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Transforming waste heat into electricity

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Transforming waste heat into electricity ( transforming-waste-heat-into-electricity )

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2 BINE-Projektinfo 13/2011 More electricity with fewer components The standard technology for utilising waste heat is provided by ORC turbines. Their intermediate thermal oil circuit means that they often use complicated system technology. Fraunhofer researchers in Oberhausen have now devel- oped turbines that dispense with thermal oil. However, turbines have a narrow optimal operating point, which with lower loads in partial load operation considerably reduces their efficiency. With the ORC process using a steam expansion engine, on the other hand, the high temperature difference in the heat exchanger (HE) leads to a greater overall efficiency, which enables the electricity production (Pel) to be dou- bled for the same heat provision (Fig. 1). The high efficiency is due to the high enthalpy gradient in the engine. That means that a considerable amount of the energy avail- able in the waste heat is transformed into usable energy. Interesting in energy terms is the further utilisation of the waste heat in the two ORC modules: the cooling water from the condenser is also available as usable heat for drying and heating. This has enabled the demonstration plant for the steam expansion engine in Fenne in Saar- land to utilise more than 90% of the heat energy used. In contrast to turbines, here it has been shown that the same engine can be used for various output levels between 100 and 200 kW. This enables variable balancing of the different pressure and temperature levels. Schmidt refers to the experience gained in Fenne: “When the cooling water temperature increases in summer, we can adjust the expansion ratio and thus the output of the gas piston engine. A turbine would have to be idle.” Demonstration plant as development hub The power plant site at Fenne is home to one of the world’s largest engine systems for generating electricity from coalmine gas, which consists mainly of methane (CH4). It is released during underground hard coal min- ing and flows out of disused mines. The demonstration plant, which is connected to one of fourteen 3-MW coalmine gas engines, began operation at the beginning of 2009. A maximum heat output of 1.3 MW is derived from the exhaust gas system (marked red in Fig. 2). The exhaust flow has a temperature of more than 400 °C. In a 7 m-high evaporator, the heat is largely transferred to ethanol as the working fluid. It flows in the green-coloured circuit and reaches temperatures of up to 300 °C. “On hearing the word ethanol, it is its combustibility that comes to mind,” says Michael Schmidt. “But that applies to many organic working fluids. In Fenne, we have invested a quarter of our development work in control and regu- lation technology that enables secure operation without supervision in accordance with recognised industrial standards.” Here the highest safety requirements apply, since the operation takes place in closed rooms and the power plant is subject to mining law. The cooling water flows from the condenser (blue) at a maximum tempera- ture of 90 °C. It is used for further purposes such as heating, drying and cooling (after adsorption). In Fenne, the cooling water heats the condensate for other power plant blocks. Field test started “We have developed a highly efficient ORC process for waste heat flows between 200 and 500 °C, which in Germany is suitable for up to 500 industrial enterprises Heat provision (related to the ambient temperature of 20 °C) =97kW ORC module turbine Pth =1.5MW;Tin =410°C 320 °C 250 °C P =200kW el ORC module Steam expansion engine P el GG Superheater Recuperator Condenser 240 °C 270 °C 70 °C 120 °C Condenser Pth = 778 kW CHP-supplied useful heat Pth = 378 kW CHP-supplied useful heat Fig. 1 Comparison of the temperatures in °C and the electrical power (Pel ) between turbines and steam expansion engines. Source: DeVeTe Condenser Steam engine Operator Media pump Induced draught fan Exhaust gas heat exchanger Evaporator and superheater Fig. 2 Layout of the demonstration plant. Source: DeVeTec and around 1,000 larger CHP plants with capacities exceeding 1.2 MW,” says Schmidt in reference to the field test that is now beginning with four different plant types. “In contrast to turbines, our engine is not dependent on the rotational speed, which means that the fluctuating heat flows are also highly suitable for partial-load uses. Instead of electricity, we can also sell mechanical shaft power that enables, for example, compressed air to be produced from exhaust heat.” The field test will show how the overall system copes after many thousands of operating hours. In addition to a biogas plant near Bremen, the waste heat from melting tanks in the glass industry and from casting kilns in the metal and chemical industries shall be used in test plants. “All this is being conducted with proven industry partners to ensure that we achieve market maturity in 2013 as planned,” emphasises Schmidt. The six ORC modules with their steam expansion engines shall then be transferred to a metal foundry in Bad Kitzingen, where a project funded by the German Federal Environment Agency has just begun. By launching the first large-scale use of the innovative plant combination for utilising exhaust heat, the company wants to reduce its CO2 emissions by more than 8,000 tonnes a year. In addition to electricity, the use of steam expansion engines instead of conventional turbines and the use of ethanol instead of silicon oil as the ORC working fluid will also enable compressed air to be produced. For the field test, the developers are installing the ORC module in a stand- Exhaust gas heat exchanger (thermal oil) Exhaust gas heat exchanger (alcohol)

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