Integration methodologies for solar-assisted heat pumps

Italian scientists have assessed the technical potential of vapor compression heat pumps assisted by photovoltaic-thermal systems. They analyzed two main configurations of this combination describing the advantages and disadvantages of both solutions. The cheapest and easiest to deploy system is a single-source direct-expansion (DX) configuration for hot water for space heating or domestic hot water.

A group of scientists from Italy’s Politecnico di Milano has analyzed the existing integration methodologies to combine a photovoltaic-thermal (PVT) unit with a vapor compression heat pump (HP) in a hybrid system for hot water and heating applications in buildings.

This combination, according to the researchers, has dual advantages. PV can power the HP’s compressor and, at the same time, transfer heat from the solar collector to the HP, which can then operate at a higher temperature, improving overall system efficiency. Furthermore, this configuration may also provide active cooling for the PV panels, which would also increase the performance of the PV system and, in a virtuous feedback loop, the HP itself.

Two different main system configurations were considered by the Italian group: a single-source direct-expansion (DX) system, where the PVT system is operated as the HP evaporator; and a dual-source indirect-expansion (IDX) one, where a heat exchanger (HX) is interposed between the PVT unit and the HP, also known as a solar-assisted heat pump (SAHP).

Single source

In the first configuration, the system does not provide cooling and produces hot water for space heating or domestic hot water (DHW). Crucial for its optimized operation is a real-time compressor frequency control, which adjusts the compressor operating frequency according to real-time radiation.

“Single-source DX-PVT-SAHP is generally adopted for hot water production, but few examples of PVT collectors operated as a condenser during night-time are present in literature,” the scientists explained, noting that this solution is characterized by easier construction and lower complexity and cost, thanks to the absence of any intermediate heat exchanger between the HP and the PVT unit. “However, this characteristic makes direct-expansion configurations more vulnerable to the solar radiation variability, due to the influence on overall performances caused by the mismatch with compressor speed.”

Despite this higher dependence on solar radiation, this solution is claimed to provide the highest heat recovery.

Dual-source

In the second configuration, which is intended at covering the building’s entire thermal needs, a second heat source is provided by a second evaporator operated in parallel with the PVT system.

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“The presence of an additional heat source, generally air, improves system performances under adverse operating conditions, when the solar energy recovery is not enough to operate the machine,” the academics emphasized. “The IDX-SAHP configuration allows a more flexible arrangement of system sub-components, thus the employment of thermal storage for solar energy or load management.”

The main advantage of this system is represented by a more stable heat gain from the solar side and a lower dependence on weather conditions.

“Dual-source IDX-PVT-SAHP systems are the most feasible solution in climatic contexts where cooling needs are relevant, being able to cover all the building thermal needs,” the research team further explained. “Those systems are also more suitable to be coupled with ground-source HP, which are very popular in heating-dominated regions, where air-source HP systems are less diffused.”

Prospects

The researchers specified it is very difficult, for either configuration, to define reference sizing approaches, as variables like climatic conditions and building loads have a big impact on the ratio among the HP rated power, the PVT system size and the storages’ size.

Looking forward, the scientists said they want to work on increasing the technology readiness level (TRL) of both configurations. The TRL measures the maturity of technology components for a system and is based on a scale from one to nine, with nine representing mature technologies for full commercial application.

“More experimental works are also needed to demonstrate the actual performances of the whole system in real case-study buildings, also comparing different sizing criteria according to climatic conditions and building loads,” they concluded. “At the same time, ad additional effort is desirable in the development of specific tools able to properly model each component of the system and the system itself as a whole.”

The proposed approach is described in the paper Photovoltaic-thermal solar-assisted heat pump systems for building applications: integration and design methods, published in Energy and Built Environment.

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