Exergy Assessment of Hybrid District Heating Systems Using Resource Exergy Analysis (REA)

Abstract

In this paper six supply systems for a simulated energy demand scenario are compared using resource exergy analysis (REA). The analysis is complemented with an assessment of greenhouse gas emissions. The six scenarios (S) that are compared are • S1: decentralized gas boilers, • S2: low temperature district heating using 50% heat from a block CHP plant and 50% heat from a gas boiler, • S3: decentralized air-source heat pumps, • S4: a very low temperature district heating network with a central large heat pump and decentralized electrical boilers, • S5: a cold district heating network using heat from the ground as a source for decentralized water-water heat pumps and • S6: deep geothermal low temperature district heating network. The electricity used in all scenarios is assumed to come from PV panels that are newly built on the district. Thus, all results for hybrid energy systems can be considered best-case scenarios. Scenario 1 and Scenario 6 are intended to provide reference scenarios for the evaluation of the considered hybrid energy systems. S1 allows a comparison with the current status quo. S6 is there to show how good hybrid energy systems can be if compared to one of the best thermal district heating sources. The results of the performed analysis show that hybrid energy networks can be among the most resource saving and low carbon heat supply solutions possible. In order to achieve this outcome, it is important to ensure that the electric load generated by these systems is covered by additional GHGE-free power supply. In comparison to natural gas boilers hybrid energy systems can save more than 70% of resource exergy and over 90% of GHGE. All considered hybrid energy systems produce similar savings, so that the decision on what type of hybrid energy system is best for a given community largely depends on the heat demand density, the potential for heat networks or air-water heat exchangers and the availability of suitable heat sources apart from life-cycle cost considerations. While hybrid energy networks can be among the top solutions for decreasing resource exergy consumption and GHGE, they are not the only technology suitable for resource saving and climate friendly heat supply. Scenario 6 (Deep geothermal heat) shows that district heating using suitable thermal sources can match or even outperform best case hybrid energy networks. However, since thermal sources that can directly provide heat at the temperature levels required by the building stock can be locally limited, hybrid energy networks are one of the key technologies to supply heat to areas with high heat demand density.

REA: Resource Exergy Analysis – A Key to Climate Sustainability

“…It has already been shown that primary energy factors are not suitable for a sensible comparison of low-carbon technologies [9]. The sample analysis in this article demonstrates the effectiveness of REA in enabling climate sustainability when looking at heating systems using low-carbon energy sources.…”