| Abstract |
For low temperature geothermal heat sources, Binary ORC power plants are often the preferred technical choice. Several studies have been trying to demonstrate the purpose of such a choice and now help to determine the most appropriate fluid. Once the temperature of the hot source is fixed and the fluid chosen, pressure and temperature range of the working fluid remains so large that it makes it complicated to find the most productive cycle. The influence of pressure and temperature conditions of the organic fluid at the turbine inlet must be studied. The method described hereafter proposes a way to restrict the possible working conditions upstream of the turbine. The cycle is plotted on an enthalpy temperature diagram which represents both brine cooling and air warming through vaporizer and condenser respectively. This restricted area is bound by some technical considerations with regards to turbo-expanders. The outlet pressure of the turbo-expander is determined by the condensing temperature, which is itself determined by an offset above the cooling medium temperature: ambient air. Considering the isobar corresponding to the fixed condensation temperature, this isobar represents all possible expander outlet points. Each temperature on this isobar corresponds to an end of polytropic expansion. An isentropic efficiency close to 0.9 is calculated according to Cryostar experience. One of these polytropic expansions is tangent to the saturation curve and determines the left boundary for the possible inlet. However, at the right of it, no expansions will cross the biphasic area (liquid droplets), where liquid ingress could damage turbo-expander. Moreover for each of these possible expansions there is an upper limit pressure ratio, allowed by the turbine. The range of these starting points determines a new bound for a one stage turbo-expander. Finally a more innovative approach is to use the Exergy concept. Exergy is for a determined cold sink temperature T0 (ambient air), the compound state function H-T0*S (H enthalpy, S entropy). Exergy represents the part of energy convertible into mechanical energy; as opposed to the Anergy which must get back to the cold sink. Assuming that for a given temperature, a corresponding enthalpy (or a pressure) exists where the Exergy is at a maximum, this point becomes a target for the cycle. So the curve of maximal Exergy is plot into the enthalpy temperature diagram and shows a new indicator for the process engineer. At the right of this curve the pressure is too low with regards to the temperature to reach the Exergy maximal. Worse, the fluid out of the turbine becomes superheated and requires for the condenser heat capacity to grow, thus fan consumption to grow too. At the left of this curve the pressure could grow to increase the mechanical power, but the fluid reaches the temperature with loss of Exergy. The pressure is too high for the temperature and the energetic cycle efficiency is degraded by the increase of pump power. Unfortunately, however the global efficiency decreases at the left of the maximal Exergy curve, there is an opposite effect into the vaporizer. Increasing the pressure minimizes the pinch through the vaporizer. By keeping this pinch constant (e.g. 5°C for economical reason: Surface of heat exchanger), the ratio of brine used to heat the ORC fluid could be reduced. Thus for the same flow of brine, the flow of ORC could be increased. The brine is more efficiently used; more heat is retrieved from it. The benefit of pushing the pressure above the max Exergy point must be balanced with better cooling of the brine. To measure the benefit of increasing pressure above the pressure corresponding to the highest Exergy at fixed pressure, Global Exergy efficiency is estimated. This estimation is done by weighting consumption with coefficients (e.g. fans consumption is weighted by the product of all electrical conversion to fan). This last work is performed for several fluids and temperatures and presented as follows. |