Development of a heat pump system for mobile applications

Abstract

The heating and cooling sector is one of the biggest sectors at this moment due to drastic fluctuations in climate parameters. Even though this sector helps in making sure everyone gets comfortable temperatures at the home, office, etc., there are environmental consequences attached to them. These consequences are in terms of emissions of greenhouse gases. Almost around 80% of the current greenhouse gas emissions are due to the demand for energy, mostly in the form of electricity and heat. Having looked at these emissions, the demand for heating and cooling in residential and commercial spaces is increasing day by day. The systems which provide the desired effect of cooling and heating use different working mediums i.e. fossil fuels and renewable sources which correspond to 75% and 22% usage in the EU currently. So, to reduce the consumption of fossil fuels, heat pumps came into the picture.
A heat pump uses a refrigerant through which heat transfer is carried out to attain the desired temperature at a particular location. The properties of these refrigerants and other component-based parameters affect the COP (Coefficient of Performance) of the system. For a mobile system, i.e. a portable heating and cooling device used for military camps, agricultural purposes currently use fossil
fuels which lead to enormous emissions, thus, these systems have to be redesigned by using vapor compression technology. One of the constraints is being able to use a refrigerant with GWP (Global Warming Potential i.e. a measure of the amount of heat a greenhouse gas traps in the atmosphere
comparing it with the same amount of CO2) of less than 150. Aspen model was created to assess different refrigerant behavior under some constraints like a cooling capacity of 40 kW, a heating capacity of 60 kW, and an air-air system with an airflow rate of less than 10000 m3/h, these requirements are
obtained from fossil fuel-based mobile systems, as these systems are air-air systems and system is kept outside the desired place, hoses have to be used to provide the air into and out of the system, thus, the specific air flow rate has to be achieved to get the desired temperature inside the desired place, similarly, 40-60 kW is a need for these systems due to high capacity applications. Sensitivity analysis of the aspen model was carried out by changing ambient temperature from 21 to 40 °C for cooling and -5 to 15°C for heating respectively, to quantify the behavior of refrigerants R290, R1234yf, R1234ze, R454C, R455A, R1270, R600a, R717, R516A. Exergy analysis was carried out along with the COP
calculations to examine the second law efficiency for all the refrigerants to see the feasibility of these refrigerants being an alternative option for widely used R410A refrigerant. By varying a few parameters considered for the heat pump systems, optimization and sensitivity analysis resulted in finding the most
suitable refrigerant for this system. For ease of calculations, a few assumptions and constraints were considered while simulating the system.
The maximum COP value obtained for the refrigerants R290, R1270, R1234yf, R1234ze, R454C, R455A, and R516 for cooling effect are 1.75, 1.77, 2.09, 1.91, 2.18, 2.25 and 2.21 respectively and similarly for heating effect are 3.61, 3.26, 2.49, 2.72, 3.56, 3.67 and 3.68 respectively. These COP values are obtained when the pressure drop across the heat exchangers is zero, so, the values with a
pressure drop of 0.5 bar will fluctuate a bit from these values. If not COP, it will surely affect all other component parameters. Apart from COPs, second law efficiency is also a crucial parameter to analyze the system performance compared to a Carnot system i.e. an ideal system. Exergy analysis helped
in quantifying the second law efficiency of the system for each refrigerant, which is calculated by the ratio of the actual COP of the system and Carnot COP of the system (Calculated by taking the source and sink temperature values). The maximum second-law efficiency values obtained for the refrigerants
R290, R1270, R1234yf, R1234ze, R454C, R455A and R516 for cooling effect are 0.32, 0.27, 0.2, 0.16, 0.29, 0.31 and 0.21 respectively and similarly for heating effect are 0.4, 0.41, 0.37, 0.45, 0.405, 0.42 and 0.41 respectively. So, by considering all the COP and second law efficiency values it can be
concluded that a suitable refrigerant for this system with good performance as well as safe to use and handle is R455A, and other potential refrigerants being R454C almost similar performance with respect to R455A, and R290 and R1270 single component refrigerants with the concern of higher flammability
can be used with further optimizations in the model.