(467g) Working Fluids for Geothermal Orc - Processes

One source of renewable energy is geothermal heat. In order to use it for the production of electricity one type of conversion process is the Organic Rankine Cycle (ORC-process). The principle of this process is the same as that in a classical steam power plant with, however, an organic substance as working fluid instead of water. In considering the efficiency of a geothermal plant with hot water as primary heat source one has to take into account a) the thermal efficiency η_th of the ORC-cycle and b) the amount of heat q_WOwhich can be transferred from the geothermal water flux of 1 kg/s to the organic working fluid. Note, that the latter can be strongly limited due to a pinch point in the heat exchanger. The electric power results from the equation P_el= m_W q_WO η_th with m_W being the mass flux of the hot water.

In order to determine η_th and q_WO the thermodynamic data of the working fluid are required which are supplied here by the BACKONE equation [1]. This equation requires for pure fluids five substance specific parameters and for mixtures only one binary paramter. Its application to geothermal energy conversion was made by suggestion of Geoforschungszentrum Potsdam and of dezentral Berlin which uses it for dynamical simulations of geothermal ORC-cycles.

In a first step we determined the thermal efficiencies η_th of 31 pure fluids in ORC cycles which work between 100°C and 30°C. For different process designs ? without and with superheating of the vapor, without and with internal heat exchange (IHE) ? as well as for pressures up to 20 bar and for supercritical pressures the thermal efficiencies were calculated with BACKONE. Results for three representative working fluids are:

· R600a (iso-Butan), critical temperature T_c = 135.0°C, overhangig dew line, turbine entrance 19.98 bar, 100°C: η_th = 0.121 without IHE, η_th = 0.124 with IHE.

· R152a, T_c = 113.5°C, bell-shaped dew line, turbine entrance 20,00 bar, 72,59°C: η_th = 0.088 (wet vapor with vapor content x = 0.96 at turbine exit).

· R143a,T_c = 72.7°C, turbine entrance 45.00 bar (supercritical), 100°C: η_th = 0.091 without IHE.

Next, we determined in a pinch analysis the heat q_WO which can be transferred from hot water entering the heat exchanger with 120°C to the working fluid. We assumed the temperature difference at the pinch point to be 10°C and a hot water flux of m_W = 30 kg/s. Then we determined the potential supply of electric power by using P_el = m_W q_WO η_th.

· For R600a the quantities η_th, q_WO and P_el were determined as functions of the evaporation temperature. With decreasing evaporation temperature η_th becomes smaller, whilst q_WO becomes larger because of the lower pinch point temperature. The product of both quantities yields a maximum value P_el for the turbine entrance parameters 11.99 bar, 74.39°C which yield η_th = 0.091, q_WO = 215.0 kJ/kg, and the power P_el = 587 kW.

· For R 152a one obtaines for the above turbine entrance parameters (20.00 bar, 72.59°C) q_WO = 218.22 kJ/kg and with η_th = 0.088 one obtains the power P_el = 576 kW.

· For R143a one obtains for the supercritical cycle and the above turbine entrance parameters (45.00 bar, 100°C) q_WO = 260.50 kJ/kg and with η_th = 0.091 one obtains the power P_el = 712 kW.

Summarizing we conclude that the supercritical process is optimal as in the pinch analysis the cold composite does not show a kink. We expect similar good results at pressure up to 20 bar for mixtures [1] as they exhibit a temperature glide during evaporation.

[1] U. Weingerl, M. Wendland, J. Fischer, A. Müller and J. Winkelmann. The Backone family of equations of state: 2. Nonpolar and polar fluid mixtures. AIChE-Journal 47, 705 ? 717 (2001).