A. Gonzalez1, M. Izquierdo1* and A. Moreno2

1 Instituto de Ciencias de la Construction Eduardo Torroja (IETCC)-CSIC
Serrano Galvache 4, 28033 Madrid, Spain

2 Universidad Carlos III, Madrid, Spain
* Corresponding Author, mizquierdo@ietcc. csic. es

Abstract

A thermodynamic model of a direct expansion solar assisted heat pump (DX-SAHP) was developed in order to simulate its behaviour in space heating applications. In this study, four different configurations of DX-SAHP were simulated using meteorological data from Madrid. The predicted results for this region indicate that long-term performance of the system ranges from 2.08 and 2.41 and that any of the studied configurations may be considered as an available solution to saving energy and CO2 emissions. Simulation results also indicate that in working with underfloor heating and R134a as refrigerant, DX-SAHP systems are more efficient. On the other hand, the proposed model permits us to know the number of panels needed to install in a specific space heating application.

Keywords: solar assisted heat pump, direct expansion, residential heating, energy saving

1. Introduction

Mitigation of global warming involves taking actions to reduce greenhouse gas emissions associated to residential buildings. Thus, it is essential to improve energy efficiency in buildings, what firstly means decreasing energy consumption and secondly substituting fossil energy sources for renewable ones. In a few words, it is necessary to research on efficient and environmentally friendly systems for heating and air conditioning applications in buildings. In this regard, both heat pumps and solar energy systems are receiving much attention. On the one hand, heat pumps may save energy in heating applications because they augment input energy with heat collected from the environment; as well, they may provide summer cooling in a rather efficient way. In spite of these advantages, viability of heat pumps has been questioned because of several aspects, for example, its reduced capacity and efficiency for low ambient temperatures. On the other hand, solar energy systems present not only some attractive advantages but also a series of problems: solar energy is known to be non-polluting and inexhaustible, but also intermittent and dispersed. Then, one can think about combining heat pumps and solar energy systems to reduce the disadvantages that each one has in operating separately. In this way, several researches have proposed the solar-assisted heat pump (SAHP) as an attractive solution for saving energy in residential applications.

From the review of existing literature, one extracts that SAHP can be operated in two basic configurations. In conventional one, a solar collector and a heat pump are integrated trough an intermediate closed loop, from the collector to the thermal storage. The circulating fluid (usually water or air) transfers solar energy to refrigerant in heat pump [6, 9, 10]. In the second configuration, so-called direct expansion solar-assisted heat pump (DX-SAHP), the collector and

evaporator are combined into one unit, where the refrigerant from the condenser is expanded and gets evaporated by incident solar energy and/or ambient air energy [3, 5, 7, 8, 11]. According to Chatuverdi [3], the DX-SAHP presents several advantages over the conventional SAHP. First, this system offers a superior thermodynamic performance since a higher heat transfer occurs when refrigerant is boiling in collector/evaporator. Next, system cost is lower due to elimination of the evaporator and the intermediate heat exchanger used in conventional SAHP. Finally, collector life is longer because the use of a refrigerant as working fluid avoids freezing and corrosion problems associated with water collectors.

Although several researches concerning DX-SAHP have been performed for last years, not many of them discuss space heating applications of the system, nor make an estimation of total energy absorbed by collectors during heating season. On the contrary, most of these studies are focused on water heating applications trying to predict work conditions which make the system COP highest. The present work intends to develop a simplified model which enables to simulate the dynamic behaviour of a DX-SAHP in space heating applications. This simulation is permitting us to estimate, for example, the thermal performance of the system, the heat absorbed by collectors or the electrical energy consumed by the system. In knowing the energy consumed, associated greenhouse gas emissions will be estimated and compared with those ones corresponding to other conventional space heating systems: natural gas and gasoil boilers. Meteorological data used in simulation are from a weather station located in Madrid, which represents one of the most populated regions in Spain. Finally, the study considers the use of two different refrigerants, R134a and R407C, and two heating systems, fan coil and underfloor heating.