Panagiotis Tsekouras[23], Mario Motta[24], Aristotelis Aidonis1, Dimitris Chasapis1,
Christina Hatzilau[25], Constantinos Balaras4

1Center for Renewable Energy Sources (CRES)

19th km Marathonos Ave., GR-190 09 Pikermi, Greece
Email: ptsek@cres. gr. aidonis@cres. gr. chasapis@cres. gr
2 Politecnico di Milano. Dipartimento di Energia — Piazza Leonardo da Vinci. 32 — 20133 Milano — Italia

Email: mario. motta@polimi. it

3 National Technical University of Athens (NTUA). Heroon Polytechniou 9. GR 15780 Zografou

Athens. Email: hatzilau@central. ntua. gr

4 National Observatory of Athens (NOA). Institute for environmental Research & Sustainable
Development. Group Energy Conservation. I. Metaxa & Vas. Pavlou. GR 15236 P. Penteli. Greece

Email: costas@meteo. noa. gr

Introduction — State of art

The market of the solar combi-systems (providing heat for both domestic hot water — DHW — and space heating) is continuously growing. especially in central and northern Europe. In Europe in 2006 the market share of combi-systems had been about 5% [Weiss et al. 2008]. In Austria. solar combi — systems had a market share of almost 40% until 2007 [ESTIF. 2007] while in Germany. the market share of combi-systems has reached 45% in 2007 [Koldehoff. 2008]. Moreover. the use of solar assisted air conditioning (SAC) is reaching the market level. with significant growth in European commercial and residential buildings [Balaras et al. 2007]. Recently. various small scale SAC systems appeared on the market [SOLAIR. 2008]. The combination of solar heating and cooling seems very promising and can increase the total solar fraction1.

The main “drawback” of solar combi systems has been the fact that excess summer solar heat couldn’t be utilized. thus making the system economically less attractive. and creating some technical problems related to stagnation (the condition when there is available solar radiation but no load and the liquid in the primary loop usually vaporizes). Since high building cooling loads generally coincide with high solar radiation. the excess solar heat can be exploited by a combi+ system (“combi+” or “combi plus” is the term used to identify the combination of solar heating and cooling systems).

Nevertheless. even in a common solar combi+ system. there is still some mismatch between the availability of solar energy and the loads. especially during the intermediate “low load” seasons in spring and autumn (low or even no heating and cooling space loads. only domestic hot water load). Moreover. the consecutive days in winter with low solar radiation. actually create a limit for the system’s solar fraction for space heating and DHW. Therefore. even a solar combi+ system can

hardly reach particularly high solar fractions (e. g. over 80% in terms of total solar fraction and over 70% in terms of partial space heating fraction).

A “solution” for the utilization of the excess solar heat during the “low load” periods is the use of a seasonal storage tank (SST), filled with water. There are several SST systems in operation [Schmidt, 2004]; most of them are quite large (thousands of m3 of water equivalent storage), typically ranging between 1.5 and 4 m3 per unit surface of solar collectors.

This paper focuses on an investigation of how to achieve particularly high solar fractions (over 90%) for heating and cooling of buildings through an optimized SST. The size of the SST is small compared to common practice and has some innovative aspects that are described in the following sections.

The work reported here-in is part of the ongoing European project “HIGH-COMBI” (Contract No: TREN/07/FP6EN/S07.68923/038659) which aims at developing high solar fraction systems by an innovative combination of optimized solar heating, cooling and storage technologies (www. highcombi. eu). In the frame of the project, five demonstration plants will be constructed in Greece, Italy, Austria and Spain using different technologies, components and control strategies in order to achieve high solar fractions’ values. Demonstration plants’ monitoring data will be analyzed, the simulation and design tools will be validated and the plants’ performance will be evaluated. A market analysis will be carried out in order to estimate the potential penetration for these systems in the European heating and cooling market. The sectors of the implementation are all medium and large buildings end-users having heating and cooling loads throughout the year.