Efficiency of heat and work in a regional energy system

Abstract: One of the largest flows of energy in Swedish municipalities is the fuel-energy flow through the regional combined heat and power (CHP) plant. The customer products from this flow are mainly electricity to the electricity grid and heat to the building sector. There are many ways to describe and examine this fuel-energy flow, and there are many perspectives. This thesis presents one perspective. It is a top-down, analytical and numerical perspective on the efficiency of heat and work in a regional energy system. The analysis focus on the present situation in Linköping municipality and aims at describing the energy efficiency improvement potential. Three subsystems are considered, the regional production of electricity, the regional production of heat, and the regional public transport by bus. These three systems are physically all heat engines i.e. engines that derive work and/or heat from fuel combustion processes. It is important to notice that the analysis in this thesis does not describe the theoretical improvement potential, that potential is considerably higher than the implementable potential, but of no practical use. Instead the analysis is as far as possible based on real world measured efficiencies and efficiency values of best practice (Best available technology).The analysis shows that hardware investments at the CHP plant can improve the electricity generation efficiency and thereby reduce CO2 emissions. The investments are in high pressure turbines, medium pressure turbines and preheaters. The size of the improvement is hard to quantify because it depends partly on unknown factors in the surrounding electricity market. In the studied system CO2 reduction could be as high as 40 - 60 %.The regionally produced biogas would be used more efficiently if it were used in the local combined cycle gas turbine instead of being used in internal combustion engines in buses. The buses would instead be electrically driven. This use of biogas would create a better integrated fuel-energy flow and reduce heat losses.Another improvement is to reduce the system temperatures in the district heating system. The study shows that the efficiency gains, because of lower system temperatures, would increase electricity production by about 1 – 3%, and that greenhouse gas emissions would be reduced by 4 – 20%. However, these improvements are dependent on demand side investments in the district heating system and are therefore slow to implement.