Keywords: energy recovery renewable hot gases lng power plant two-phase turbine geothermal aqua
Energy recovery is developing considerably these days. However, there are still many little exploited avenues.
Aqua thermal heat
Most public and private buildings are heated with fuel, natural gas, coal, biomass or electricity. We are currently witnessing a slow reduction in the consumption of diesel fuel given its very negative impact on the greenhouse effect while natural gas still retains an important place given its lesser impact on the environment. Aerothermal energy has developed somewhat in recent years for private residences mainly because of its reversibility (reversible air conditioning) but remains badly suited to medium or large buildings. Aquathermal energy would present many advantages, mainly a significant reduction in electricity consumption (approximately four times less). Although it is relatively accessible to a large fraction of the population and represents a great energy potential (comparable to nuclear plants), it remains under exploited.
In many industrial processes, the hot gases resulting from combustion or energy production are released into the atmosphere, resulting not only in considerable energy loss but also in some cases in the discharge of toxic gases. These discharges concern the residual heat at the end of the expansion process in a steam turbine of a nuclear plant, the combustion gases exhausting a gas turbine or a fuel engine, the gases released by boilers or even gases emitted by road vehicles. In many situations these gases are of the order of 100 ° C. The use of a thermal cycle would make it possible to recover part of the available energy. The Carnot’s cycle efficiency being relatively low, the use of a thermal cycle could be justified if it was accompanied by a gas treatment (depollution).
Association of LNG and power plants
A very specific case of using a motor cycle at the combustion exhaust is the association of an LNG terminal and an industrial site producing combustion gases. The LNG terminal represents a cold source of -160 ° C. Its association with hot gases at 100 ° C would allow the realization of a motor cycle with a Carnot efficiency approaching 70%. If the sea water was selected as a hot source, the Carnot efficiency would slightly reduce, from 70 to 60 %, but would certainly benefit of a much greater simplicity.
Two phase turbines
A greater generalization of the two-phase turbine would allow a simplification of industrial processes leading to the replacement, by a single machine, of an assembly consisting of a separator vessel, a liquid let down (valve or turbine) and a gas expander. This helico axial machine which can operate in the whole range of a two-phase regime (from all gas to all liquid) would allow energy recovery under good performance conditions.
Geothermal energy from CO2 storage
Carbon dioxide captured during a petroleum production process or downstream a combustion unit is sometimes injected into the ground for long time storage. When the storage is carried out at a great depth, there is an opportunity for recovering the thermal energy contained into the ground through a thermodynamic cycle. Although the gas is always injected into the ground at high pressure, the cycle operation is explained, below, in a simplified schematic not requiring any compression unit. The gas is operated in a phase transition cycle where it is, firstly, naturally injected (basic assumption) into the storage reservoir taking into account a high hydrostatic pressure (dense or liquid phase), secondly, heated into the reservoir transforming it into a gas phase with a high pressure and high temperature, thirdly, extracted through a production well with a lower hydrostatic pressure providing an excess in pressure at the surface level. It is the difference in manometric heads between the injection and the production wells which permits the expansion of the high pressure gas though a turbine (expander) providing mechanical or electrical energy