Supercritical CO2 operating in a closed-loop recompression Brayton cycle has the potential of equivalent or higher cycle efficiency versus supercritical or superheated steam cycles at similar temperatures . The current applications of the supercritical CO2 Brayton cycle are intended for the electricity production only and the questions which are related to the building of CHP plants based on Supercritical CO2 technology were not considered yet.
CHP is the concurrent production of electricity or mechanical power and useful thermal energy (heating and/or cooling) from a single source of energy. CHP is a type of distributed generation, which, unlike central station generation, is located at located at or near the point of consumption. Instead of purchasing electricity from a local utility and then burning fuel in a furnace or boiler to produce thermal energy, consumers use CHP to improve efficiency and reduce greenhouse gas (GHG) emissions. For optimal efficiency, CHP systems typically are designed and sized to meet the users’ thermal base load demand. CHP is not a single technology but a suite of technologies that can use a variety of fuels to generate electricity or power at the point of use, allowing the heat that would normally be lost in the power generation process to be recovered to provide needed heating and/or cooling. This allows for much greater improvement in overall fuel efficiency, therefore resulting in lower costs and CO2 emissions. CHP’s potential for energy saving is vast.
It should be noted that CHP may not be widely recognized outside industrial, commercial, institutional, and utility circles, but it has quietly been providing highly efficient electricity and process heat to some of the most vital industries, largest employers, urban centers, and campuses. While the traditional method of separately producing useful heat and power has a typical combined efficiency of 45 %, CHP systems can operate at efficiency levels as high as 80 % (Figure 1) .
Taking into consideration the high efficiency of fuel energy utilization of CHP plants and the high potential of the supercritical CO2 technology, the latter should be also considered as the base of future CHP plants. The comparison with traditional Steam based CHP plants also should be performed.
The study of CHP plant concepts were performed with the use of the heat balance calculation tool AxCYCLE™ .
CHP can use a variety of fuels, both fossil- and renewable-based. However the consideration of all varieties of fuel is a quite complex task and for simplicity the only fossil fuel based CHP plants will be considered in the scope of this paper.
Fossil-fired Steam Power plants always include a Steam Generator (SG), which represents a complex heat exchanger for the heat transfer from the flue gas to the main working fluid (steam). A slightly simplified scheme of a typical water SG is shown on Figure 2. Consideration of SG as a complex heat exchanger allows to organize the process of heat transfer between the fluids more effectively and to take into the consideration an efficiency of SG heat utilization at the step of the conceptual cycle design.
Consideration of SG for a single S-CO2 cycle is especially important because of the small temperature difference in the main heater due to a high degree of the heat recuperation. Such a fact is necessary to take into consideration with S-CO2 power cycle coupling for each specific heat source. Some issues of supercritical CO2 power cycle coupling to the different heat sources were presented in the report .
Typical Steam Rankine cycle CHP plants are usually based on a condensation steam power cycle. In such an embodiment the cogeneration is implemented by the adjustable steam extraction for heat production. In dependency with the useful heat purposes the extraction as well as the heated water may have different parameters. For urban centers or campuses heating, the typical steam pressure of the extraction is about 0.2 MPa due to the value of the condensation temperature at this pressure (120.21°C.) Steam from this extraction is heating the water for the consumers. After the heating the water usually has a temperature around 100°C. The typical pressure of the water for the consumers is about 1 MPa. The mentioned parameters were used as the base in this study.
In order to accommodate supercritical CO2 technology to CHP conception, a lot of configurations and approaches were considered and the following two approaches with two embodiments for each were selected as the most interesting ones:
1. Steam Rankine cycle CHP plant with bottoming supercritical CO2 cycle
- – Combined Complex Steam-S-CO2 CHP Plant
- – Combined Simple Steam-S-CO2 CHP Plant
2. CHP plant with single supercritical CO2 working fluid
- – Cascaded Supercritical CO2 CHP Plant
- – Single Supercritical CO2 CHP Plant
The cogeneration steam turbine unit T-250/300-23.5 was taken as a base for comparison. The performance warranty for both electrical power and combined heat and power modes is presented in
table 1 .
In order to correctly compare the proposed concepts of S-CO2-based CHP plants with the ordinary CHP unit presented above we use the same total heat consumption and live steam conditions for the combined Steam-S-CO2 schemes. However, for the S-CO2 schemes the live pressure of S-CO2 was optimized and was consequently different. The efficiency of the steam turbine cylinders as well as the CO2 turbine and the compressor were taken as 90 %. The efficiency of the alternators was assigned as 98% and the heat losses for the regenerative heaters of the steam cycle part as well as for the S-CO2 recuperators were defined as 1 % from the transferred heat amounts.