Operational optimisation of a complex trigeneration system connected to a district heating and cooling network

The combined production of electricity, heat and cold by polygeneration systems ensures maximum utilization of resources by reducing emissions and energy losses during distribution. Polygeneration systems are highly integrated systems characterized by the simultaneously production of different services (electricity, heating, cooling) by means of several technologies using fossil and renewable fuels that operates together to obtain a higher efficiency than that of an equivalent conventional system. The high number of distribution technologies available to produce electricity, heating and cooling and the different levels of integration make it difficult to select of the optimal configuration. Moreover, the high variability in the energy demand renders difficult the selection of the optimal operational strategy. Optimization methodologies are usually applied for the selection of the optimal configuration and operation of energy supply systems. This paper presents a scenario analysis using optimization models to perform an economic, energetic and environmental assessment of a new polygeneration system in Cerdanyola del Vallès (Spain) in the framework of the Polycity project of the European Concerto Program. This polygeneration system comprise high-efficiency natural gas cogeneration engines with thermal cooling facilities and it will provide electricity, heating and cooling for a new area in growth known as Alba park including a Synchrotron Light Facility and a Science and Technological park through a district heating and cooling network of four tubes. The results of the scenario analysis show that the polygeneration plant is an efficient way to reduce the primary energy consumption and CO₂ emissions (up to 24%).     

On the potential trade-offs between energy supply and end-use technologies for residential heating

In Sweden, where district heating accounts for a significant share of residential heating, it has been argued that improvements in end-use energy efficiency may be counter-productive since such measures reduce the potential of energy efficient combined heat and power production. In this paper we model how the potential trade-offs between energy supply and end-use technologies depend on climate policy and energy prices. The model optimizes a combination of energy efficiency measures, technologies and fuels for heat supply and district heating extensions over a 50 year period. We ask under what circumstances improved end-use efficiency may be cost-effective in buildings connected to district heating? The answer hinges on the available technologies for electricity production. In a scenario with no alternatives to basic condensing electricity production, high CO₂ prices result in very high electricity prices, high profitability of combined heat and power production, and little incentive to reduce heat demand in buildings with district heating. In contrast, in a scenario where electricity production alternatives with low CO₂ emissions are available, the electricity price will level out at high CO₂ prices. This gives heat prices that increase with the CO₂ price and make end-use efficiency cost-effective also in buildings with district heating.     

Influence of supply and return water temperatures on the energy consumption of a district cooling system

In a district heating and cooling system, for example, the Beijing combined heating, cooling and power (CHCP) system studied here, high temperature water generated by cogeneration plants circulates through a network between the plants and the heating substations. In heating substations, high temperature supply water from the network drives absorption chillers for air-conditioning in the summer, satisfies space heating demands in the winter and provides domestic hot water using heat exchangers throughout the year. This paper studies the significant effect of the parameters, i.e. the supply and return water temperatures in the network, on the CHCP system energy consumption for cooling and for domestic hot water.     

Conversion of individual natural gas to district heating: Geographical studies of supply costs and consequences for the Danish energy system

Replacing individual natural gas heating with district heating based to increasing shares of renewable energy sources may further reduce CO₂-emissions in the Danish Building mass, while increasing flexibility of the energy system to accommodate significantly larger amounts of variable renewable energy production. The present paper describes a geographical study of the potential to expand district heating into areas supplied with natural gas. The study uses a highly detailed spatial database of the built environment, its current and potential future energy demand, its supply technologies and its location relative to energy infrastructure. First, using a spatially explicit economic model, the study calculates the potentials and costs of connection to expanded district heating networks by supply technology. Then a comprehensive energy systems analysis is carried out to model how the new district heat can be supplied from an energy system with higher shares of renewable energy. It can be concluded on the basis of these analyses that the methods used proved highly useful to address issues of geographically dependent energy supply; however the spatio-economic model still is rather crude. The analyses suggest to expand district heating from present 46% to somewhere in between 50% and 70%. The most attractive potential is located around towns and cities. The study also suggests that CO₂-emissions, fuel consumption and socio-economic costs can be reduced by expanding district heating, while at the same time investing in energy savings in the building mass as well as increased district heating network efficiency.     

Effect of saturation‐temperature on the performance of a vapour‐compression refrigeration‐cycle working on different refrigerants using exergy method

In this study, the behaviour of a vapour‐compression refrigeration cycle, for different refrigerants such as NH₃, R‐12, R‐22 and HFC‐134a was investigated using the exergy method. The cooling load of the plant and the saturation‐temperature of the cold chamber were held constant, whereas the saturation‐temperatures of the evaporator and the condenser were varied from 303 to 313 K and 258 to 248 K, respectively. The irreversibility rates (or exergy destruction rates) of sub‐regions for the whole cycle, using energy and exergy analysis, were determined for each refrigerant. The effects of changes in the saturation‐temperature in the condenser and evaporator on the irreversibility rate of the cycle were obtained for each refrigerant. The relations between the total irreversibility rate of the plant and the irreversibility rate of the condenser and the evaporator were determined for different values of saturation temperatures of the condenser and the evaporator. The COP of the cycle and the rational efficiency were determined for each of the refrigerants and compared with each other. Among the refrigerants used, R‐12 was found to be the most economical refrigerant as compared with the others. Copyright © 2005 John Wiley & Sons, Ltd.     

Review of carbon dioxide (CO2) based heating and cooling technologies: Past,present, and future outlook

Refrigerants bearing high global warming potential (GWP) and ozone depletion potential (ODP) were outlawed or facing time-beared permission under the Montreal (1987), Kyoto (1997) protocols, F-Gas law (2015), Paris Accord (2016), and recent Kigali Amendment to Montreal Protocol (2019). In order to modify followed by the paradigm shift of existing heating and cooling systems, American Society of Heating, Refrigeration, and Air-Conditioning Engineers (ASHRAE) envisaged natural and synthetic refrigerants (Synrefs) are under investigation globally. Carbon dioxide (CO₂) is a popular natural refrigerant (Natref) replacing Synrefs used in commercial heating, ventilation, air conditioning, and refrigeration (HVAC&R) systems globally. A rampant rise is observed in markets of Asia (Japan and China), North America (the United States and Canada), the Australian continent, and Africa (South Africa) by reaching 20 000 CO₂-stores around the globe. The European markets are leading the utilization of CO₂ in the heat pump, and refrigeration, CO₂ based markets estimate 14% of the total food retail stores (400 m²). Japan is the second leading market of CO₂ heat pump and refrigeration with more than 10 200 CO₂ condensing units. CO₂ transcritical systems have a share of more than 10% in only European market (large stores); however, their share is less than 10% of total stores in other major markets of the world. New pump and compressor-driven transcritical CO₂ systems integrate ejectors, condensers, and booster systems to reduce energy consumptions, enhance efficiency, efficacy, and coefficient of performance. This article reports a critical review of the CO₂ based heating, cooling, and refrigeration system and presents updated literature along with barriers and challenges on commercial use of Natref-based heating and cooling applications worldwide.     

Preliminary investigation of a transcritical CO2 heat pump driven by a solar‐powered CO2 Rankine cycle

In this paper, a transcritical carbon dioxide heat pump system driven by solar‐owered CO₂ Rankine cycle is proposed for simultaneous heating and cooling applications. Based on the first and second laws of thermodynamics, a theoretical analysis on the performance characteristic is carried out for this solar‐powered heat pump cycle using CO₂ as working fluid. Further, the effects of the governing parameters on the performance such as coefficient of performance (COP) and the system exergy destruction rate are investigated numerically. With the simulation results, it is found that, the cooling COP for the transcritical CO₂ heat pump syatem is somewhat above 0.3 and the heating COP is above 0.9. It is also concluded that, the performance of the combined transcritical CO₂ heat pump system can be significantly improved based on the optimized governing parameters, such as solar radiation, solar collector efficient area, the heat transfer area and the inlet water temperature of heat exchange components, and the CO₂ flow rate of two sub‐cycles. Where, the cooling capacity, heating capacity, and exergy destruction rate are found to increase with solar radiation, but the COPs of combined system are decreased with it. Furthermore, in terms of improvement in COPs and reduction in system exergy destruction at the same time, it is more effective to employ a large heat transfer area of heat exchange components in the combined heat pump system. Copyright © 2012 John Wiley & Sons, Ltd.     

Applications of artificial neural networks for refrigeration, air-conditioning and heat pump systems—A review

In this paper, an attempt has been made to review the applications of artificial neural networks (ANN) for energy and exergy analysis of refrigeration, air conditioning and heat pump (RACHP) systems. The studies reported are categorized into eight groups as follows: (i) vapour compression systems (ii) RACHP systems components, (iii) vapour absorption systems, (iv) prediction of refrigerant properties (v) control of RACHP systems, (vi) phase change characteristics of refrigerants, (vii) heat ventilation air conditioning (HVAC) systems and (viii) other special purpose heating and cooling applications. More than 90 published articles in this area are reviewed. Additionally, the limitations with ANN models are highlighted. This paper concludes that ANN can be successfully applied in the field of RACHP systems with acceptable accuracy.     

Exergetic comparison of wind energy storage with ice making cycle versus mini-hydrogen economy cycle in off-grid district cooling

For the feasible and continuous utilization of intermittent wind and solar energy sources for electricity generation in district energy systems in hot-climates, where cooling loads are dominant, ice storage may be an option. In this study, the rationality of the ice storage system for wind energy was investigated using the Rational Exergy Management Model, REMM for two options and compared with a base scenario, which comprises a wind turbine system, grid connection, conventional chillers, and the district cooling system. The main objective is to minimize exergy destructions and thus to improve the exergy performance. The first ice storage option is composed of wind turbines, deep chillers for ice making, ice storage tanks, and the district cooling system. The second option is similar to the first option but it also includes a ground-source heat pump upstream the deep chiller. These options were also compared against a mini-hydrogen economy (District size) alternative, which encompasses a hydrogen-water cycle with excess renewable energy-powered PEM electrolysis unit, hydrogen tank, fuel cell, absorption chiller, gas compression chiller, and the district cooling system. These two options and the hydrogen-water cycle alternative were compared in terms of their REMM efficiency, First and Second-law efficiencies, and the primary energy ratio. A new Sustainability Performance Index, namely SPI was also defined. SPI is the product of the REMM efficiency, First-Law Efficiency, and the load coincidence factor, CF of wind energy. In order to establish a realistic application background for the comparisons, first a nearly-net-zero exergy farmland (nZEXF) utilizing biogas, cogeneration, solar photovoltaics, heat, absorption cycle, ground-source heat pump, Organic Rankine Cycle, and wind turbines was introduced as a model. The primary objective of this study is to determine the best option with the least avoidable CO₂ emissions responsibility of the systems considered in terms of the REMM efficiency in thermal or hydrogen storage systems. Results have been compared in terms of SPI with the base scenario and it has been concluded that the second option (SPI = 0.88) is better than the first option (SPI = 0.38). However, hydrogen storage is an even better alternative with an SPI value of 1.06. These figures according to REMM with the coincidence factor being considered, mean that the avoidable CO₂ emissions may be reduced by up to 54% compared to the base case. Hydrogen cycle option may also be used with the same effectiveness in district heating, while ice storage options are limited to district cooling only. This paper provides the relevant theory, shows the fundamental calculations about the option rankings based on a unit cooling load, makes recommendations for future district energy systems, and refers to a conceptual hydrogen economy driven city.     

Green energies and the environment

Globally, buildings are responsible for approximately 40% of the total world annual energy consumption. Most of this energy is for the provision of lighting, heating, cooling, and air conditioning. Increasing awareness of the environmental impact of CO₂ and NO_x emissions and CFCs triggered a renewed interest in environmentally friendly cooling, and heating technologies. Under the 1997 Montreal Protocol, governments agreed to phase out chemicals used as refrigerants that have the potential to destroy stratospheric ozone. It was therefore considered desirable to reduce energy consumption and decrease the rate of depletion of world energy reserves and pollution of the environment. This article discusses a comprehensive review of energy sources, environment and sustainable development. This includes all the renewable energy technologies, energy efficiency systems, energy conservation scenarios, energy savings and other mitigation measures necessary to reduce climate change.     

Cooling System Design

Cooling water systems and refrigeration systems are required to provide cold utility to chemical processes. Little attention has been placed on the structure and system interactions between cooling systems and processes. In this article, an integrated design and retrofit method for cooling systems is reviewed. For cooling water system design, a new design methodology has been developed for debottlenecking of cooling water systems. Reuse of cooling water between different cooling duties enables cooling water networks to be designed with series arrangements. This allows better cooling tower performance and increased cooling tower capacity. The interactions between the performance of the cooling tower and the design of cooling water networks are explored systematically. For refrigeration system design, a systematic design and retrofit method has been proposed for refrigeration systems using pure or mixed refrigerants. A graphical exergy analysis method, the z -H diagram, is introduced to help engineers obtain understanding and insights into complex refrigeration systems. The systematic synthesis method proposed in this article can greatly improve operating efficiency for refrigerant systems.     

Proposal and thermodynamic performance study of a novel LNG-fueled SOFC-HAT-CCHP system with near-zero CO2 emissions

The cold energy in many liquefied natural gas (LNG) satellite stations is directly carried away by air or seawater. This causes cold energy waste and environmental cold pollution. To solve this problem, a combined power, heating and cooling system (CCHP) driven by LNG is established based on solid oxide fuel cell (SOFC) and humid air turbine (HAT), namely SOFC-HAT-CCHP system, in which, not only can the waste cold energy cool compressor inlet air to decrease power consumption, but supply cold energy for the cold storage and CO₂ recovery. Based on FORTRAN and Aspen Plus, the thermodynamic performance calculation models and the simulation work of the new system are carried out, such as the exergy and energy analysis, as well as the effects of the selected important variables. The results indicate that total exergy efficiency and total power efficiency are 64.7% and 54.4%, and the total thermal efficiency is 79.1%. Besides, the capture rate and purity of the CO₂ are 98.7% and 98.9% respectively. The novel system is environmental protective, energy-saving and efficient, which may provide a new direction to reasonably utilize the waste cold energy in LNG satellite stations.     

Development and comparison of two expander cycles used in refrigeration system of olefin plant based on exergy analysis

In this study, two typical types of low temperature expander cycles for cold section of olefin plant are designed and simulated. In addition, the best refrigerant is selected as the working fluid and the matching of the heating and cooling curves in heat exchangers is also analyzed. Furthermore, the second law efficiency is calculated for systems, as well as the exergy destruction and exergy efficiency for the system components, leading to determination of sources of irreversibility. The results show that the expander cycle with one cooling stage surpasses the expander cycle with two cooling stages. The main irreversibility exists in the expander due to its large pressure difference. Eventually, the significant parameters of the expander cycle with one cooling stage are optimized and the best cycle for the separation system is introduced. The net power of the expander cycle with one cooling stage is 3549 kW, the flow rate is 47.35 kg/s and the overall exergy efficiency is 50.18% for the considered cycle in this research.     

Regional energy system optimization – Potential for a regional heat market

Energy supply companies and industrial plants are likely to face new situations due to, for example, the introduction of new energy legislation, increased fuel prices and increased environmental awareness. These new prerequisites provide companies with new challenges but also new possibilities from which to benefit. Increased energy efficiency within companies and increased cooperation between different operators are two alternatives to meet the new conditions. A region characterized by a high density of energy-intensive processes is used in this study to find the economic potential of connecting three industrial plants and four energy companies, within three local district heating systems, to a regional heat market, in which different operators provide heat to a joint district heating grid. Also, different investment alternatives are studied. The results show that the economical potential for a heat market amounts to between 5 and 26 million EUR/year with payback times ranging from two to eleven years. However, the investment costs and the net benefit for the total system need to be allotted to the different operators, as they benefit economically to different extents from the introduction of a heat market. It is also shown that the emissions of CO₂ from the joint system would decrease compared to separate operation of the systems. However, the valuation of CO₂ emissions from electricity production is important as the difference of emitted CO₂ between the accounting methods exceeds 650 kton/year for some scenarios.     

System analysis in a European perspective of new industrial cooling supply in a CHP system

In the municipality of Södertälje two large industries use much of the electricity, district heating (DH) and chilled water in the area. The Södertälje energy system is not isolated, however, but is connected to the DH systems of southern and central Stockholm, and a change in the Södertälje energy system will also influence the connected energy systems in Stockholm. The cooling demand in Södertälje is currently covered by lake water cooling and compression chillers, but in order to reduce the use of electricity, conversion to absorption cooling or increased lake water cooling can be considered. The large combined heat and power (CHP) plant in Södertälje is not used to its full potential today, but investment in absorption cooling and/or a cold condenser unit integrated with the CHP plant could increase the plant’s operation hours. In this paper the system effects of introducing new industrial cooling supply in Södertälje has been investigated through optimizations of a model including both the industries and the district heating supply in Södertälje and Stockholm. The results show that, independently of whether condensing power production is feasible in the CHP plant or not, investments in both increased lake water cooling and absorption cooling are profitable. A sensitivity analysis of how energy market prices affect the results shows that even though the system cost will change depending on energy market prices, the optimum cooling technology mix will remain the same. However, a sensitivity analysis of the transfer DH capacity between the Södertälje and Stockholm energy systems shows that if the transfer DH capacity is increased, absorption cooling will be less profitable since more heat can be sold from Södertälje to Stockholm while at the same time reducing the use of fuel resources.     

集成太阳能辅助供热的600 MW高背压热电联产机组的运行及优化

为了解决太阳能辅助燃煤发电系统与太阳能辅助抽汽供热系统的余热损失增多问题,提出一种太阳能辅助高背压热电联产系统的优化改造方案。利用EBSILON软件采用数值模拟的方法,对改造前后机组的整体性能和改造后机组在不同发电功率、背压、热网供回水温度工况下的收益变化进行了分析,对比了改造前后机组的(火用)效率与经济性差异。结果表明：改造后的太阳能辅助高背压热电联产系统节煤更多,机组回收了集成太阳能产生的余热后供热能力增强;改造后的机组在低发电功率、较高背压和较低热网供回水温度时节煤更多;在低发电功率、较低背压及较高热网供回水温度时机组的供热能力更强;改造后的太阳能辅助高背压热电联产系统(火用)效率更高、系统的经济效益更佳。     

A district heating system based on absorption heat exchange with CHP systems

In order to decrease the energy consumption of large-scale district heating systems with cogeneration, a district heating system is presented in this paper based on absorption heat exchange in the cogeneration system named Co-ah cycle, which means that the cogeneration system is based on absorption heat exchange. In substations of the heating system, the temperature of return water of primary heat network is reduced to about 25°C through the absorption heat-exchange units. In the thermal station of the cogeneration plant, return water is heated orderly by the exhaust steam in the condenser, the absorption heat pumps, and the peak load heater. Compared with traditional heating systems, this system runs with a greater circuit temperature drop so that the delivery capacity of the heat network increases dramatically. Moreover, by recovering the exhausted heat from the condensers, the capacity of the district heating system and the energy efficiency of the combined heat and power system (CHP system) are highly developed. Therefore, high energy and economic efficiency can be obtained.     

Heat Roadmap Europe: Combining district heating with heat savings to decarbonise the EU energy system

Six different strategies have recently been proposed for the European Union (EU) energy system in the European Commission's report, Energy Roadmap 2050. The objective for these strategies is to identify how the EU can reach its target of an 80% reduction in annual greenhouse gas emissions in 2050 compared to 1990 levels. None of these scenarios involve the large-scale implementation of district heating, but instead they focus on the electrification of the heating sector (primarily using heat pumps) and/or the large-scale implementation of electricity and heat savings. In this paper, the potential for district heating in the EU between now and 2050 is identified, based on extensive and detailed mapping of the EU heat demand and various supply options. Subsequently, a new ‘district heating plus heat savings’ scenario is technically and economically assessed from an energy systems perspective. The results indicate that with district heating, the EU energy system will be able to achieve the same reductions in primary energy supply and carbon dioxide emissions as the existing alternatives proposed. However, with district heating these goals can be achieved at a lower cost, with heating and cooling costs reduced by approximately 15%.     

Experimental investigation of a multi-function heat pump under various operating modes

Heat pumps have been spotlighted as efficient building energy systems because they have great potentials for energy reduction in building air conditioning and reducing CO₂ emission. In this study, a multi-function heat pump which has the functions of heating, cooling, and hot water supply was designed and its performance was investigated according to operating modes. In the cooling-hot water mode, the capacity and COP were enhanced as compared to other modes because the waste heat from the outdoor heat exchanger was utilized as useful heat in the indoor heat exchanger. In the heating and hot water supply mode, the compressor speed should be increased to get appropriate heating and hot water capacities. For all operating modes, the system could be optimized by adjusting the superheat.     

A district heating system based on absorption heat exchange with CHP systems

In order to decrease the energy consumption of large-scale district heating systems with cogeneration, a district heating system is presented in this paper based on absorption heat exchange in the cogeneration system named Co-ah cycle, which means that the cogeneration system is based on absorption heat exchange. In substations of the heating system, the temperature of return water of primary heat network is reduced to about 25°C through the absorption heat-exchange units. In the thermal station of the cogeneration plant, return water is heated orderly by the exhaust steam in the condenser, the absorption heat pumps, and the peak load heater. Compared with traditional heating systems, this system runs with a greater circuit temperature drop so that the delivery capacity of the heat network increases dramatically. Moreover, by recovering the exhausted heat from the condensers, the capacity of the district heating system and the energy efficiency of the combined heat and power system (CHP system) are highly developed. Therefore, high energy and economic efficiency can be obtained.