Subatmospheric micro gas turbines (MGTs) based on the inverted Brayton Cycle (IBC) are a promising solution for certain applications. They have an advantage over conventional gas turbine cycles in that they do not require a fuel gas compressor and that their turbo charger components have larger wheel diameters and thus higher efficiencies than a compressed version at the same output. Furthermore, the power electronics function more efficiently due to the low shaft speeds. Another advantage of subatmospheric gas turbines is their ability to pre-heat fresh air using waste heat from exhaust gases fed through the vent stack, improving thermal efficiency. Fresh air and exhaust gases also share the same vent stack. One disadvantage, however, is the larger and thus more expensive recuperator, despite the output remaining the same.
In terms of micro cogeneration systems, this power plant system enables the use of more readily available, and more cost effective turbo charger components from the automotive industry due to its low air mass flow rate in comparison to pressurized micro gas turbines. Early estimates show that it could make sense, technically and financially, to operate an MGT-based micro cogeneration system.
The lack of a fuel compressor is particularly advantageous when using weak gas with very low heating values or weak gas with hydrogen content. Possible increases in efficiency can be seen most clearly at lower outputs. When burning weak gas with a hydrogen content, security aspects play a particularly important role, complicating the design and manufacture of the fuel compressor and making it significantly more expensive.
Unlike in conventional gas turbine cycles based on the Brayton Cycle, in subatmospherically operated micro gas turbines (see figure 1), the ambient air is first fed through the recuperator (4). Once in the recuperator, the air is heated using waste heat from the exhaust gases before being fed into the combustion chamber where it is mixed with the fuel. Combustion takes place here under atmospheric conditions. The hot exhaust gases are then decompressed to below atmospheric pressure in the turbine (2) and fed through the recuperator (4). In order to achieve the highest compressor efficiency possible, the flue gas must be cooled further once it has passed through the recuperator. To this end, it is fed through a heat exchanger and/or cooler at ambient pressure. The largest proportion of the turbine’s output is used for compressing the air used in operation. Excess mechanical output is converted to electrical energy by a permanent magnet generator.
Micro Gas Turbine-Based Micro Cogeneration Plant
Generally speaking, the heat and electricity needs of detached and semi-detached houses as well as apartment blocks are currently supplied separately from one another. While electricity is taken from the public network, heating and warm water requirements are usually met using oil or gas boilers. Over 40% of Germany’s total energy consumption can be contributed to heating requirements.
A high percentage of plants used for the generation of power for heating and warm water are no longer state-of-the-art. Just 20% currently use a gas condensation boiler. This makes urgent renovation of the country’s heating methods indispensible if climate targets are to be reached.
An alternative to the separate generation of electricity and heat is offered by so-called micro cogeneration systems, also called power-generating heating systems, in an output range of less than 3 kWel. Thanks to the connected electricity and heat production, the fuel can be used in a more efficient and therefore cost-effective and environmentally friendly way. In comparison to separate electricity and heat production, the CO2 emissions produced by cogeneration plants are up to 50% lower.
Micro gas turbines offer an alternative to piston engines and fuel cells as the basis for micro cogeneration plants. However, using the currently available, cost-effective turbo charger components from the automobile industry, only micro gas turbine-based micro cogeneration plants with an electric output of around 3kW and higher are feasible.
Using such a plant for a detached house, for example, is unlikely to prove economically viable, because the electrical and heating output far exceeds the house’s actual requirements. This leads to short operating times and high, inefficient injection rates into the public power network.
In contrast to pressurized micro gas turbines with similar component sizes, a subatmospherically operated micro gas turbine-based cogeneration system makes operational power output rates of less than 3 kWel feasible for the first time, due to its low air mass flow rate.