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Improving Heat Pump Performance with Solar Energy - By : Pierre-Luc Paradis, Daniel R. Rousse, Louis Lamarche, Hakim Nesreddine,

Improving Heat Pump Performance with Solar Energy


Most air conditioning systems can be reversed, i.e. they can be used as a heating system. In such a scenario, it is possible to use solar energy to maximize the system’s heating performance. Coupling is even more interesting when photovoltaic modules are used. We find ourselves in a situation where the performance of each component can be simultaneously improved.

 

Pierre-Luc Paradis
Pierre-Luc Paradis Author profile
Pierre-Luc Paradis holds a PhD in Mechanical Engineering from ÉTS. His research focuses on the modeling and design of thermal systems, solar energy and heat pumps.

Daniel R. Rousse
Daniel R. Rousse Author profile
Daniel R. Rousse is a professor in the Mechanical Engineering Department. His research interests include sustainable and renewable energy, Measurements in energy, and transfer, conservation, conversion and production of energy.

Louis Lamarche
Louis Lamarche Author profile
Louis Lamarche is a professor in the Mechanical Engineering Department at ÉTS. His research interests include heat transfert, simulation of energetic and mechanical systems, geothermal systems and instrumentation.

Hakim Nesreddine
Hakim Nesreddine Author profile
Hakim Nesreddine is a researcher at Hydro Québec's LTE (Laboratoire des technologies de l’énergie). His research focuses on refrigeration systems, heat recovery, energy efficiency and decentralized energy generation.

Building façade peppered with air conditioners

Header image purchased on Istock.com. Protected by copyright.

Heat Pumps

We see more and more residential buildings equipped with air conditioning to maintain the comfort level of occupants during hot summer days. In the case of multi-unit buildings, it is not uncommon to see decentralized systems where each dwelling has its own system, as shown in Figure 1. Typically, a heat exchanger with a fan (also called a fan coil unit) is installed inside the apartment and the rest of the system is located directly on the balcony or on a wall bracket (Figure 1). The external part of the system includes a compressor and a second heat exchanger equipped with a fan that rejects heat into the outside ambient air during summer. These systems are often reversible: in addition to air conditioning, they can serve as a heating system. The system then acts as a “heat pump” since its function is to transfer heat from a cold environment to a warm one. In the summer, the heat pump keeps apartments cool while it is hot outside; in the winter it does the opposite. However, the performance in heating mode is affected by the outside ambient air temperature: the colder it is outside, the lower the performance of the heat pump will be, while at the same time, the building’s heating needs increase.

Building packed with individual air conditioners

Figure 1 Multi-unit buildings equipped with air conditioning systems (photo taken in the Trois-Rivières area)

Coupling with Solar Collectors

To maintain the heat pump’s heating performance despite the decrease in temperature of the outside ambient air, solar collectors can be used, as the outdoor unit of the heat pump is usually installed on the balcony. If the facade of the building is exposed to the sun for much of the day, the glass panes of the balcony railings could simply be replaced with solar collectors. The idea is to use solar collectors to increase the heat pump evaporator capacity, consisting of a heat exchanger and a fan. This way, the sun’s energy is reclaimed, in addition to drawing in the cold outside air to evaporate the heat pump’s refrigerant (heat transfer fluid).

Photovoltaic Module

The advantage is twofold in the case of a photovoltaic (PV) module, a type of solar collector known to convert solar radiation into electricity. The efficiency of a PV module is about 15%. This value fluctuates according to the technology used, but it currently does not exceed 25%. However, such low efficiency leads to parasitic heating of the PV module being used. Its temperature can easily reach 50 °C under rated operating conditions.

Advantages of Coupling

Increasing the temperature of PV cells is detrimental as it reduces their electrical conversion efficiency. Thus, the performance of both the heat pump and the PV module are affected by a temperature variation. By combining these two systems, the heat of the solar collector can be used to boost the heat pump’s heating performance. At the same time, the solar collector’s operating temperature is reduced to maximize its electrical performance.

PV/T Solar Evaporator

To do this, a heat exchanger can be installed on the back of the solar collector, as shown in Figure 2a) and Figure 2b). The heat pump coolant evaporates behind the collector and reclaims its thermal energy. This constitutes a hybrid photovoltaic/thermal (PV/T) solar evaporator that produces both electricity and heat. In Figure 2a), a coil-shaped tube is used as the exchanger. This geometry has the advantage of providing a uniform and constant flow throughout the exchanger. The tube is attached to the back of the panel using an epoxy adhesive containing aluminum particles to maximize its thermal conductivity, as shown in Figure 2b).

Heat exchanger assembly

Figure 2a) Coil-shaped heat exchanger installed at the back of a photovoltaic module; b) Epoxy glue used to attach the heat exchanger behind the solar collector

Types of Solar Collectors

Figure 2c) shows various solar collector concepts being tested on the Michel-Trottier HélioLAB rooftop of Pavilion A at Montréal’s École de technologie supérieure. On the left, the solar collector is simply made of a black-painted steel sheet and a coil-shaped tube glued to the back. In the centre, a hybrid PV/T solar collector includes PV cells, a steel sheet and a coil. On the right, a standard PV module (without heat exchanger) serves as a reference for electrical energy production. The solar collectors on the left and centre are connected in parallel to a CO2 transcritical heat pump prototype, developed in recent years. More details can be found on the Abscisse Énergie website.

Solar panels installed on the École de technologie supérieure rooftop

Figure 2c) Different types of solar collectors installed on the rooftop of Pavilion A at Montréal’s École de technologie supérieure

Additional Information

The three types of solar absorber plate related to the three different solar collector geometries were modeled in detail in the following article:

The photovoltaic/thermal hybrid solar evaporator simulation is described in the following article:

Finally, the use of a CO2 heat pump and a hot water storage tank for hot water production has also been studied in the following article:

Pierre-Luc Paradis

Author's profile

Pierre-Luc Paradis holds a PhD in Mechanical Engineering from ÉTS. His research focuses on the modeling and design of thermal systems, solar energy and heat pumps.

Program : Mechanical Engineering  Renewable Energy and Energy Efficiency Engineering 

Author profile

Daniel R. Rousse

Author's profile

Daniel R. Rousse is a professor in the Mechanical Engineering Department. His research interests include sustainable and renewable energy, Measurements in energy, and transfer, conservation, conversion and production of energy.

Program : Mechanical Engineering 

Author profile

Louis Lamarche

Author's profile

Louis Lamarche is a professor in the Mechanical Engineering Department at ÉTS. His research interests include heat transfert, simulation of energetic and mechanical systems, geothermal systems and instrumentation.

Program : Mechanical Engineering 

Author profile

Hakim Nesreddine

Author's profile

Hakim Nesreddine is a researcher at Hydro Québec's LTE (Laboratoire des technologies de l’énergie). His research focuses on refrigeration systems, heat recovery, energy efficiency and decentralized energy generation.

Author profile


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