1. 2 Blackett Laboratory, Department of Physics, Imperial College London
AN energetic and economic analysis of hybrid PV and solar-thermal systems for the combined provision of domestic hot water and power in the UK
Ilaria Guarracino1, Maria Herrando1, Ned Ekins-Daukes2, Christos N. Markides1
1 Clean Energy Processes (CEP) Laboratory, Department of Chemical Engineering, Imperial College London
2 Blackett Laboratory, Department of Physics, Imperial College London
1. MOTIVATION
Solar-based renewable systems capable of delivering domestic hot water, space heating and/or cooling, and electricity have a significant potential to contribute to UK and European targets of: (1) a 20% reduction of greenhouse gas emissions; and (2) an increase in the proportion of final energy generation from renewable sources to 20% by 2020, while at the same time decreasing the primary energy consumption in the building sector. A hybrid PV/Thermal (PV/T) system is designed to provide both a thermal output (in the form of hot fluid stream) and an electrical output with the following advantages over separate (side-by-side) PV and thermal systems:
Combined generation of heat (hot water) and electricity from a common area, also leading to a more aesthetically pleasing solution that provides greater architectural uniformity than separate PV and solar thermal systems with a different appearance
Higher PV conversion efficiency and electrical output due to the cooling effect of the circulating fluid
Reduced installation costs (and cost per unit energy) due to the fact that only one system has to be installed
Various PV/T collector designs are available depending on the application. The sheet-and-tube collector water-type design, or PVT/w, operating under forced circulation has been found to perform with high thermal efficiency in cold climates and is an easily adaptable PV/T configuration.
2. ANNUAL PERFORMANCE OF THE PV/T COLLECTOR
The performance of a PV/T system designed for the provision of hot water and electricity in a 3-bedroom (4 person) terraced house in London (Fig. 1) is evaluated on a diurnal basis (Fig. 2). The outputs are then integrated over a whole year. The system demonstrates a coverage of up to 37% of the annual household demand for hot water and 51% of the annual demand for electricity [1]. The best collector performance is obtained for a PV/T collector fully covered with solar cells (P = 1) and operating under low cooling circulation flow-rates (Vp = 20 L/h) (Fig. 3) [1].
FIGURE 2
In a typical day in July, the electricity demand is completely covered by the PV/T system with a surplus of electricity exported to the grid during hours of high irradiance (net electricity Enet > 0, Fig. 2(a)). On the other hand, only 55% of the total hot water demand is covered by the system. The hourly thermal energy provided by the system is always smaller than the total thermal energy required by the household (QPVT < Qt, Fig. 2(b)).
Single glazed
Unglazed
Double glazed
FIGURE 5
A double-glazed collector allows a higher thermal efficiency when compared to a single-glazed or unglazed collector due to the reduction of the convective losses.
FIGURE 6
The electrical efficiency is reduced with glazing due to the optical losses (mainly reflection), which are reduced with AR coating. Spectrally selective coatings (indium oxide coating (ITO) or an aluminium-doped zinc oxide (AZO)) reduces the emission in the infrared spectrum.
FIGURE 7
When the tube spacing is increased the thermal and electrical efficiencies deteriorate due to the higher collector temperature.
4. ECONOMIC ASSESSMENT
PV/T systems are not yet economically competitive with the conventional systems for DHW generation and with PV-only systems due to their high investment cost (~£9,000 vs. £4,000 for 3 kWp) [2]. The forms of incentives available in the UK for the subsidy the PVTs are: the Renewable Heat Premium Payment (RHPP), the Feed-In-Tariffs (FIT) for the electricity generation, and the Renewable Heat Incentive (RHI) for non domestic applications.
FIGURE 9
When the thermal output is taken into account in for economic consideration, the levelised production costs (LPC) and the levelised coverage costs (LCC) of the PV/T systems show that PV/Ts are better alternative than PV-only systems.
Tcout
Tcin
Twin TL TT G
PV/T collector
Bypass
branch
Storage tank
Auxiliary
heater
Load
Mains
Solar radiation G
FIGURE 1
The hot water generated in the solar collector heats up the storage tank. An auxiliary heater ensures the temperature of the water to the load to be at the required temperature.
FIGURE 8
The discounted payback time of the PV/T system is years, which is longer than that of a conventional solution. The payback time of a PV-only system (considering only the cost for electricity provision) is 7 years.
(a)
(b)
FIGURE 10
The application of the RHI closes the difference between the cumulative costs of the PVT and the PV only over the time, thanks to the annual payments for the production of the hot water.
(c)
FIGURE 3
The hot water demand covered DCHW (Fig. 3(a)) has an optimum collector cooling flow-rate at around VP = 10 – 20 L/h. The electrical demand covered DCE (Fig. 3(b)) is maximised between VP = 60 – 120 L/h. The demand covered increases with the collector flow-rate VP, whereas the water outlet temperature decreases, increasing the cell efficiency. As the flow-rate increases more pumping power is required, and as result DCE decreases slightly under high flow-rate operation mode. DC decreases with the covering factor P, while DCE increases at any flow rate (Fig. 3(c)).
FIGURE 11
An alternative form of incentive would notably reduce the cumulative cost of the PV/T compared with the PV-only systems. This consists of the RHI given as a one off voucher at the beginning of the system’s lifetime, assuming a constant annual thermal output and a system’s lifetime of 20 years, together with the RHPP. The discounted payback time can be reduced to one or two years with the application of the RHI or with the alternative form of incentive respectively.
3. MODULE DESIGN
A numerical model of the collector is used to study in more detail the effect of the module design parameters on its thermal and electrical efficiencies and to evaluate the temperature locally on the solar cell. This study assesses the effect of two design parameters: the collector glazing and the tube spacing (W/D ratio).
FIGURE 4
The temperature between two adjacent pipes in a sheet-and-tube collector is not uniform. It exhibits a maximum half-way between the pipes (x = 0) and a minimum above each pipe.
REFERENCES
1. Herrando, M., Markides, C.N., Hellgardt, K., A UK-Based Assessment of Hybrid PV and Solar-Thermal Systems for Domestic Heating and Power: System Performance. Applied Energy (122)
2. Herrando, M., Markides, C.N., A UK-Based Assessment of Hybrid PV and Solar-Thermal Systems for Domestic Heating and Power: Economic Considerations and Lessons for Policy Development. Journal of Energy Policy (submitted).
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