Photovoltaic heat-pumps for cooling applications

Resumen

In the light of the exhaustion of fossil fuels and of the fight against the climatic change, it is necessary to increase the penetration rates or renewables energies in the energetic system, as well as its electrification. This situation offers a variety of new niches for photovoltaic systems. In particular, heat pump systems powered by photovoltaic generators permit to supply the heating and cooling thermal demands with a technology that only consumes electricity. Furthermore, this electricity comes from a renewable source. Cooling applications are especially favorable for this technology because of the better matching between generation and demand, which permits to reduce or even eliminate the storage needs. This work presents a new technical solution for photovoltaic heat pump systems, with a stand-alone configuration without batteries. This way, electric storage can be substituted (if needed) by thermal storage, which generally is cheaper and more reliable. This solution has been validated with an experimental prototype, where two different control algorithms have been implemented for the compressor of the heat pump. One of them has the objective of maximizing the photovoltaic utilization; the other one has the objective of maintaining a constant temperature in a room. With the data obtained from the tests implemented in this prototype, a series of indicators, some of them proposed specifically in this work, have been calculated. These indicators permit to evaluate separately the performance of the heat pump unit (thorough the EER and the SPF), of the photovoltaic generator (thorough the PR, the PRPV,STC,ref and three utilization ratios for identifying the causes of hypothetical low PR values), the integration of both components (thorough the SPFPV-HP,STC.ref) and how renewable the system is (thorough the SFPV and the SCR for not stand-alone configurations). The analysis of the results shows the good performance of the prototype, and how it could be improved. It has also been evaluated the stability of the system against solar power fluctuations due to cloud-passing. A total of 75% of the cloud-passing events during the tests have been resisted. Finally, an economic feasibility assessment has been performed, comparing the stand-alone solution developed in this work with a self-consumption configuration and an only grid-powered system. This evaluation shows a high economic profitability of photovoltaic heat pump systems. The stand-alone solution allows to reduce the electricity costs, not only for the consumption but especially because the power access term can be eliminated. Thanks to this, the economic profitability of installing a photovoltaic system would be higher than for a self-consumption solution. The technical solution developed and validated in this work is particularly relevant for industrial applications, where it is feasible to power the compressor of the heat pump directly with a photovoltaic generator and a frequency converter. Some examples of cooling applications are the conservation of food and medicines or the control temperature in industrial ambiences. Additionally, this solution is also valid for heating applications like food drying (cereals, carnic products…), which makes it feasible for a big amount of market opportunities.