Determination of energy quality for hydronic heating systems

The quality of a hydronic energy system isinfluenced by supply and return temperatures.Instead of using the exergy term, a TEA(Temperature and Energy Accumulated) curveis introduced for evaluating an examplifiedplanned 100 GWh district heating systemoutside Oslo.Main focus is on normal running conditionsoff the design point. The following questionswere raised when analyzing the energysystem:how could we make a design that improvesthe use of low temperature energy?is a design with low flow rate, i.e. lowpipeline costs, compatible with the use oflow temperature energy?can we visualize the quality of the energyused through the distributed temperaturesand the corresponding quantity of energy?A spreadsheet analysis tool was developed andused for the heating plant design andoptimization.This paper presents the TEA curve andshows how evaluations may be done. Differentdesign temperatures ranging from 60 C/50 Cto 110C/40C are included in the discussion.Focus on low return temperatures is oftenmore important than avoiding high supplytemperatures at design conditions. The mainfindings are:The TEA curve demonstrates that it is auseful tool in order to consider the energyquality of a hydronic heating system.Temperature mixing with shunt valves andrecirculation of supply water into the returnwater pipeline has a negative effect on thepossibility to use low temperature energysources. High temperatures should beutilized when possible.A 75 C/40 C design improves the level ofenergy use compared to a 60 C/50 Cdesign.