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Effect of Supply/Return Temperature at Heat Production Efficiency

Effect of Supply/Return Temperature at Heat Production Efficiency

Effect of Supply/Return Temperature at Heat Production Efficiency

I already have a graph showing the effect of supply/return temperature (to/from a district heating system) on the heat production efficiency (at a cogeneration cycle), taken from the IEA Report - Annex X:

Interpreting the graph; while the heat demand (and other parameters) being same, supplying heat at high temperature (for example 90 °C) cause higher electricity loss in cogeneration plant when compared to supply with lower temperature (same example, this time with 60 °C). Assuming the return temperature same at 40 °C for both cases (which in real not true but here the purpose to explain the temperature effect), 100 MW heat supply results in 12.5 MW electricity loss with 90°C supply temperature (z factor %12.5 from the graph) while 7.5 MW electricity loss with 60°C supply temperature (z factor %7.5).
Another interpretation: in case supply temperature is fixed at 60°C and the heat demand is same, if the return temperature (from the consumers) is reduced from 50°C to 30°C (by increasing the performance at the consumer site - in both same heat demand), then the z factor changes from %8 to %4, which as a result cause the electricity loss reduce from 8 MW to 4 MW, all respectively.

I don't want to see the interaction with other parameters (in-cycle or with outside parameters i.e. outdoor temp.) due to the outlet/inlet temperature for the heat production plant (in other words of the same; supply/return temperature for the district heating). What I want is, as in the graph above (z factor - that is for extraction-condensing turbine):

Would you please provide me function/graph showing quantitatively the direct effect of outlet/inlet temperatures on the performance/efficiency/COP of the heat production plant (at any type)? Qualitative facts with showing the principles behind "why low temperature production increases the efficiency/COP at heat production" can also be rewarding.

Why I need this is because one of my colleagues blocked my studies with some imputations saying that "the benefit with low temperature supply is not true for heat production plants rather than cogeneration". I need some proof to show my superiors!

“In life, the truest guide is science” – Mustafa Kemal Atatürk

RE: Effect of Supply/Return Temperature at Heat Production Efficiency

Easiest way to prove this is to draw out the two systems and do the maths over the whole cycle to compare where the energy goes, then calculate your own coefficient of performance. Your heat load is given by M.Cp.dT, the electrical energy extracted by the change in enthalpy over the turbine times the efficiency.

District heating load is to a first approximation independent of the hot water temperature (Cp roughly constant, you select dT, buy bigger radiators for lower temperature). Turbine power extracted is based on difference between enthalpy in (constant) and enthalpy out (lower for lower temperatures), so you can extract more electricity per unit heating. You then have a colder return stream to your steam generator of course, you're not getting anything for free.

If electricity is much more valuable than heating my house, then being able to take a greater fraction of the energy available makes sense. I think that your colleagues point is that if my main aim is to provide heat for another process, dropping the supply/return temperatures may not be optimal.


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