Experience: the EURAC study case

In Bolzano, the capital of the most northern Italian Province, three buildings are equipped with solar collectors assisted by one Combined Heat and Power generator. For one of them, which is the seat of EURAC, a large amount of information could be collected thanks to a monitoring system which has worked since 2005.

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The main features of the EURAC energy facility are reported in Table 1. Fig. 1. and Fig. 2 show the plant layout respectively for winter and summer operation mode. A more detailed description of the system can be found in [3].

Table 1.Main features of the SHC-CHP installation in EURAC, Bolzano

Heat Production Facility

Solar Collectors — Gross Area

615 m2

1 Cogeneration Unit

180 kWe/ 330 kWth

2 Condensing Boilers

350 KWth each

Cold Production Facility

1Absorption chiller

300 kWc

2 Compression chillers

315 kWc each

Storage tanks

2 Solar tanks

5,000 l each

1 Cold tank

5,000 l

Fig. 1.Layout of the SHC-CHP installation at EURAC, Bolzano : winter operation mode.

One critical aspect within this plant is the presence of a hydraulic junction where all the hot and cold streams are mixed, in particular the ones of the cogenerator and the solar loop which often have different temperatures, especially in winter. Besides increasing entropy generation, mixing flows at different temperatures can decrease both collectors’ and cogenerator’s efficiency, in winter and in summer as well. In fact, in winter solar fraction usually has a temperature lower than the one of
cogenerated heat. Hence, solar fraction can be stored and used for SDHW supply or mixed. If it is mixed, collectors’ efficiency can be negatively affected by a too high mean temperature in the hydraulic junction which is due, on one hand to the high temperature provided by the cogeneration, on the other hand to the high temperature returning from the distribution system (high temperature radiators are included). On the contrary, in summer, high temperatures are delivered by the solar loop, increasing the main temperature in the hydraulic junction. In this case, when the absorption chiller works at partial loads (i. e. the “V Abs” in Fig. 2. reduces the mass flow entering the generator of the absorption chiller, thus the mass flow between the hydraulic junction and the valve is recirculated), the engine cooling stream temperature risks to be too high. The cogenerator is put in alarm and it switches on/off continuously, first because it is controlled by the electricity demand, secondly because it has no heat storage. Furthermore, the nominal hot mass flow entering the generator of the absorption chiller is higher than the sum of the nominal flows of the cogenerator and the solar loop, thus the boilers have to be used if the absorption chiller has to be run at nominal conditions. Whenever not all of them work and the cooling peak load is reached at the same time, colder flows from the bottom of the hydraulic junction enter the absorption machine and the inlet temperature is decreased.

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Fig. 2. Layout of the SHC-CHP installation at EURAC, Bolzano: summer operation mode.

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