The aim of buffer storage systems is the compensation of fast transients in solar radiation which usually result from passing clouds. These systems should protect the components of the power plant from the effects of sudden variations in thermal load. Characteristic features of buffer storage systems are short reaction times and high discharge rates, while the capacity is only in the range of 5-10 minutes. The function of energy storage for extended discharging periods will then be fulfilled by storage systems as previously proposed for oil and DSG parabolic trough plants.

Steam accumulators show the characteristic properties of buffer storage systems. Fig. 11 shows the basic scheme of a sliding pressure (Ruths-type) steam accumulator: pressurized, saturated water is used to store sensible thermal energy. During the discharge process, the pressure is decreased and saturated steam is generated using the sensible thermal energy from the liquid water volume.

Steam accumulators have been used for decades in process industry and power plants, applications cover a pressure range from a few bars up to 120 bar; characteristic storage capacity is 20-30 kWh/m3 [5].

There are different options for charging a steam accumulator; the energy in the storage volume can be increased by condensation of superheated steam or by feeding saturated liquid water into the steam accumulator. If a heat exchanger is integrated into the liquid water volume the steam accumulator can also be charged by a different fluid than water which might be at a lower pressure.

The amount of saturated steam provided during the discharge process of the steam accumulator depends on the initial pressure and the extent of the pressure drop. Fig. 12 shows the volume-specific amount of saturated steam released during discharge for various initial pressures depending on end pressure.

A cost effective approach for integration of buffer storage capacity is the combination of the steam accumulator with other components of the power plant; Fig. 13 shows the simplified scheme of a parabolic trough power plant. The collector field is operated in the recirculation mode, i. e. wet steam from the collector field flows into a steam drum where the liquid phase
is separated from the gas phase. The volume of the steam drum can be used to store saturated water; by variation of the water level the energy content can be changed.

Steam accumulators can also be used for parabolic trough power plants using a thermal oil as a heat transfer medium in the solar collectors if the energy provided by the collector field is used in a steam process; here, the steam accumulator is integrated in the secondary loop. Fig. 14 shows a parabolic trough power plant with thermal oil in the solar collectors; the steam accumulator is used as a heat exchanger between oil loop and water/steam loop. Heat from the solar field is used to heat the liquid water volume of the steam accumulator indirectly. The energy content of the heat exchanger/steam accumulator is related to the water level

In a Ruths-type steam accumulator the steam pressure drops during discharge. For some applications, a storage system providing steam at constant pressure is advantageous. One option to avoid a pressure drop is the application of a separate flash evaporator (Fig. 15): the saturated liquid water taken from the steam accumulator is depressurized externally, cold water is fed into the storage vessel to keep the water level constant, mixing of hot and cold water must be minimized, thermal stress resulting from filling the pressure vessel with cold water must be considered.

Another option for constant pressure storage is the integration of phase change material (PCM) into the storage vessel partly replacing the liquid water (Fig. 16). Here, the thermal energy associated with the phase change between liquid and solid state is used for isothermal energy storage. PCMs usually exhibit a low thermal conductivity so layers of this material must be thin to ensure a sufficient heat transfer rate. One option to fulfill this demand is the encapsulation of PCM in small containers placed inside the liquid volume. Using PCM is not only attractive regarding the avoidance of thermo mechanical stresses resulting from temperature transients, the characteristic volume-specific storage capacity of PCMs is in the range of about 100kWh/m3. Compared to the corresponding value for water (20-30kWh/m3), the integration of PCM helps to increase the storage capacity of a given pressure vessel.

Although steam accumulators exhibit only a small storage capacity, the availability of these buffer storage systems can contribute to reduce the investment costs for storage capacity if they are combined with storage systems intended for longer periods of discharge. By reducing the requirements regarding response time and discharge rate the specific costs for storage systems with several hours of heat capacity can be reduced.

Acknowledgement

Part of the work presented in this paper has been funded by the German Federal

Environment Ministry under the contract code PARASOL/WESPE and part by the European

Commission within the 5th Framework Programme on Research, Technological

Development and Demonstration under contract no. ENK5-CT-2001-00540.

The authors are responsible for the content of this publication.

[1] Solar Energy Laboratory (LABSOLAR) — Florianopolis BSRN station.

[2] We would like to point out that the objective function is the collector gain on a daily (or monthly or yearly) basis. It is not absolutely necessary to be able to correctly describe the momentaneous collector performance in every timestep of operation.

[3] This process has been discussed in section 5.

[4] For collectors with a biaxial incident angle behaviour the incident angle in east west direction has to be considered

[5] the sum of the absolute values of the difference in calculated and measured power per time step divided by the sum of the measured power per time step must be less than 5% (equation 5).

[6] V. Weitbrecht, D. Lehmann, and A. Richter. Flow distribution in solar collectors with laminar flow conditions. Solar Energy, 73(6):433-441, 2002.

[7]eff = = г (equ — 2)

PTin ‘ Ac PTin ’n’ reff

The effective cross section for the flow in vertical direction was assumed as circular with the radius reff. To further improve the model accuracy, a constant, hoffset, was additionally taken into account:

[8] Presently a PhD candidate at Queen’s University, McLaughlin Hall, Kingston, ON, CANADA. K7L 3N6 Email — mesauita@me. queensu. ca

[9] Companhia Energetica de Minas Gerais, Av. Barbacena, 1.200, Belo Horizonte, MG, BRAZIL.30161-970

[10] 23456789 10

Solar irradiation on collector plane [kWh/(m2 d)]

[12]random order not according to the order in the presented diagrams and tables

[13] Corresponding author ph: +61(0)2 93515979 fax: +61(0)2 93517725 email: d. buie@physics. usyd. edu. au

[14] An air cooled condenser is used for both options. This will be not the case for the real plant. Since this investigation is a comparison between two technologies and not an investigation of a single option this difference is not that important.

[15]1 (PWH1-C