Italian researchers have reviewed different system configurations for photovoltaic-thermal solar-assisted heat pumps in buildings. They say that using the PV-thermal collector as the heat pump’s evaporator results in the highest heat recovery, but a dual-source system with a separate heat exchanger is the most promising solution to cover all thermal needs.
Researchers from the Polytechnic University of Milan have given an overview of different configurations of solar-assisted heat pump (PVT-SAHP) systems in buildings. While most research has historically focused on thermal solar-assisted heat pumps, the academics noted that “the use of photovoltaic-thermal (PVT) solar collectors is expanding, which may be exploited by heat pumps, with benefits for both systems.”
The team only focused on vapor-compression heat pumps and PVT-SAHP systems for water heating or cooling – the most common application in buildings. They said they only considered water-based flat plate PVT collectors because they “are the most efficient PVT technology and the most feasible for HVAC systems.”
The academics classified the PVT-SAHP systems according to the way heat pumps use solar energy. There are direct-expansion systems (DX), where the PVT collector is used as the heat pump evaporator, or indirect-expansion systems (IDX), where a heat exchanger is interposed between the PVT and the heat pump. These can be further differentiated between single-source configurations, where solar energy is the only heat source for the heat pump, and dual-source systems, where there is another heat source in addition to solar.
Single-source direct-expansion PVT-SAHP systems are an efficient, simple solution for water heating, according to the study. Its applications for cooling are however limited, due to the lack of an additional, cold, source like air or geothermal. Experimental work shows an average coefficient of performance (COP) between 2.7 and 7 for such systems, registered during experiments in favorable environmental conditions.
The absence of an intermediate heat exchanger makes their construction easier and lowers costs. However, using the PVT collector as the heat pump’s evaporator makes the system vulnerable to variable solar radiation.
“Therefore, complex control strategies under unstable weather conditions are necessary, as well as the use of a variable-speed compressor,” the scientists said.
They noted that it is crucial to choose a refrigerant that fits the needs of both the PVT collector and the heat pump, to ensure that the wet refrigerant does not enter the compressor. Dual-source indirect-expansion PVT-SAHP systems, on the other hand, could address many shortcomings of these systems above, but they might be less efficient. These systems report average COPs of 2.3 and 4.5, but the measurements were taken in longer-term experiments.
“The principal advantage of IDX-PVT-SAHP systems is the more stable heat gain from the solar side, since the refrigerant evaporates in a water-to-gas heat exchanger under more stable conditions,” the researchers said.
They said that the separate heat exchanger also allows for the shortening of refrigeration lines, reducing refrigeration mass with economic and environmental gains. The systems also allow for a more flexible configuration and the extra heat source increases the heat pump’s output. This opens up the possibility of adding thermal water storage on either the heat pump side or user side.
The second heat source makes dual-source indirect-expansion PVT-SAHP systems the better solution for cooling. They are a comprehensive solution to cover all building thermal needs, including heating, cooling, and domestic hot water.
“Those systems are also more suitable to be coupled with ground-source heat pumps, which are very popular in heating-dominating regions,” the researchers said.
They claimed that flat-plate PVT collectors with direct lamination of PV cells over roll-bond absorbers guarantee the best compromise between efficiency, reliability, manufacturing complexity, and cost.
“The integration with heat pump systems is confirmed to enhance both electric and thermal performances due to the active cooling, which is relevant in both direct- and indirect-expansion system configurations,” the academics concluded.
They shared their findings in “Photovoltaic-thermal solar-assisted heat pump systems for building applications: Integration and design methods,” which was recently published in Energy and Built Environment.
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