Primary exergy efficiency—Effect of system efficiency environment to benefits of exergy savings

Researchers from Aalto University Department of Energy Technology- School of Engineering analyzed the benefits of Exergy Analysis and Primary Energy Efficiency in energy improvement of whole energy chain despite not modelling all of it. Laukkanen et al. showed that system environment affects  the benefits of exergy savings in a system level depending what production does the exergy saving replace. Their work appeared in Energy and Buildings Journal.

One of the ways used to improve production process is to directly improve energetic performance of a system in question. Second law of thermodynamics can be used to analyze energetic performance using Exergy Analysis.

Exergy Analysis deals with the maximum useful work potential a system has at a given state and environment. Hence, exergy level of a system in same state is different at different environments.

Another method namely Primary Energy Efficiency is based on First Law of Thermodynamics which considers all primary energy input to a production system that is required for yielding a certain product at system boundary. Primary Energy Analysis is beneficial in comparing efficiencies of the same product with different production chains which compares the amount of primary energy the overall chain has consumed. It can be concluded that Primary Energy Efficiency is product-oriented analytic method while Exergy Analysis is process-oriented analytic method.

Laukkanen et al. (2016), combined the benefits of Exergy Analysis and Primary Energy Efficiency as a holistic energy efficiency indicator in analyzing energy improvement in whole energy chain despite not modelling all of it. The combination of these approach is known as Primary Exergy Analysis (PeXa) which is generally a basic exergy analysis method where losses outside the studied process are calculated with factors obtained from Primary Energy Efficiency. This means that exergy of a product that is made up from many production routes depends on the portion of these routes used to make up the product.

Factors for inputs and products are obtained from databases or are calculated according to standards (for example in Combined Heat and Power (CHP) the European Union Standard EN 15316-4-2-5). Physical and chemical exergy values of flows were calculated neglecting nuclear, magnetic, electrical and interfacial effects.

PeXa method was applied in a small district heating system which produces District Heating (DH) and electricity (P). The system has a plant producing both DH and electricity (Combined Heat and Power) and a plant producing heat only (Heat Only Boiler) and efficiency of Heat and Power and Heat Only Boiler were assumed to be constant.

Exergy and energy values of flow were required for calculation of the PeXa-factor for District Heating and PeXa efficiency of District Heating in CHP and Heat Only Boiler processes assuming that Primary Energy Factor (PEF) of fuel and electricity is 1.1 and 3 respectively.

Results show that when PEF_EL=3 at a constant 1MW increase in DH load, PeXa DH factors are lower or efficiencies higher in Heat and Power plant showing that constant DH load increase will be provided with the CHP plant. When PEF_EL=1.2 at a constant 1MW increase of DH load, all additional heat is provided by HOB plant if objective is to maximize the systems District Heating PeXa-efficiencies while additional heat is provided by Heat and Power plant if objective is to maximize exergy efficiency.

From Laukkanen et al. (2016), the above results concluded that if the objective is to maximize HOB+CHP efficiency, exergy analysis is enough, but if the objective is to maximize system (CHP+HOB+society), PeXa should be used instead. This conclusions led to the fact that PeXa efficiency indicates magnitude of benefits to society which exergy analysis can’t provide alone.