J. Cadafalch

Universitat Politecnica de Catalunya (UPC), Departament de Maquines i Motors Termics (MMT), Escola
Tecnica Superior d’Enginyeria Industrial i Aeronautica de Terrassa (ETSEIAT),
c/ Colom 11, 08222, Terrassa, Spain, e-mail: jcadafalch@mmt. upc. edu

Abstract

This work presents publishable information of the European-Craft project “OPICS — Optimised Integrated Collector Storage: Low-Cost Solar Thermal Systems for Houses and Offices” that finished on 2005. The project was funded in part by the European Commission and involved the companies DISOL and FOELCA from Spain, CIVILMAC from Portugal, SUNSTRIP from Sweden and UPC University. More than 7000 different configurations of integrated collector storage systems (ICSs) were simulated investigating different parameters such as the absorbing surface, single glazing, transparently insulated covers or water and phase change material store. Pre-industrial OPICS prototypes were constructed and tested following ISO procedures to assess the credibility of the results. Researched carried out proved that improved efficiencies can be achieved in the OPICS prototypes with respect to standard ICSs, and that they can be used in applications where standard ICSs are not able to give a reasonable performance such as space heating in moderate climates.

Keywords: DWH, ICS, TIM, PCM

1. Introduction

The integrated collector storage (ICS) is a solar thermal system in which the functions of collection and storage of solar energy are performed within the same device. The integration of collector and storage elements leads to compact equipment simplifying the connection pipes and avoiding the use of intermediate elements such as pumps. Therefore, ICS is one of the simplest and cheapest solar thermal systems. In fact, the first thermal solar systems constructed were ICSs and during all the XXth century this kind of systems were installed all over the world.

As in the ICS the thermal storage is directly exposed to the ambient, the system heat losses are high. This problem, together with other problems always inherent in solar thermal systems as freezing of the thermal fluid or overheating, have limited up to now the applicability of the ICS to domestic hot water applications of familiar houses in warm climates.

Technologies as transparent insulation materials (TIM) and phase change materials (PCM) make it possible to improve the performance of the ICS maintaining low costs. Transparent insulation materials allow reducing the thermal loss through the cover while preserving good optical performance, and phase change materials can increase the energy storage in the collector avoiding overheating and freezing problems.

The design, study and optimisation of ICSs is much more difficult than the study of other solar thermal components, because many different complex and coupled physical phenomena are involved in only just one device: radiation heat transfer with spectral and angular dependence, heat conduction in the solid parts, heat convection in the thermal fluid in the storage tank and in the air

cavities, heat storage, etc… This difficulty is even increased when new technologies such as TIM or PCM are used.

One the other hand, one of the major drawbacks to be solved in solar thermal is the integration of the equipment in buildings. Most solar thermal systems and elements do not take into account the shape and the design of the buildings in which they will be installed, and generally the occupied space is not optimised and the visual impact is very criticised specially from architectonical points of view.

An European-Craft project called “OPICS-Optimised Integrated Collector Storage: Low-Cost Solar Thermal Systems for Houses and Offices” was carried out from 2002 to 2005. The project was funded in part by the European Commission and involved 4 European Companies (DISOL and FOELCA from Spain, CIVILMAC from Portugal and SUNTRIP from Swdend) and the UPC University, see [1].

The goal of the project OPICS was to develop Integrated Collector Storages with the following features:

• Use of Transparent Insulation Materials (TIM) and Phase Change Materials (PCM).

• Improvement of the efficiency with respect to available ICSs.

• Modular design to facilitate integration and to reduce installation costs.

Major developments of the project are two pre-industrial prototypes called OPICS1 and OPICS2. They both use a TIM-cover and have a modular rectangular shape that facilitates their integration in buildings. Main difference is the store. In the OPICS1, energy is stored in a 75 l rectangular water tank with internal mechanical elements to prevent deformation with pressure. On the other hand, in OPICS2 energy is stored in a rectangular PCM store with an internal compact heat exchanger that permits to remove the accumulated energy. The prototypes were designed using virtual prototyping tools [2,3] and were measured following standard procedures [4,5].

This work gives an overlook of the OPICS projected including the strategy, methodology, a description of the major developments and an assessment of the results achieved.