Exergetic evaluation of heat pump booster configurations in a low temperature district heating network
Abstract
In order to minimise losses in a district heating network, one approach is to lower the temperature difference between working media and soil. Considering only direct heat exchange, the minimum forward temperature level is determined by the demand side, as energy services are required at a certain temperature. As domestic hot water is required at a temperature range where legionella is no longer a threat, forward temperatures in a traditional low temperature district heating network cannot be lowered beyond approximately 55 ºC. One solution is to boost the temperature of the forward tap water stream with a heat pump, as the remaining heat demands are often not required at temperature levels as high as the tap water. The scope of this work is to evaluate the power consumption and second law efficiency of booster heat pumps for tap water production in a low temperature district heating network. The heat pump and storage arrangement is evaluated based on a tapping sequence from the Danish standards (DS439). Based an initial investigation of possible designs, three configurations have been chosen for the evaluation. Of the three heat pumps, two are implemented on the primary side to boost the network stream, and one is intended to increase the temperature of the tap water directly. Results show that one of the three configurations are superior to the two remaining, when considering temperature levels of forward stream between 35 ºC and 47 ºC. The overall results remain the same regardless of heat exchanger sizes and the isentropic efficiency of the compressor used in the heat pump. The superior configuration shows exergetic efficiencies higher than 0.5 when forward temperatures is around 45 ºC.
Info
Conference Paper, 2012
UN SDG Classification
DK Main Research Area
Science/Technology
