene gives the best energetic and exergetic performance. The results of the study also show that the main source of exergy destruction occurs in the solar collector where 74.9% of the total exergy loss is destructed. Next to collectors, 18.2% of the total destructed exergy occurs in the condenser

Designing of a solar heating system involves appropriate sizing of different components based on predicted solar insolation and heating load demand. But, it is a complex problem due to unpredictable weather data components. A number of design methods are available for solar heating systems. In this paper, f-chart method has been used due to its simplified design procedure, analysis and low cost in selecting the sizes and type of solar collectors and in estimating the annual thermal performance of solar heating system. Using this method, the design of liquid solar heating system and the estimation of the fraction of total heating load (domestic water and space heating load) that will be supplied by solar energy for a family of six in Riyadh have been conducted. The study includes the effect of different collector areas and storage capacity per square meter of collector area and collector tilt angle on fraction of the load supplied by solar energy. It has been found that increasing the collector area results in an increasing of annual load fraction supplied by solar energy. It has also been seen that increasing the specific storage capacity results in small increase in solar load fraction and the effect is more visible during the summer than during the winter. The result of the study reveals that collector configurations with lower tilt angles are better during the summer and higher values of angles are better during the winter. The optimal annual collector configuration tilt angle which gives the maximum solar load fraction has been found to be 30o