In most cases in which solar thermal systems were monitored, the failure detection consisted of analysis of measurement data by an expert. An expert with enough experience can recognize if a system is performing as expected based on analysis of data. A state-of-the art example is provided in the Austrian demonstration project Optisol (OPT), in which 10 large solar thermal systems were built and monitored for ca. 1 year [2].

In the Optisol project an integrated approach was used for designing, building and monitoring the systems. The monitoring part consisted of a so called optimization phase of two months and a consecutive routine supervision of one year. During the optimization phase many weaknesses of the solar supported heating system were recognized by analysing the temperature profiles of the systems. 35 faults in installation, design or operation were detected in 9 systems, several of these faults were related to the auxiliary heating system. In the routine supervision monthly energy balances and yearly

[13] Introduction

The characterization of collector efficiency is the fundamental tool for long term thermal performance calculation, i. e. collector yield, and for design of solar thermal systems. It is, thus, one of the most important inputs in software tools aiming at the design of solar thermal systems.

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[15] Introduction

Solar air collectors systems may have a number of advantages compared to solar collectors using a liquid heat transfer fluid. Just to mention some of them: solar air collectors are safe with respect to stagnation, because air as the heat transfer medium is not affected or destroyed by high temperatures. Air also does not boil or evaporate and no vapour pressure is built up in the solar loop under stagnation conditions. There is no need for membrane expansion vessels in the solar loop. Air as heat transfer fluid does not cost anything and does not need to be exchanged.

But also disadvantages exist, such as the necessity of larger heat exchanging areas, more voluminous air ducts compared to water pipes and (depending on system applications) a possibly higher auxiliary energy demand for the transport of energy by air.

With respect to system aspects, it may be mentioned that solar heated air can directly be used to heat residential — and office-buildings or industrial factory halls. The solar heat can also be stored, using suitable air-to-water heat exchangers together with water storage tanks or directly in other storages such as pebble bed storages.

[16] The specific heat capacity of air does not only depend on temperature, but also on the humidity of the air, see figure 3. The steep decreases at low temperatures denote the range in which the air is saturated and the water vapour condenses. In the range of the normal operating conditions, the value of cp increases at a given temperature by 1.6% when the absolute humidity is decreased from 1g/m3 to 20 g/m3.