Novel energy station by natural fluid CO 2

Abstract

The concept of an energy station is a very general term, as long as it can play the role of energy supply. Now it mainly refers to a heating station or a heat exchange station. Yet the concept of a distributed energy station corresponds to the current centralized energy supply system (such as central heating, large grid power supply). The distributed CCHP (combined cooling, heating, and power) system is a comprehensive cascade energy utilization solution. It is the most sustainable way to mitigate climate change and improve energy efficiency. This system can be operated independently or in parallel, which can meet the needs of users with different power loads. However, there are still many conventional energy stations using fossil fuels (natural gas as the main fuel) to drive gas turbines or internal combustion engine generators, resulting in carbon emissions. Along with the development of modern technology, the impact of fossil fuels on our natural life has become well known: they are environmentally unfriendly or hazardous, incur high operating costs, depleate nonrenewable energy resources, and so on. This is the first point that I want to emphasize about the current conventional energy station. The second disadvantage of a conventional energy station is regarding the thermodynamic cycle, or more specifically, the complete seperation of heat and cold. In addition to the above two points, another important point is the problem caused by the wide use of hydrochlorofluorocarbon (HCFC) and CFC refrigerants. As the environmental and ecological pressure increases, many refrigeration applications are required to substitute these with other natural refrigerants as the working medium. The principal aim of this perspective article is to discuss the negative factors of the conventional energy system (eg, working fluids, heat source, power generation mode, etc) that is being used in recent years and to put forward a new concept of an energy station run by natural fluid CO2. We look forward to solving the problems brought about by fossil fuels by using renewable energy and a new thermodynamic cycle by using natural fluid CO2, thereby solving the environmental problems due to freon or other refrigerants and saving energy as far as possible. Owing to atmospheric pollution and the high cost of burning coal and oil, we take natural gas as the main fuel, shown in Figure 1 as an example. As shown in the figure, gas fuel enters the gas turbine generator to produce electricity to meet the power demand of users. After power generation, the discharged waste heat from the system is recovered by the recuperator, and the appropriate equipment (waste-heat boiler or waste-heat direct fired machine, etc) produces cold water for air-conditioning systems in summer and hot water in winter. Simply, the boiler is used to boil water and the high-temperature flue gas is used to generate electricity. A gas-fed boiler can also be used to supply hot water and for heating. In the general CCHP, the temperature is often very high, and large quantities of pollutants are produced during combustion. However, the cold and heat that have to be produced do not need such high temperature and result in very large loss due to thermodynamic difference. Use of a grid for power generation also results in energy and economic loss, so the primary energy utilization rate of the whole system is greatly reduced. This kind of energy station produces a lot of carbon emission, needs many devices, and occupies a huge area of land. Furthermore, the cold and heat are separated, which leads to lower efficiency. The working medium required to drive the thermal cycle of the entire system is known as the refrigeration working fluid. This medium can undergo a thermodynamic cycle of gasification, absorption, and condensation in the refrigeration system to achieve cooling or heating. There are many kinds of the working fluids, such as ammonia and freon, but the widely used HCFCs and CFCs cause many problems such as global warming and ozone layer depletion and even human health issues. According to the Kyoto Protocol and the Montreal Agreement, freon refrigerants will be gradually phased out by 2030. The energy station proposed in this paper will use transcritical CO2 as the natural fluid to run the refrigeration/vapor compression cycle. Because CO2 is a greenhouse gas, in the conventional energy station we need to reflect on how to effectively use and manage CO2. Therefore, we propose the new energy station that does not emit carbon during the supply; on the contrary, it makes full use of carbon. The natural transcritical CO2 cycle can be understood through Figure 2A and the power cycle through Figure 2B, both powered by renewable energy. From Figure 2A, it is seen the input power is renewable energy, which powers a compression process, and the output are cooling and heating simultaneously. The energy recovered from the cooling process can be transported to a higher temperature and heating can be achieved. No boiler is needed for heating. In this cycle, both cooling and heating can be done using renewable energy sources. No CO2 emission occurs. Instead, CO2 is used as a useful resource. The generated energy is transported by the compressor to simultaneously achieve both cold and heat. As shown in Figure 2, the cooling process from −10°C can be converted to heating process at 80°C, and there is only a small energy loss or even no loss. This is the reason for using carbon dioxide, which can greatly improve the energy utilization efficiency, thus making the process cycle efficient. Furthermore, in the power supply system in Figure 2B, the natural fluid CO2 is heated by renewable energy and then used to supply power. The produced heat at the outlet of the power supply can reach 100°C, or even higher, which can supply sufficient heat to produce heating load or cooling load. In this cycle, there is also no carbon emission, but CO2 is used as a resource. Unique thermodynamic and heat transfer properties of CO2 are promising in many new energy systems. As an ecologically safe and natural refrigerant, CO2 is abundant all over the world, and if it is used to establish the thermodynamic cycle, it has the potential to alleviate energy shortage. It can greatly reduce carbon emission (ozone depleting potential, ODP = 0; global warming potential, GWP = 1) and provide environmental and personal safety. In addition, because of the good heat transfer characteristics, the thermodynamic and transport properties of natural CO2 are very conducive for it to act as a working fluid. This process reduces the lower temperature limit of waste heat recovery and utilization, reduces the unit energy consumption of commercial and industrial products, and contributes to energy conservation and emission reduction. In Figure 3, we can see the P–h diagram of the CO2 refrigeration cycle: the isothermal and isentropic lines of CO2 are also included. The specific data is determined by the required heat, so this figure is only for reference and understanding the characteristics of the thermal cycle. A small cycle shown in the upper part of Figure 3 represents the transcritical CO2 power cycle, and the large cycle represents the CO2 transcritical heating and cooling cycle. The regional or district energy stations will use more types of heat storage, renewable energy, and surplus energy (waste to energy, industrial surplus energy). Power can be obtained from solar energy, wind power, tidal energy, and so on. Trends are changing from a single source to multiple sources, and distributed heating is shifting from from high temperature to low temperature areas. We hope that the power generation part of the proposed energy station will be realized by renewable energy such as CSP Gen3. Thus a lot of energy can be saved through this kind of triple generation system. For an understanding of the thermodynamic cycle of the proposed energy station, one can refer to Figure 4. Research shows that combined cycles with both renewable and nonrenewable energies as the heat source give good performance. Nevertheless, small-scale power generation needs single cycles for high performance and has economic advantages.1 When the expansion work is restored, the performance of the systems with different subcooling methods has no significant difference. The internal heat exchanger is selected as the optimal subcooling mode for winter operation, whereas the cooling tower is recommended as the appropriate subcooling mode in summer.2 Other than that, considering the metal degradation in the equipment used for concentrating solar power (CSP) at relevant temperatures of S-CO2 environment, it has been shown that a nickel-based alloy is a proper structural material for S-CO2 with CSP integration.3 It will be better if we can configure the high-pressure expander with high Pin and low Tin, low-pressure expander with low Pin and high Tin, and low a Tout air cooler.4 A two-stage or multistage compression cycle (high coefficient of performance (COP), low power consumption, eg, DORIN) system with recuperator, high-efficiency heat exchanger, as well as optimal condensing and evaporating temperatures and exhaust pressure can increase the cooling capacity. We need to pay attention to improved water requirements (frosting and corrosion, water content <0.1%), sufficient safety valve (condensing unit), etc. The hybrid cascade refrigeration cycle can give both cooling temperature with high COP and energy efficiency at the same time. It has been shown that the amount of energy loss in the gas cooler, compressor, and ejector accounts for a large percentage of the total energy loss.5 Results has proved that the turbine power generator can completely cover all power demands of the compressor and pump, which indicates that the turbine inlet temperature and condensation temperature have positive effects on heating capacity.6 The annual primary energy consumption (PEC) can be remarkably decreased by using dedicated mechanical subcooling (DMS) transcritical CO2 system for space heating and cooling all year round.7 Although we have laid the above foundation, this technology also faces some difficult challenges. With CO2 as a resource in this cycle, the temperature range of CO2 evaporation is usually 0°C to −50°C, but still some industries (fishing freezers, bio-storages) require lower temperatures. At present, the CO2 thermodynamic cycle technology has made great progress in small-scale units, such as refrigeration and heating in supermarkets or markets, and application in power stations, but larger scale application of this technology has to be developed. The heat source used in most systems is fossil fuels, which wastes large amounts of nonrenewable resources, apart from causing environmental pollution and personal injury. The application of a renewable energy-powered CO2 system in an energy supply station has many advantages, but its industrial application is facing many challenges: for example, challenges in system complexity, technical requirements, high-pressure and high-temperature risks, which also result in the increasing demand on material used for components and equipment, flow dynamics, and heat transfer of near-critical, supercritical, and transcritical CO2. From the basic flow and heat transfer to the equipment inside must be controlled by talented engineers who have been trained in this field and can understand the engineering management procedures. Transcritical CO2 turbine is a special device, but there is also supercritical flow and heat transfer in the turbine, so it is necessary to consider what kind of material will be safer to use. If we use solar energy, it is necessary to optimize the materials to be used in the turbine, which is very tough in the CO2 thermal cycle at temperatures 1000°C to 1500°C. Because heat is sufficient during the day but not at night, intelligient storage problem should be considered too. This system is unlike the conventional energy station, which burns oil or coal in very stable state, because we make use of renewable energy such as solar energy, wind energy, and so on, and there are dynamic problems to be considered and controlled. The heat source, thermodynamic cycle, rectifier voltage regulation control, and system safety protection also need to be paid much attention (Figure 5). By using a lot of CO2 as the resource, carbon emissions will be greatly reduced, and it can even be viewed as negative carbon emission. A natural CO2 energy station is sure to produce huge dividends to society, because it can be used as an alternative to fossil energy, resulting in clean air and a healthier life. The high efficiency of the system can achieve better energy saving, along with providing a lot of cold and hot supplies, even as comprehensive cold and hot at the different temperatures. The successful establishment of such an energy station can also solve many employment problems through increased demand from a new industry. It would help people to train efficiently, and get employed as soon as possible, and engage in an industry in the environmental protection and safety system (Figure 6). This paper presents a new concept of energy and resource application and introduces a unique optimal energy station run by natural CO2 for a healthier human life. A low-carbon ecosystem and better climate will lead to better world safety. In this scheme, CO2 is used as a resource to achieve cooling, heating, and power production without any carbon emission. Thus this natural CO2 energy station is much superior to any existing conventional energy stations. Furthermore, the heat source of the power generation is a renewable, economic, and safe resource. Therefore, we hope that the establishment of such energy stations will be very meaningful to achieve the four major goals of energy conservation, efficiency, safety, and environmental protection. This work was supported by the National Natural Science Foundation of China (No. 51776002) and the Beijing Engineering Research Center of City Heat. Data available on request from the authors.