PCM simulations in a solar combisystem

C. Sole, M. Medrano, M. Nogues, L. F. Cabeza*

GREA Innovacio Concurrent Edifici CREA, Universitat de Lleida, Pere de Cabrera s/n, 25001-Lleida (Spain)

Phone: +34-973 003576, Fax: +34-973 003575 Corresponding Author, [cabeza@diei. udl. cat

Abstract

Simulations were done to check the behaviour of a PCM integrated into the storage tank of a solar combisystem (system that provides energy for DHW and space heating (SH)). The meteorological conditions chosen for the simulations were the Spanish climate of Madrid. The building selected is a single-family two storey house with an energy demand of 60 kWh/m2-year. These simulations were done with a tool developed within the framework of the Solar Heating and Cooling (SHC) Programme of the International Energy Agency (IEA). The main topics studied in these simulations were the behaviour of the PCM and the storage configuration. The aim of these simulations was to check whether the advantages of PCM observed experimentally in a real solar pilot plant for DHW demand are still valid in a more complex system such as a solar combisystem. It was shown that the position of the output to the heating system is a crucial point for the final performance of the system since its position benefits the DHW demand or the heating demand. With the system designed as it is and the control applied (typical differential control for simple water tank), no better results were obtained for a PCM-water store compared to a water store regarding the performance indicators used. Penalties are accounted when the demand of DHW or space heating is not fulfilled. It is seen that the introduction of PCM helps to decrease ore completely avoid the penalties for not reaching the comfort conditions in the demand.

Keywords: Phase Change Material, simulation, combisystem, energy storage

1. Introduction

Obtaining high storage energy systems seems to be one of the key parameters to promote the use of renewable sources. Due to the mismatch between energy generation and demand, the storage set-up is essential.

Phase Change Materials (PCMs) seem to be one of the most promising techniques that might lead to this high energy storage performance. A PCM is a material which stores or supplies heat at its melting/solidification temperature using its high thermal energy storage density per unit volume as a consequence of its latent heat, which is higher than the sensible heat. It is possible to use the latent heat of solid-gas, solid-liquid and liquid-gas transformation, however, only the solid-liquid transformation is used due to its lower volume variation [1].

The real behaviour of some PCMs was tested in the pilot plant of the University of Lleida [2-3] with a daily DHW demand. Some of the advantages of PCM were experimentally tested and showed its capability of reheating the cold water surrounding the PCM containers after a partial draw-off of the storage tank. Another advantage is that the temperature of the water next to the PCM module keeps constant or decreases slower than the water with no interaction with the PCM [4].

Simulations with a combisystem (system that provides DHW and space heating demand) have been carried out to test the PCM behaviour under such a complex system. The chosen climatic data were from the city of Madrid and the building simulated had an energy demand of 60 kWh/m2-year.

The aim of these simulations was to check if the advantages observed in a DHW system were still kept in a more complex system that provides DHW and space heating demand as it is a combisystem.

This work is included in the framework of the Task 32 of the Solar Heating and Cooling program of the International Energy Agency (IEA), and the system and boundary conditions were previously defined in this work group [5]. A simulation tool with TRNSYS was designed to perform easy and fast simulations by non-expert users to get clear and easy-to-understand results.

Fig. 1 shows and sketch of the system implemented and designed in the framework of the Task 32 of the Solar Heating and Cooling (SHC) program of the International Energy Agency (IEA). The system can be divided in three different parts: the solar loop, the storage system and the auxiliary system. The system also simulates the behaviour of a single family house with its corresponding DHW demand and space heating demand. The combisystem has to guarantee a DHW temperature demand of 45 °C all the year and a temperature inside the building above 19.5 °C (heating demand).

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