Scientists in Thailand have simulated a photovoltaic-thermal assisted indirect expansion heat pump and measured its performance under different cold water temperatures and tank sizes.
Researchers from Chiang Mai University in Thailand have evaluated the performance of a photovoltaic-thermal assisted indirect expansion heat pump system (IDX-PVT-AHP) designed for hot water production in the tropical climate of Chiang Mai.
“The application of the IDX-PVT-AHP system in the tropical climate of Chiang Mai poses a notable challenge due to the high humidity in the region, which can lead to moisture accumulation on PVT panels,” the team said. “To overcome this problem, it is essential to control the cold water temperature and determine the optimal size of the cold water storage tank to avoid excessive cooling, while also identifying the correct number of PVT modules.”
The system works by circulating water through the PVT modules, absorbing heat before entering the cold water storage tank. Inside the tank, a coil acts as a heat pump evaporator, transferring heat from the plate heat exchanger, which acts as a condenser, to the hot water storage tank. In addition, the PVT modules generated electricity to power the heat pump compressor, water pump and additional heaters. When the electricity generated by the PVT modules is insufficient, the system draws additional power from the grid.
In the first simulation experiment, the system configuration consisted of 1,000 liters of hot water storage and 1,500 liters of cold water storage, connected to three PVT modules. Using monocrystalline cells, it achieved a maximum power of 550 W and an efficiency of 21.3%, with a thermal peak of 1,436 W. The compressor had a cooling capacity of 5.7 kW and used R-134 refrigerant. Four cases for the minimum cold water temperature in the tank were tested under the 2023 climatic conditions for Chiang Mai: case 1 at 18 C, case 2 at 21 C, case 3 at 24 C and case 4 at the true dew point. Dew point temperature is the temperature at which condensation begins to form in the air.
“The lowest energy consumption was achieved when the cold water storage tank set point was 18 C; this condition caused an excessive number of hours causing water vapor condensation (302 h/y), potentially accelerating the degradation of the PVT module,” the results showed. “Of the scenarios evaluated, maintaining the cold water storage tank at the dew point temperature offered the most favorable trade-off between minimizing electricity consumption and reducing the risk of water vapor condensation on the PVT modules.”
In a second experiment, the temperature of the cold water in the tank was kept constant at the actual dew point, while the number of PVT modules and the size of the cold water tank were varied. As for the PVT, they were the same 550W monocrystalline modules, connected in pairs or triplicate. The size of the water tank was set to 500 l, 750 l, 1,000 l or 1,500 l. The aim of this optimization was to find the size that provided the shortest payback time for heating 1,000 l of water to 60 C.
The academics also found that increasing the number of PVT modules improved system performance and reduced reliance on auxiliary heaters, while increasing the size of the cold water storage tank slightly improved thermal buffering but extended the payback period.
They concluded that the optimal configuration consisted of three PVT modules, a 1,000 liter hot water storage tank and a 1,500 liter cold water storage tank, effectively balancing energy efficiency, reliability and economic efficiency in tropical conditions. They also noted that using a dew point temperature setpoint reduced the duration of water vapor condensation to just seven hours per year, while keeping annual electricity consumption low at 7,315.5 kWh and the payback period was 3.36 years.
The system was presented in “Evaluation of cold water temperature and tank size affects the performance of a PVT assisted heat pump system for hot water applications”, published in Case studies in thermal engineering.
This content is copyrighted and may not be reused. If you would like to collaborate with us and reuse some of our content, please contact: editors@pv-magazine.com.
