Operating Principles and Performance
First and second laws of thermodynamic govern the operating principles of heat pumps. Particularly, heat pumps operate in reversed cycles unlike the heat engines (Fig. 1.1) (Chwieduk 2012). The purpose of heat engines is to generate work (W) out of a heat source (Q2), while heat pumps purpose is to upgrade heat from a low temperature heat source (Q1) to a high temperature heat sink (Q2). It must be said that heat pumps can be used as heating device or refrigerator, reversing the refrigerant flow. In order to comply with the first law of thermodynamics, heat engines have to discharge an amount of heat (Q1) to a heat sink, while heat pumps have to receive W. Obviously, T2 temperature is greater than T1 (Fig. 1.1).
Fig. 1.1 Operating principles of heat engines (power cycle) and heat pumps (Reverse cycle) (Chwieduk 2012)
As regard heat pumps system, two efficiency indexes can be formulated in accordance with Fig. 1.1 for estimating heating or cooling performance. They are respectively the coefficient of performance (COP) and the energy efficiency ratio (EER).
From Eqs. 1.1 and 1.2, it can be concluded that EER is lower than COP. However, this can mislead. Indeed, second law of thermodynamic states that heat cannot be transferred from a lower temperature to a higher temperature without consuming energy. Appling this law to heat pumps (Fig. 1.1), Eq. 1.3 can be formulated.
Considering that T2 temperature is greater than T1, W has to be big enough to counterbalance the algebraic sum of the first and the second term in Eq. 1.3. Thus, performances of the heat pump are influenced by temperatures levels of both heat source and heat sink. Fixing the heat delivered or removed from the building, closer the temperatures of the heat sink and heat source are, less is the work needed. Depending on the climate conditions and building energy requirements, EER values can be greater than COP values, if these temperatures are closer during the cooling period than during the heating season.
Figure 1.2 describes both heating and cooling operating modes of a SL-GSHP. Particularly, main components can be identified:
Fig. 1.2 SL-GSHP operations: a heating mode, b cooling mode
- • Compressor;
- • Condenser;
- • Expansion valve
- • Evaporator;
- • Ground heat exchanger (GHX).
These are responsible of the four changes of state shown in Fig. 1.3, which represent a vapour compression cycle in the pressure-enthalpy diagram. Particularly, segment 1-2a refers to non-isentropic compression, the portion of the segment 2a-3 inside the bell-shaped curve to condensation (isobaric and isothermal heat rejection), 3-4 to isenthalpic expansion and 4-1 to isobaric and isothermal evaporation. Basically the GCHPs interact with the soil extracting or releasing heat in it as a function of the operating mode. With regard to heating operations Fig. 1.2a heat is extracted from the ground, due to the evaporation of the refrigerant at low pressure, and it is released into the building, condensing the refrigerant at high pressure. On the contrary as regard cooling operations, switching the position of the GHX with the condenser in the refrigerant cycle (Fig. 1.2b), heat is released into the ground, refrigerant condensates, and extracted from the building, refrigerant evaporates.
The performance of a GCHP are estimated as the ratio between produced useful energy, heating or cooling energy and energy consumed, generally electricity. In accordance with Fig. 1.3, COP and EER can be expressed, as:
However, COP and EER of a real heat pump are lower than values estimated using Eqs. 1.4 and 1.5. Three are the main reasons (Chwieduk 2012). Working fluid
Fig. 1.3 Pressure-enthalpy diagram for a heat pump cycle operating in heating mode and using the refrigerant R410a (Girard et al. 2015) is often subcooled in order to ensure a fully liquid state; point 3 of Fig. 1.3 is moved further left, falling into the liquid region. This increases the work to be provided to the heat pump. Temperature differences between heat source and heat sink are higher due to the presence of heat exchangers. Compression process has a certain isentropic efficiency and motor driving the compressor have efficiency lower than one.
The main difference between SL-GSHP and DX-GSHP is the refrigerant cycle. Indeed in such heat pump (DX-GSHP), the refrigerant is in direct thermal contact with the ground, interacting with the soil through the walls of the pipes, where it circulates. Instead in the system (SL-GSHP) depicted in Fig. 1.1, the refrigerant interacts through a heat exchanger, usually located within the heat pump, with a mixture of water and anti-freeze liquid. The mixture, referred to as brine, circulates in the GHX thermally interacting with the soil. The additional heat exchanger and the related circulation pump, for running this additional loop, make SL-GSHPs more inefficient than DX-GSHPs. Moreover, SL-GSHPs excavation costs are also a bit higher since they need longer GHX. Although these disadvantage, only SL-GSHP solution has been considered in this book, due to the fact that SL-GSHPs are more widely applicable. Indeed, DX-GSHPs require more system design and have technical and environmental complications such as compressor starting, potential ground pollution, and high refrigerant charge (Hakkaki-Fard et al. 2015).
Even so, despite the solution adopted, generally GSHPs have better performance, about 20-30% more, than air source heat pump, which is the most used heat pump typology in the world (Jean-Christophe Hadorn 2016). This is due to the more stable thermal conditions of the ground compared to air. Indeed, especially in location with extreme weather conditions or during days with highest heating or cooling energy demands, the ground represents a more efficient energy source since its undisturbed temperature is close to the mean yearly ground surface temperature. Therefore, it is warmer and colder than air temperature respectively in winter and in summer. Although initial GSHPs capital cost are 30-40% higher than air source heat pumps, its reliability is very long about 20-25 years (Jean-Christophe Hadorn 2016).