S. Heft*, P. Di Lauro, S. Rose and M. Rommel

Fraunhofer Institute for Solar Energy Systems ISE, Heidenhofstrahe 2, 79110 Freiburg, Germany
* Corresponding Author, Stefan. Hess@ise. fraunhofer. de

Abstract

There is a high potential for the integration of solar generated heat into industrial processes or systems for air-conditioning. Important aspects to select the suitable collector technology for a specific application are the required working temperature of the solar loop and the corresponding effectiveness of the collector. But also the energy gained by a specific collector technology throughout the yearly operation has to be simulated before a clear decision can made.

Considerable work was done within IEA-Task 33/IV, SHIP (= Solar Heat for Industrial Processes), to collect information, initiate new developments and characterise collectors for medium temperature applications (80°C — 250°C).

At the lower medium temperature level (80°C — 150°C) plannings often apply vacuum-tube collectors because of their low heat losses. As a result of the previous SHIP — work, also advanced flat-plate collectors look promising. If the area for mounting the collectors is not restricted, advanced flat-plate collectors can offer technical and/or economical benefits compared to vacuum-tube collectors at the required temperature.

Additionally, there are many possibilities to develop collectors up to 250°C if concentrating reflectors are used. Low-concentrating CPC collectors do not require sun-tracking and achieve the highest possible concentration within their acceptance-angle. Concentrating solar radiation can reduce heat losses per absorber area and therefore achieve higher temperatures, even if the collector is not evacuated. A proper orientation is important to assure these features throughout the yearly operation.

Keywords: concentrating solar collector, SHIP, solar cooling, process heat

1. Introduction

Solar thermal collectors are usually the most cost intensive component of the heat production sub-system of solar supported processes. For a first estimation of the suitable collector technology for a given process, the most important aspects are:

• The efficiency of different collector-types at the required working temperature of the solar loop

• Necessary collector area per nominal heating/cooling capacity in m2/kW and the collector costs in EUR/m2

Methods for an estimation of these values are given in [1] and [2]. In the past, planners of solar thermally driven cooling systems or solar process heat applications often simply compared flat — plate collectors to evacuated tube collectors. Because the required temperatures for the support of industrial processes are highly different, the following discussions will be made on the basis of

different technologies for solar air conditioning. This way the application fields discussed about can be restricted to certain temperature levels.

For absorption cooling systems standard flat-plate collectors often can not reach the required working temperature economically. Therefore vacuum tube collectors have been used in recent developments. The main drawback of this collector type is its significantly higher costs [3]. Considerable work was done within IEA SHC-Task 33/ Solar Paces IV, SHIP (Solar Heat for Industrial Processes), to initiate new developments and characterise collectors for medium temperature applications (80°C — 250°C). The Task started in December 2003 and ended in October 2007. The work carried out in the SHIP project showed, that as an alternative to vacuum tube collectors also advanced flat-plate collectors look promising to reach higher temperatures. Additionally, there are many other possibilities to reach higher working temperatures by the reduction of optical and thermal losses [4]. This paper gives a brief overview of possible mechanisms to reduce the optical and thermal losses and shows some of the developed collectors with their efficiency at different working temperatures.