What’s a Heat Pump?
A typical heat pump is made of different components:
- A refrigerant: a heat transfer fluid which evaporates or condenses under pressure variations;
- An evaporator: a heat exchanger which gathers heat in the ambient environment by evaporating the refrigerant. It’s known as the cold side of the heat pump;
- A condenser: a heat exchanger which rejects heat in the ambient environment by condensing the refrigerant. It’s known as the hot side of the heat pump;
- A compressor: it vacuums refrigerant vapor from the evaporator side and increases its temperature and pressure;
- An expansion valve: it creates a pressure drop as the refrigerant flows through a small aperture.
The heat pump Coefficient Of Performance (COP) is a general indicator for the heat pump efficiency. Using energy and entropy balances, one can show that a decrease in the temperature of the cold sink (TL) increases the compressor electrical consumption (W) for the same amount of heat transferred to the hot side (QH). It means that when the ambient outdoor temperature decreases, the compressor electrical consumption increases to continuously provide heat to the building. Furthermore, the hot side temperature (TH) is generally relatively stable at around 20°C to guarantee thermal comfort to building occupants. The following figure shows the mathematical equations describing the heat pump system performances..
Photovoltaic (PV) solar collectors
A photovoltaic (PV) solar collector is a solar panel (as shown below) able to convert incident solar radiation to electricity. Different solar cell technologies exist but the most common uses silicon. The produced electricity is dependent on the cell technology, the level of solar radiation (sunny or cloudy day), the design of the panel, the working cell temperature, the area of the solar collector, and many other factors. The following figure presents a typical 980 mm x 1650 mm silicon solar collector made of a matrix of 60 solar cells.
A PV solar collector is only able to convert a small part of the incident solar radiation in electricity. The remaining incident energy is converted into parasitic heat which increases the solar cells temperature. This heat is eventually lost in the outdoor ambient environment. The electrical conversion efficiency (ηelec) decreases with an increase in the working cells temperature. The following equation presents this decrease in efficiency related to the working cells temperature (Tp) in comparison with a reference temperature (T0) which is usually 25°C.
A calculation example of the impact of heat on a monocrystalline silicon PV panel is also shown above. The typical photovoltaic conversion factor (η0) of these cells is usually 15% for the reference temperature (T0) of 25°C. When the solar collector is exposed to the sun, its temperature can easily reach 50°C. This increased working temperature decreases the photovoltaic performances by 1.66% and the solar collector will only able to convert 13.3% of the sun power into electricity. Then, if the incident solar radiation is approximately 1000W/m2, the 1.6m2 collector only produces 215W.
Why use CO2 as a refrigerant ?
The heat transfer fluid used in a heat pump system is called refrigerant. There are actually multiple fluids that could be used. In the early days of the heat pump system, natural refrigerants like hydrocarbon (HC), carbon dioxyde (CO2) and ammonia (NH3) were used. The invention of synthetic refrigerants led to their widespread use in most systems. The cloro-fluoro-carbon family (“CFC”) is the most known. unfortunately, these synthetic fluids have a large ozone depletion potential (“ODP”). As such, the Montreal Protocol (1989) was widely adopted by countries to protect the ozone layer. This Protocol bans the use of CFC in developed countries since 2009, and will prohibit the use of R-22 gas effective 2020. Another environmental problem related to the use these refrigerant fluids concerns their Global Warming Potential (“GWP”). The Kyoto Protocol (2005) aimed to address this issue, however it has not been widely respected. For example, Canada signed the protocol but hasn’t adopted constraining measures to reduce GWP of commercialized fluids.
The following figure shows the environmental impacts of different refrigerants based on 2 indicators. ODP is a ratio based on R-11, a CFC, which has a value of 1. All other refrigerants’ ODP is compared to the R-11. GWP is a ratio based on CO2, a well known global warming gas, which has a value of 1. On the figure, the grey areas represent the approximate range of ODP and GWP which are subject to legislation adopted by various countries following the Montreal and Kyoto protocols. Natural refrigerants such as hydrocarbon (HC), ammonia (NH3) and carbon dioxyde (CO2) are shown on the white area of the figure. They all have an ODP of zero and low GWP.
Many synthetic fluids have been developed by the industry since the R-11 with lower ODP and GWP ratios; nonetheless, their impacts on the environment are still unknown. Some of today’s systems use R-22 but starting from 2020, it will not be possible to repair and refill these systems. As such, alternative systems have to be developed.
The use of CO2 as refrigerant appears to be a long term solution because it is usually gathered from outside air and does not produce any negative impacts on the environment in case of leakage. This is an advantage in comparison to synthetic refrigerants which could be banned by future legislation should any negative impact be uncovered.