With an increasing share of power generated by renewable energy sources, fluctuations in grid power supply due to varying solar radiation or varying wind speeds are increasing, too. Unlike many other renewable energy technologies solar thermal power plants can supply power dependent on the demand using thermal energy storages.

In central receiver systems/solar power towers with open volumetric receiver air is sucked through a porous structure (absorber), which is heated by concentrated solar radiation. By this the air is heated up to 800 °C. In normal operation the air heat is used to run a steam cycle for power generation in a conventional steam turbine-generator system. During periods with excess of solar energy a part of the air current can be also used to charge a thermal storage.

For high-temperature storage up to approximately 800 °C systems using packed beds of ceramic bodies or ceramic bricks have been analysed so far. These storage systems are featured by a simple configuration, but involved with higher costs for the storage material.

In view of storage material costs the sand storage concept for solar power towers with open volumetric receiver has been developed, as shown in Fig. 1:

[12] Introduction

Phase change materials offer a great potential for energy saving in cooling applications as well as efficient energy storage at different temperature levels. Commercially available applications of PCM are e. g. passive solid materials which are used in building walls to shave temperature peaks in the afternoons and are recooled by outside air during the night [1]. In active cooling applications, the material is integrated in storages, e. g. encapsulated water (ice storage) or paraffin panels and the refrigerant medium circulates around it thus withdrawing or releasing energy [2, 3].

By contrast Phase Change Slurries are emulsions or suspensions of PCM which circulate in the heat/cold network. So the energy is taken directly to the consumer (e. g. chilled ceiling). This technology has the advantage that the problem of slow heat transmission due to the poor heat transfer coefficients and low thermal conductivity of pure macroencapsulated PCM is reduced significantly. The disadvantage is the higher viscosity which can cause problems with small pipe diameters.

Presently PCS are used on major scale only in Japan [4]. There mainly ice-slurries are employed — a suspension of liquid and frozen water with additives preventing the accumulation of ice particles. With

[14] During the cooling process of PCM the initiation of crystallization, usually requires temperatures well below the melting point. This temperature difference is called subcooling.

[15] Q = dt*m*cw =14 °C * 500kg * 1,16Wh/kg = 8,12kWh

[16] V. M. van Essen, H. A. Zondag, R. Schuitema, W. G.J. van Helden, C. C.M. Rindt (2008), Materials for thermochemical storage: characterization of magnesium sulfate, Proceedings Eurosun 2008.

[17] A. N. Kalbasenka (2008), Basis calculations of reactor concepts, ECN memo Acknowledgement

This project has received financial support from the Dutch Ministry of Economic Affairs by means of the EOS support scheme. The work on thermochemical heat storage is part of the long-term work at ECN on compact storage technologies.

[18] Introduction

A novel solar powered 10 kW absorption chiller was developed and tested at the ITW in recent years. To extend the hours of operation and to achieve a high chiller efficiency for air conditioning purposes, a small ice store was designed, constructed and experimentally investigated.

The ultimate aim of this project is to offer an efficient and commercially viable solution for solar driven cooling of small residential and commercial buildings such as single occupancy houses and small office buildings. In a first series of measurements several of the offices of the Institute of Thermodynamics and Thermal Engineering (ITW) of the University of Stuttgart are cooled by a

[21] A. Hauer, Beurteilung fester Adsorbentien in offenen Sorptionssytemen fur energetische Anwendungen, Doctoral Thesis, Technische Universitat Berlin, Fakultat III Prozesswissenschaften, 2002.

[22] E. Lavemann, A. Hublitz, M. Pelzer, Betriebsergebnisse einer solarunterstutzten Flussigssorptionsanlage in Singapur (German), Proceedings 4th Symposium „Solares Kuhlen in der Parxis“, Stuttgart, April 3-4, 2006.

[23] The term “total solar fraction” is defined by the following equation: SFtotal = 1 — Eaux/Edemand. where Eaux is the

energy supplied from the auxiliary heater and Edemand is the sum of the energy needed to meet the heating and

cooling thermal comfort in the building and the energy to heat up the domestic hot water.

[26] Floor space heating has proved to be much more effective (compared to fan-coils) in terms of solar system efficiency. Most probably the floor elements will prove to increase the system’s efficiency furthermore, if they are used for cooling as well. However, since “floor cooling”, in order to be effective, needs some dehumidification device (and, consequently, complicated controls and simulations), we have chosen to use conventional fan-coils for cooling in summer. The “floor cooling” configuration (perhaps with a limited number of fan-coils for dehumidification) seems very promising and is, at the moment, under investigation.

[27] Currently, for the simulations there is no possibility to bypass the SST and deliver energy directly from the collectors to the chiller. However, this bypass could increase system’s performance and should therefore be implemented as the optimization procedure evolves.

[28] In the framework of High-Combi, the use of low cost materials as additional insulation of the SST has been investigated by laboratory measurements. The experimental results will be available soon in the project’s web site.

[29] One may argue that some winters may be more severe (or have less solar radiation) than others and, therefore, an auxiliary boiler is needed. In order to handle such fluctuations the solution is a slight increase the collectors’ field area.

[30] By fully charging the DHW tank: i) some energy is taken away from the SST, and ii) there is a reduction of the collectors’ efficiency (compared with their efficiency when charging the SST), due to the higher operating temperatures. However, the amount of energy lost by the SST in order to achieve 100% solar fraction for DHW is about 720 kWh and may, therefore, be achieved with an addition of a few (about 2) m2 of solar collectors.

[31] Higher solar fraction

Most Danish natural gas fuelled CHP-plants are now operating in a free electricity marked, where one of the main sources of income comes from regulation of electricity production. The regulation is needed because of a large fraction of windmill produced electricity in the Danish system.

That means normally less running hours for the engines and 25-50% of the heat produced in a natural gas boiler. Natural gas is an expensive fuel that very often means production prices of more than 60 €/MWh.

Therefore solar heat is an attractive alternative, but if the solar fraction exceeds app. 10%, the ac­cumulation tank is too small for electricity regulation. The CHP-plants are thus eager to find solu­tions making it possible to increase the solar fraction and at the same time keep the regulation pos­sibility. In the following 4 new projects with higher solar fraction are described.