IEA Solar Heating and Cooling Programm - NachhaltigWirtschaften.at
IEA Solar Heating and Cooling Programm - NachhaltigWirtschaften.at IEA Solar Heating and Cooling Programm - NachhaltigWirtschaften.at
IEA SHC Task 38 Solar Air Conditioning and Refrigeration Subtask A Report, November 2009 Serial flow, "Low Flow" ∆M ges = 10 - 15 l/m 2 h related to collector area Parallel flow, "High Flow" ∆M ges= 50 l/m 2 h related to collector area ∆M ges = ∆M col = 10 - 15 l/m 2 h ∆M ges = 4 * ∆M col = 50 l/m 2 h ∆p ges = 4 * ∆p col ∆p ges = ∆p col ∆T ges = 4 * ∆T col = 40 °C ∆T ges = ∆T col = 10 °C ∆M col ∆Τ ∆p ∆M col ∆Τ ∆p ∆M col ∆Τ ∆p ∆M col ∆Τ ∆p ∆M col ∆Τ ∆p ∆M col ∆Τ ∆p ∆M col ∆Τ ∆p ∆M col ∆Τ ∆p ∆M ges ∆M ges Fig. 7: Hydraulics for the collector field for low flow serial (left) and high flow parallel (right) systems. In all cases care has to be taken to maintain an equal resistance in all parallel lines. This can be partly achieved by a “Tichelmann” arrangement where the added length of the inlet and outlet pipes is the same for all parallel collectors (Streicher 2008). Many existing solar combisystems (without cooling function) work with the low-flow principle in the collector loop. The advantage is that the necessary temperature lift for domestic hot water preparation and space heating can be reached in one step in the solar collector if the solar radiation is high enough. Therefore, the auxiliary heater has to be operated less frequently. When connecting such a system to a thermally driven chiller, this can be a problem because the temperature difference between flow and return of the hot side of the chiller is typically small (~ 6 K). Therefore it would make more sense to operate the collector loop (at least for the cooling mode) at this small temperature difference rather than the roughly 30-40 K that are typical for low-flow solar combisystems due to the domestic hot water (DHW) demand. During the summer high flow operation would be favourable for solar cooling but low flow operation would be favourable for DHW production. One option to overcome this dilemma is to switch the collector loop at least between low-flow operation in winter to high-flow operation in summer. However, care has to be taken to ensure proper turbulent flow and acceptable pressure drops in the collector circuit as well as on the balanced flow through all parallel collector tubes. There are in principle four options: • The collector hydraulics stay the same in summer and winter but the mass flow through the collector varies by switching the pump speed or the type of pump. In this page 14
IEA SHC Task 38 Solar Air Conditioning and Refrigeration Subtask A Report, November 2009 case there is either a high pressure drop in the collector loop during high flow operation, that results in a high electricity demand of the collector pump or the collector loop is designed for high flow and during low flow operation the flow regime in the collector is laminar which reduces the collector efficiency by a few percentage points and may additionally result in non-uniform flow in the parallel collector tubes. As high flow has about four times the volume flow of low flow, the pressure drop is about 16 times (4 2 ) higher in high flow than for low flow (according to Bernoulli’s law with dp ~ (ρv 2 )/2. The electricity demand is even 64 times higher (P el = (dV * dP)/ η ~ v 3 . Here also the question arises, that maybe two different pumps for high flow and low flow have to be chosen (high pump head and high volume flow and low pump head at low volume flow). • The other possibility is to change the hydraulics of the solar collector. This can be done by installing switching valves that connect more collectors in series for low-flow operation. Again probably two pumps are needed (one for high volume flow and low delta T and one for low volume flow and high pump head). • A third possibility would be to drive the collector field only in high flow and adjust the hydraulic system for space heating accordingly (e.g. no stratifiers in the tank etc.), which gives less useful solar gain in winter but makes the plant cheaper. • A fourth possibility would be to drive the collector between high and low flow. This assures equal flow in the collectors but gives slightly less solar yield during the heating and cooling season. Concluding, it becomes obvious, that there is room for plenty of optimization for solar thermal driven cooling systems coupled to solar combisystems. page 15
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<strong>IEA</strong> SHC Task 38 <strong>Solar</strong> Air Conditioning <strong>and</strong> Refriger<strong>at</strong>ion Subtask A Report, November 2009<br />
case there is either a high pressure drop in the collector loop during high flow<br />
oper<strong>at</strong>ion, th<strong>at</strong> results in a high electricity dem<strong>and</strong> of the collector pump or the<br />
collector loop is designed for high flow <strong>and</strong> during low flow oper<strong>at</strong>ion the flow regime<br />
in the collector is laminar which reduces the collector efficiency by a few percentage<br />
points <strong>and</strong> may additionally result in non-uniform flow in the parallel collector tubes.<br />
As high flow has about four times the volume flow of low flow, the pressure drop is<br />
about 16 times (4 2 ) higher in high flow than for low flow (according to Bernoulli’s law<br />
with dp ~ (ρv 2 )/2. The electricity dem<strong>and</strong> is even 64 times higher (P el = (dV * dP)/ η ~<br />
v 3 . Here also the question arises, th<strong>at</strong> maybe two different pumps for high flow <strong>and</strong><br />
low flow have to be chosen (high pump head <strong>and</strong> high volume flow <strong>and</strong> low pump<br />
head <strong>at</strong> low volume flow).<br />
• The other possibility is to change the hydraulics of the solar collector. This can be<br />
done by installing switching valves th<strong>at</strong> connect more collectors in series for low-flow<br />
oper<strong>at</strong>ion. Again probably two pumps are needed (one for high volume flow <strong>and</strong> low<br />
delta T <strong>and</strong> one for low volume flow <strong>and</strong> high pump head).<br />
• A third possibility would be to drive the collector field only in high flow <strong>and</strong> adjust the<br />
hydraulic system for space he<strong>at</strong>ing accordingly (e.g. no str<strong>at</strong>ifiers in the tank etc.),<br />
which gives less useful solar gain in winter but makes the plant cheaper.<br />
• A fourth possibility would be to drive the collector between high <strong>and</strong> low flow. This<br />
assures equal flow in the collectors but gives slightly less solar yield during the<br />
he<strong>at</strong>ing <strong>and</strong> cooling season.<br />
Concluding, it becomes obvious, th<strong>at</strong> there is room for plenty of optimiz<strong>at</strong>ion for solar<br />
thermal driven cooling systems coupled to solar combisystems.<br />
page 15
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