System, methods and credible data for calculating primary energy efficiency for energy systems with special focus on energy systems applying CHP-technology with bio based fuel

Abstract

During the last few decades, reduced energy consumption, increased energy efficiency and utilization of renewable energy have gained interest while different energy efficient energy conversion technologies have matured. Combined heat and power (CHP), which utilizes biomass, has been promoted as a mean to reduce dependency of fossil fuels, increase use of renewable energy, improve overall energy efficiency and reduce importation of fossil fuels both by the European Union (EU) and associated countries.

Primary Energy (PE) and CO2 equivalent emissions were selected as the most relevant parameters to describe the environmental impact of an energy system (EN 15603). This means that the energy supplied to a building shall be calculated as the sum of kWh energy used per energy carrier multiplied by the Primary Energy Factor (PEF) for each energy carrier. The Primary Energy Factor takes into account all the energy that is needed to deliver 1 kWh power and/or heat to the end user, while “CO2 equivalents” refers to the equivalent effect of all greenhouse gases that is emitted from extraction to delivery expressed by a CO2-value. The Intergovernmental Panel on Climate Change (IPCC) has decided to make official the term “CO2 equivalent,” which consists of emissions from CO2, CH4 and N2O.

A model was developed for calculating Primary Energy and CO2 emissions for heat from district heating grids from combined heat and power plants utilizing biological material as fuel.

The model is based on energy calculations and Life Cycle Assessment (LCA) methods. The embodied energy and CO2 equivalents are calculated by LCA methods based on information from producers, contractors, district heating companies and the Ecoinvent database. The district heating grid, used in the case studies, is created from an average Norwegian district heating grid, with variable flow and temperatures in each of the four seasons: winter, spring/fall and summer. Heat is supplied to a district heating grid with different energy densities, where supply and return temperatures vary for each different season. Heat loss and the loss of head are calculated for each season as well. The part load efficiencies of the CHP-plants are calculated by SteamPro and GTPro. Usually, yearly average efficiencies are used in those calculations/standards. Therefore, this study examined the impact by utilization seasonal efficiencies instead of annual values in calculating PEF and CO2 equivalents for the four different CHP-plants.

Research in this field has typically utilized average values for fuel. This study applies the impact of both biogenic and biomass with variable moisture levels.

The results demonstrate both the importance of detailed calculation and the need for a strict standardized calculation method. Utilization of different fuels will also have an additional impact on the efficiency of the CHP-plant and thereby the total PEF for heat delivered in a district heating system. In addition, allocation methods such as alternative production (instead of the power bonus method) will reduce the impact of exported power, thus promoting energy efficient systems and/or renewable energy systems with low CO2 equivalent emissions.

Furthermore, the study has shown the need for at least two different parameters/indicators describing the environmental impact of such allocation methods. Application of a combination of PEF and CO2 equivalents will provide a better understanding and comparison of different energy systems.

This PhD-work has been carried out as a part of the Nordic/Baltic PhD-project Primary Energy Efficiency (PEE), and was partly financed by Nordic Energy Research and partly from some leading Norwegian district heating companies.