Subatmospheric micro gas turbines based on the Inverted Brayton Cycle (IBC) are a very promising solution in certain fields of application. Compared to conventional gas turbines that operate under high pressure, combustion in the IBC takes place at atmospheric pressure; therefore, no fuel compressor is required. This is particularly advantageous when using lean gas mixtures with an extremely low calorific value or lean gas mixtures containing hydrogen.
Compared to a pressurised model with the same output, the turbo components used in an IBC have a larger wheel diameter and hence a higher level of efficiency. In addition, due to the lower throughput of air in the IBC, it can be employed in micro combined heat and power (CHP) systems (1 - 3 kW of electrical output) by using the inexpensive turbo components already available in the automotive industry. Initial estimates have demonstrated that an MGT-based micro CHP system that is technically and commercially practical is achievable.
Another advantage of subatmospheric micro gas turbines is the possibility of using the residual heat from the exhaust gas that is extracted via the flue to pre-heat the fresh air at the intake. This improves the thermal efficiency. In addition, the power electronics are more efficient due to the lower shaft rotation rates. A disadvantage, however, is the larger and hence more expensive recuperator needed to achieve the same power output as in a pressurised system.
How it works
In a conventional micro gas turbine system, the process air in the gas turbine, as shown in Figure 1, is first compressed by a radial compressor (1) and further heated in a recuperator (gas-gas heat exchanger) using the hot turbine exhaust gas (2). This form of heat recovery allows a significant increase of the electrical efficiency at typically low-pressure ratios of micro gas turbines (π < 5). The process air is then fed to the combustion chamber (3), mixed with the fuel and burned. Subsequently, the combustion exhaust gases are depressurised via a turbine (4) and cooled in the recuperator (2) and water heat exchanger (5). Although the majority (60 percent) of the turbine output is required for compressing the process air and overcoming mechanical friction losses, excess mechanical output can be converted into electrical energy, for example by using a permanent magnet generator (6) that is on the same shaft as the turbo components.
In the inverse micro gas turbine process, the flow of the individual components occurs in a different sequence than the conventional MGT cycle (Brayton cycle); see Figure 2. In an IBC-MGT, the process air is fed directly into the recuperator (2), heated and forwarded to the combustion chamber, where it is mixed with the fuel and burned (3). After exiting the combustion chamber, the exhaust gas is led through the turbine, where it expands below ambient pressure (4). Subsequently, the exhaust gas is first cooled in the recuperator (2) and then in the downstream water heat exchanger (5), before being compressed to atmospheric pressure in the compressor (1). In this inverse cycle - as in a conventional cycle the turbine again generates more energy than is required for the compression process, so that a generator (6) can be run. As a result of the compression, the exhaust gas is heated, and can thus be cooled down again in a second water heat exchanger (7) in order to exploit as much of the released energy as possible.
Exhaust gas recirculation (EGR): Increase in overall efficiency levels
One option to use the fuel in a combustion process more effectively is to use condensing heating technology. With this, the steam contained in the exhaust gas is condensed and the resulting heat that is released can be utilised. This leads to an overall increase in the thermal efficiency of the system. Although condensing heating technology is readily used in modern heating systems, their use in combination with a micro gas turbine cycle has the disadvantage that micro gas turbines are operated under a large excess of air and the proportion of steam in the exhaust gas is very small.
By partially recirculating the exhaust gases, the proportion of steam in the exhaust gas can be increased and condensed. The composition of the combustion air changes significantly depending on the proportion of recirculated exhaust gas, which consequently gives rise to changes in the combustion process.
Application areas of micro CHP systems
Currently the power and heat requirements in detached and semi-detached houses are usually provided separately. Whereas power is generally drawn from the main electricity grid, the central heating and hot water generation is usually provided through the use of oil or gas boilers. As the majority of heating systems are dated, and the heating requirements of these buildings make up over 40 percent of the total energy consumption in Germany, there is great potential for the use of decentralised CHP systems. However, at present, the use of existing micro gas turbine-based systems (e.g. MTT EnerTwin with 3 kW electric power) in a detached house, for example, has been made difficult from a commercial perspective, as the electrical output and heat produced far exceed the actual requirements. This would lead to short operating times, and a high proportion of the generated power would need to be fed back into the public grid, making it uneconomical. Using subatmospheric gas turbines, smaller output ranges can be achieved, which would make gas turbine-based CHP technology accessible for detached and semi-detached houses.
Application areas of lean gas CHP systems
When using hydrogen-containing gases in a combustion system, which needs a fuel compressor, additional safety aspects play an important role. This significantly complicates the design and manufacturing, and hence increases the costs. Eliminating the need for a fuel compressor by using lean gases with a low calorific value or lean gases containing hydrogen (e.g. wood gas or landfill gas) turns out to be particularly beneficial. The achievable increase in efficiency is particularly evident for systems in the lower power output range.
Status of the research work
To evaluate the inverse micro gas turbine-based CHP concept, feasibility studies with in-house cycle simulation tools were carried out. Thereby, the impact of the efficiency of various components, heating systems and EGR rates on the overall system were investigated. These studies showed that a first demonstrator could achieve an electrical efficiency of approximately 16 percent and an overall efficiency of approximately 91 percent at an EGR rate of 75 percent. At the moment, a micro gas turbine that has been converted for inverse operation is being operated and investigated.
Based on the acquired measurement results, the individual system components will be optimised for the inverse gas turbine cycle, and a test system will be built in a second phase. Then, in a third phase, the system will be investigated in a test run lasting several weeks. Market and prospect analyses will be carried out at the same time for the inverse CHP system. These analyses are intended to demonstrate the use of micro gas turbine-based power generators with exhaust gas recirculation as low-pollutant, low maintenance and low cost CHP systems with high levels of electrical efficiency for use in the detached and semi-detached household sector.