Performance Evaluation of Combined Heat and Power (CHP) Applications in Low-Energy Houses

The Research Centre on Zero Emission Buildings has a vision to eliminate the greenhouse gas emissions caused by buildings related to their production, operation and demolition. The concept of Zero Energy Building (ZEB) has gained wide international attention during the last few years and the government in Norway has agreed that passive house standard is to be required for new buildings from 2015 and nearly ZEBs as a standard from 2020. Combined heat and power (CHP), also known as cogeneration, is an emerging technology associated with the potential to reduce primary energy consumption and associated greenhouse gas emissions through the concurrent production of electricity and heat from the same fuel source. Until the recent focus on Net-ZEB, the heat provided by electricity production of CHP was considered as a by-product during energetic evaluation. Within the Net-ZEB concept, CHP systems are considered as a potential energy supply solution for buildings. As CHP systems have large thermal output and the heating needs of buildings are getting decreased with super insulated envelops, the integration of the CHP systems becomes challenging. The potential offered by these systems is strongly dependent on their suitable integration with the building heat loads. A simulation model is used to investigate the performance of CHP systems supplying a residential building. Analysis of the simulation results indicate that increasing the size of the storage tank does not improve the performance of the system as the heat losses becomes greater. Having less stringent requirements to the thermal comfort will improve the operation of the CHP unit, but the comfort must be maintained at an acceptable level. By adding an auxiliary gas boiler to the system, covering the heating needs outside the heating season, a system efficiency of 80% is achieved when supplying a passive house and 81% when supplying a low energy building. Compared to the systems only using CHP, these efficiencies became 78% and 79% for the passive house and low energy building, respectively. When supplying the low energy building a higher efficiency is achieved. The low energy building has higher heating needs which are a more favorable condition for the operation of the CHP. Nevertheless, the system supplying the low energy building will emit more CO2 which is not desirable in a net-ZEB context. The amount of CO2-production for different energy supply systems are calculated and compared showing that the CHP systems are more favorable when the CO2-production factor for electricity is high. Taking into account that the CO2-production factor for electricity is expected to increase over the years, as the electricity production in the world becomes greener, the CHP-technology will need further development in order to retain its position as a favorable energy supply solution in a net-ZEB context.