Performance enhancement of a photovoltaic thermal (PVT) and ground-source heat pump system

Transcription

1 Performance enhancement of a photovoltaic thermal (PVT) and ground-source heat pump system Mohammad Abu-Rumman*, Mohammad Hamdana, Osama Ayadi Mechanical Engineering Department, The University of Jordan, Amman Mohammad.Aburumman@rss.jo Abstract A new system consisting of photovoltaic thermal-ground source heat pump (PV/T-GSHP) was proposed as a solution for electricity shortage and high electricity consumption in heating buildings in Jordan. The performances of the photovoltaic system and the ground source heat pump system were studied without coupling as in real life and with coupling in a hybrid system using TRNSYS software. The results show that this PV/T-GSHP hybrid system can reduce the photovoltaic panels' temperature by more than 20 C, and improve the efficiency of electricity production by 9.5%, simultaneously. And in heating season the average coefficient of performance of the heat pumps increased from 4.6 to 6.2 with a decreament in electricity consumption by 25.7%. The life cycle cost of the hybrid system decreased by 3.9% and the net present value inceased by 13.2% compared to the two systems separated. The model of the PV/T- GSHP system was established using TRNSYS software, an optimmization was made to select the photovoltaic modules' temperature at which cooling starts, followed by technical and economical studies on both systems seperated and coupled in a hybrid system. This hybrid system can provide guidance for future related project. Keywords: renewable energy; ground-source heat pump; solar photovoltaic/thermal; TRNSYS; hybrid system

2 1. Introduction Jordan is a non-oil producing country of approximately 10 million inhabitants [1] that suffers from a severe energy crisis. In 2016, Jordan s total primary energy supply amounted to 9.62 million Ton Oil Equivalent (TOE). Nearly 90% came in the form of imports of crude oil, natural gas, and petroleum products and 5% in the form of local production of natural gas and crude oil. Only the remaining 5% was from renewable energy sources [2]. Much focus in Jordan has been given to solar photovoltaic which is one of the most widespread technologies of renewable energy generation. On the other hand, the main obstacle that faces the operation of solar photovoltaic is overheating due to excessive solar radiation and high ambient temperatures. As a rule of thumb, every 1 C surface temperature rise of the solar photovoltaic module causes a reduction in efficiency of 0.5% [3]. The residential sector in Jordan is responsible for 20% of the final energy consumption, with nearly 49% of this energy going to space heating and cooling [4]. Geothermal energy is a renewable, environment-friendly and the most common application of geothermal energy is ground-source heat pump system which can be used to cover the heating and cooling needs of the residential sector. Geothermal heat pumps have low operation and maintenance cost but a high investment cost compared to other cooling and heating products.

3 Fig. 1. Electricity consumption by sector [2] As indicated in Fig. 1 household sector consumed 45% of the electricity generated and 49% of it goes to space heating and cooling. In order to tackle this problem there are two possible solutions. The first solution is to increase the share of generated electricity from renewable energy sources to cover part of the demand and the second one is by reducing the electricity consumed by household sector. In this paper, the two solutions were applied together by increasing the generated electricity from cooling solar photovoltaic to improve its efficiency and by decreasing the consumed electricity by heat pumps for space heating with coefficient of performance increment. This paper investigated the efficiency enhancement of the photovoltaic modules by using forced water circulation cooling technique and using the heated water as inlet-source water for the heat pumps in heating mode to reduce consumed electricity and rise the COP of the heat pumps. As a result, the two systems will function more effective than each system alone. 2. System description and operation strategy The higher council for science and technology (HCST) building was taken as a case study due to the deployment of photovoltaic and ground-source heat pump systems. The photovoltaic

4 system on the roof of the Higher Council for Science and Technology (HCST) building has a capacity of 52 kwp approximately consisting of two inverters connected to 174 polycrystalline panels with a maximum power output of 305 W each. Any cooling technique used should keep photovoltaic panel's temperature low and steady. In addition, it should allow the usage of the extracted heat from the panels. Forced water circulation technique was used due to its ability to increase the photovoltaic panels efficiency effectively if adjustment flow rate is used. The ground-source heat pump system type is ground coupled heat pump with vertical-loop design. The system consists of five 60kW heat pumps, four working and one backup, with a total capacity of 300kW. The external-loop consist of 32 boreholes with a depth of 130 m each and 6 meters distance apart. Three pumps used to circulate the water through the ground heat exchanger to approximately exit with C temperature all around the year. The ground heat exchanger connected to the heat pumps transferring heat from or to refrigerant. The refrigerant goes into vapor-compression refrigeration cycle then makes heat transfer with water circulated by using ten circulation pumps inside the building to 133 fan coils for air conditioning the space. The photovoltaic panels cooling system starts working when the panels reach a temperature above 45 C using variable speed pump. Cooling the photovoltaic panels by water at full load every 1 C rise in temperature will lead to that extra electricity generated will be consumed by continuous operation of the pump so a process of optimization was done to get best results of cooling with minimum electricity consumption. The output flow of the pump is dependent on the panels temperature. The ground source heat pump (GSHP) system works as in real case. If the ambient temperature is less than 15 C then the system in heating mode, if the ambient temperature is

5 greater than 22 C then the system in cooling mode and if the ambient temperature between C the heat pumps is off and ventilation fans is on. The two systems coupled with each other if two conditions occurred at the same time, the first condition is the heat pump is working in heating mode which means that the ambient temperature is less than 15 C. The second condition is that the water's temperature comes out from the photovoltaic cooling system is greater than the water's temperature comes out from the ground heat exchanger. The normal operation that the two systems work separately unless the two conditions mentioned above occurred at the same time then the two systems coupled with each other. In normal case the cooling of the photovoltaic system uses the water coming out of the ground heat exchanger at C to cool the panels. At the same time the ground heat exchanger is used for the ground source heat pump in the cooling mode or in the heating mode if the temperature of the geothermal loop water is greater than the water output from the photovoltaic cooling system. When the two systems coupled with each other, the system working sequence become as the following: the water coming out of the ground heat exchanger at C goes for cooling the photovoltaic system then the output water enters as source inlet of the heat pump and the output water of the heat pump goes to the ground heat exchanger and the cycle repeats itself again. The evaluation took a one-year time using TRNSYS [5] simulation program to simulate the photovoltaic panels system and the ground source heat pump system performances separately and coupled together.

6 the working sequance can be summurized in the following schematic: Fig. 2. Schematic diagram about the operation sequence of the PV/T-GSHP system. Fig. 3. Shows the simulated hybrid system built using TRNSYS software with the hourly space heating load calculated using Type 12c of TRNSYS based on the typical meteorological year (TYM) weather conditions.

7 Fig. 3. PV/T-GSHP system built using TRNSYS software. Ten heat pumps were used with the same capacity of the real system and the PVT modules photovoltaic characteristics the same of real photovoltaic panels. In addition, a system validation was made to ensure that the simulated systems represent the real systems and the validation results was in an acceptable range for photovoltaic system and ground source heat pump system with a deviation of 5.2% and 4.9% respectively. 3. Cooling Temperature Optimization The cooling of the photovoltaic panels is essential to increase generated electricity and the conversion efficiency. On the other hand, the consumption of the water circulation pump electricity must be taken in consideration. In addition, the higher temperature water entering the heat pumps, the lower electricity consumed for space heating and the higher coefficient of performance.

8 Net electricity ( kwh per year) An optimization was made to select the photovoltaic panels' temperature at which cooling will start to yield maximum net electricity. The net electricity is the generated electricity from the photovoltaic system with subtracting the consumed electricity by water circulation pump and the consumed electricity by heat pumps for space heating. Using TRNSYS simulation program, the net electricity was estimated when the cooling starts at a photovoltaic panels' temperature of 20 C, 25 C, 30 C, 35 C, 40 C, 45 C, 50 C, 55 C and 60 C and it was found that the optimal photovoltaic temperature at which cooling starts is 45 C as shown in Fig Photovoltaic temperature at which cooling starts ( C) Fig. 4. Net electricity VS. starting cooling temperature.

9 Temperature in C Temperature in C 4. Simulation results and discussions In this part, TRNSYS was used to simulate and analyze the performance of each system with and without coupling. 4.1 Photovoltaic system simulation results (With and Without Cooling). The period from the beginning of June until the end of July was chosen for photovoltaic system evaluation because it's the highest temperature period around the year and if the cooling system can handle the heating of the photovoltaic panel during this period, it could handle the heating in the whole year. Fig. 5. Photovoltaic mean temperature with cooling system. Fig. 6. Photovoltaic mean temperature without cooling system.

10 Efficiency Efficiency From Figs. 5-6, it can be noted that the maximum temperature of the photovoltaic panels with cooling reaches (61 C) and without cooling reaches (83 C) in the period from the beginning of June until the end of July. The temperature decrement in photovoltaic panels' temperature led to a rise in its electricity generation by (9.5%) per year. In addition, it will decrease photovoltaic panels degradation with time. Fig. 7. Photovoltaic efficiency with cooling system. Fig. 8. Photovoltaic efficiency without cooling system.

11 Output power (W) Output power (W) From Figs. 7-8, it can be noted that the forced water circulation cooling method can reduce the temperature effect on efficiency and mainly on the lower limit in the hottest period in the year but it can t increase the efficiency above its rated value which is (18.5%). The maximum and minimum efficiency values of the photovoltaic panels with cooling is (18.3%, 15.6%) respectively and without cooling is (18.0%, 14.1%) respectively in the period from the beginning of June until the end of July. It can be noted that the bigger effect of the cooling appears on the minimum values preventing it from coming lower than (15.6%). Fig. 9. Photovoltaic output power with cooling system. Fig. 10. Photovoltaic output power without cooling system.

12 Input power (W) Input power (W) From Figs. 9-10, the summation of the photovoltaic panels' output power with cooling is (24,166 kwh) and without cooling is (22,822kWh) with an increment of (1,344 kwh) in the period from the beginning of June until the end of July. While the increment in output power for the whole year is (8,478 kwh). 4.2 Ground Source Heat Pump System Simulation Results (With and Without Coupling). The period from the beginning of January until the end of February was chosen for ground-source heat pump system evaluation because it is the lowest temperature period around the year and with the maximum heating load required, leading to the biggest effects of coupling process around the year. Fig. 11. Heat pumps input power with coupling with PV/T system. Fig. 12. Heat pumps input power without coupling with PV/T system.

13 Input power (W) Input power (W) Fig. 13. Heat pumps input power with coupling with PV/T system. Fig. 14. Heat pumps input power without coupling with PV/T system. The period of 300 hours was taken to show the detailed power consumption of the heat pumps. It is to be noted that the gaps represent the offpower status of the heat pumps. Figs , show the difference in operation hours of the heat pumps with and without coupling with PV/T system leading to a difference in electricity consumption when comparing the two cases with each other. By using the heated water from photovoltaic thermal

14 Temperature in C Temperature in C system as inlet water for the heat pumps, the electricity consumption dropped from (61,085 kwh) to (12,367 kwh) with a decrement of (4,213 kwh) in the period from the beginning of January until the end of February. While the decrement in output power for the whole year is (8,044 kwh). Fig 15. Heat pumps inlet-source water's temperature with coupling. Fig 16. Heat pumps inlet-source water's temperature without coupling. The Fig , show the difference in heat pumps source-inlet water's temperature with and without coupling with the PVT system. This difference causes the reduction in the electricity consumption of the heat pumps and the increment in the coefficient of performance.

15 The generated electricity from the photovoltaic system without cooling is (89,031 kwh per year) while with cooling it is (97,509 kwh per year). In addition, the consumed electricity by heat pumps in heating mode without coupling is (31251 kwh per year) while with coupling it is (23207 kwh per year) leading to an increase in COP from 4.6 to 6.2. It can be concluded that the two systems performances enhanced by coupling them in a hybrid system. 5. Economic Impact The Higher Council for Science and Technology (HCST) building was depending before the ground source heat pump system installation on HVAC units for cooling and on diesel boilers for heating. The cost of covering the heating capacity by diesel boilers was 6,834 JOD per year and the cost of covering the cooling capacity by HVAC units was 21,424 JOD per year. The total cost for covering heating and cooling capacities was 28,258 JOD per year while after installing the ground source heat pump, the cost of covering the heating and cooling capacities became 13,178 JOD. The ground source heat pump system costs about 258,600 JOD and the salvage value of replaced items in boiler room is 10,000 JOD. The electricity bills for the building in 2014 was 18,345 JOD and after installing the photovoltaic system the saving in electricity is 22,791 JOD per year after electricity usage increases due to the space heating became dependent on the electricity consumed by the heat pumps. The photovoltaic system costs about 52,000 JOD and the modification for converting PV to PVT costs about 27,982 JOD. Life cycle cost and net present value will show the difference between the two systems separated and the two systems coupled with each other in a hybrid system. The following factors were considered in the economic study: The initial investment of the ground source heat pump system is 258,600 JOD

16 The initial investment of the photovoltaic system is 52,000 JOD The cost of converting photovoltaic panels to photovoltaic thermal panels is 27,982 JOD Salvage value of the old boilers is 10,000 JOD Annual saving from using photovoltaic system instead of grid electricity is 22,791 JOD Annual saving from using ground source heat pump system instead of using conventional system for cooling and boilers for heating is 15,080 JOD Annual saving from using photovoltaic system after coupling 24,962 JOD Annual saving from using ground source heat pump system after coupling is 17,139 JOD The annual operation and maintenance cost for the photovoltaic system is approximately 100 JOD and for the ground source heat pumps system is approximately 200JOD Inflation and discount rates are 4.5% The life time for photovoltaic system and ground-source heat pump system is estimated as 25 years with zero salvage values. 5.1 Life cycle cost method. Life-cycle cost (LCC) was used to compare the economic effect of coupling the photovoltaic system with ground-source heat pump system. Life-cycle cost (LCC) is the sum of all the costs associated with an energy delivery system over its lifetime or over a selected period of analysis and takes into account the time value of money [6]. The basic idea of life-cycle costs is that anticipated future costs are brought back to present cost (discounted). This analysis includes inflation when estimating future expenses. In addition, the net present value was also used to compare photovoltaic system with photovoltaic thermal system. Also, to compare ground source heat pump coupled with photovoltaic thermal system and without coupling.

17 The annual energy cost over the life time is evaluated in the present time using Present Worth Factor (PWF) which is a function of inflation and discount rates as shown in Equations(1-2) ( 1 i i ) [1 (1 d i 1 d ) ], d i (1) { 1 d, d i (2) Where: i, Inflation Rate (4.5%) and d, Discount Rate (4.5%), PWF = The life cycle cost for photovoltaic system with and without coupling and the life cycle cost for ground source heat pump system with and without coupling are: LCC PV = Initial Investment + PWF. O&M Cost = 54,392 JOD LCC PVT = Initial Investment + Conversion to PVT Cost + PWF. O&M cost + PWF. Cooling Pump electricity Cost = 83,672 JOD LCC GSHP without coupling = Initial Investment + PWF. O&M cost + PWF. Heating Electricity Cost = 454,750 JOD LCC GSHP with coupling = Initial Investment + PWF. O&M cost + PWF. Heating Electricity Cost = 405,492 JOD LCC PV+GSHP without coupling = 509,142 JOD LCC PVT+GSHP with coupling = 489,168 JOD The life cycle cost of the photovoltaic system increased from (54,392 JOD) to (83,672 JOD) due to the addition cost for converting photovoltaic modules to photovoltaic thermal modules and the electricity cost for cooling circulation pump. The life cycle cost for the ground-source heat pump system decreased from (454,750 JOD) to (405,492 JOD) due the heated water effect on the heat pumps electricity consumption. The life cycle cost of the photovoltaic

18 system and the ground-source heat pump system decreased by coupling them in a hybrid system from (509,142 JOD) to (489,168 JOD). 5.2 Net present value method. The Net Present Value method can be computed for each system using Equation 3: ( ) ( ) ( ) NPV: Net present value. B o: Benefits in the 0th year. B j: Benefits at the end of period j. C o: initial capital investment in the project. C j: Costs at the end of period j. i: Interest rate(4.5%). n: Useful life of the project (25years). The net present values for each system can be computed using equation 5.3 by solving the equation for n = 25 and i = 4.5%. The salvage value of replaced items in boiler room was considered as B o (Benefits in the 0th year) and the modification for converting photovoltaic to photovoltaic thermal was included with initial investment costs. The estimated life cycle for both systems is 25 years and no salvage value were considered. 256,511 JOD 290,447 JOD The net present value of the photovoltaic system and the ground-source heat pump system increased by coupling them in a hybrid system from (256,511 JOD) to (290,447 JOD). The life cycle cost and the net present value methods proves the economic status of the both systems is better coupled with each other than each system alone.

19 Cash flow in JOD Fig. 16. shows the cash flow diagram for a period of 25 years for each system: Cash flow diagram for all systems Time in years Photovoltaic system Photovoltaic thermal GSHP without coupling GSHP with coupling Total system without coupling Total system with coupling Fig. 17. Cash flow diagram for all systems.

20 6. Conclusion A new PV/T-GSHP system was designed for applying in Jordan to solve energy problem. The system was validated and the data were analyzed. Then, the system performance was simulated by TRNSYS software, so the performance evaluation could be obtained and finally, and an economic study was conducted to study the influence of coupling photovoltaic system and ground source heat pump system together compared with the two systems separated. According to the data analyzed, generated electricity of the photovoltaic system increased by (9.5%) as well as the maximum temperature of photovoltaic modules decreased by (36%) compared to photovoltaic modules without cooling. In addition, consumed electricity by heat pumps decreased by (25.7%) and the COP increased from (4.6) to (6.2) due to coupling with photovoltaic thermal modules. The economic study showed that, due to coupling the life cycle cost for both systems decreased by (3.9%) and the net present value increased by (13.2%) compared to both systems separated. The performances of both systems were better and the economic status also were better than each system alone leading to the conclusion that the coupling of the two system is efficient, effective and economically feasible. References [1] Jordan Department of Statistics, Jordan in figures 2016, Amman, [2] Jordan Ministry of Energy & Mineral Resources, 2016 Annual Report, Amman, [3] Kumar K, Sharma SD, Jain L. Standalone Photovoltaic (PV) module outdoor testing facility for UAE Climate. Submitted to CSEM-UAE Innovation Centre LLC; [4] A. Al-Ghandoor, I. Al-Hinti, B. Akash, E. Abu-Nada, Analysis of energy and exergy use in the Jordanian urban residential sector, Int. J. Exergy 5 (2008) [5] TRNSYS, TRNSYS 16.1: A Transient Simulation Program, 2007, University of Wisconsin, Madison, USA. [6] John A D, William A B. Solar Engineering of Thermal Processes. Fourth Edition