Combined heat and power systems with dual power generation units and thermal storage

The objective of this paper is to study the performance of a combined heat and power (CHP) system that uses two power generation units (PGU). In addition, the effect of thermal energy storage is evaluated for the proposed dual‐PGU CHP configuration (D‐CHP). Two scenarios are evaluated in this paper. In the first scenario, one PGU operates at base‐loading condition, while the second PGU operates following the electric load. In the second scenario, one PGU operates at base‐loading condition, while the second PGU operates following the thermal load. The D‐CHP system is modeled for the same building in four different locations to account for variation of the electric and thermal loads due to weather data. The D‐CHP system results are compared with the reference building by using conventional technology to determine the benefits of this proposed system in terms of operational cost and carbon dioxide emissions. The D‐CHP system results, with and without thermal storage, are also compared with that of single‐PGU CHP systems operating following the electric load (FEL), following the thermal load (FTL), and base‐loaded (BL). Results indicate that the D‐CHP system operating either FEL or FTL in general provides better results than a single‐PGU CHP system operating FEL, FTL, or BL. The addition of thermal storage enhances the potential benefits from D‐CHP system operation in terms of operational cost savings and emissions savings. Copyright © 2013 John Wiley & Sons, Ltd.     

Performance analysis of CCHP and CHP systems operating following the thermal and electric load

Heating and cooling energy requirements for buildings are usually supplied by separated systems such as furnaces or boilers for heating, and vapor compression systems for cooling. For these types of buildings, the use of combined cooling, heating, and power (CCHP) systems or combined heating and power (CHP) systems are an alternative for energy savings. Different researchers have claimed that the use of CCHP and CHP systems reduces the energy consumption related to transmission and distribution of energy. However, most of these analyses are based on reduction of operating cost without measuring the actual energy use and emissions reduction. The objective of this study is to analyze the performance of CCHP and CHP systems operating following the electric load (FEL) and operating following the thermal load (FTL), based on primary energy consumption (PEC), operation cost, and carbon dioxide emissions (CDE) for different climate conditions. Results show that CCHP and CHP systems operated FTL reduce the PEC for all the evaluated cities. On the other hand, CHP systems operated FEL always increases the PEC. The only operation mode that reduces PEC and CDE while reducing the cost is CHP‐FTL. Copyright © 2009 John Wiley & Sons, Ltd.     

Evaluation of a base‐loaded combined heating and power system with thermal storage for different small building applications

The objective of this paper is to demonstrate the advantages of using a combined heating and power (CHP) system operating at full load to satisfy a fraction of the facility electric load, that is, a base load. In addition, the effect of using thermal storage during the CHP system operation (CHP‐TS) is evaluated. A small office building and a restaurant with the same floor area, in Chicago, IL, and Hartford, CT, were used to evaluate the base‐loaded CHP and CHP‐TS operation based on operational cost, primary energy consumption (PEC), and carbon dioxide emissions (CDEs). Results indicate that, in general, the use of thermal storage is beneficial for the CHP system operation because it reduces cost, PEC, and CDEs compared with a CHP with no thermal storage. The CHP and CHP‐TS operation is more beneficial for a restaurant than for a small office building for the evaluated cities, which clearly indicates the effect of the thermal load on the CHP system performance. Copyright © 2011 John Wiley & Sons, Ltd.     

Evaluation of the performance of combined cooling,heating, and power systems with dual power generation units

The benefits of using a combined cooling, heating, and power system with dual power generation units (D-CCHP) is examined in nine different U.S. locations. One power generation unit (PGU) is operated at base load while the other is operated following the electric load. The waste heat from both PGUs is used for heating and for cooling via an absorption chiller. The D-CCHP configuration is studied for a restaurant benchmark building, and its performance is quantified in terms of operational cost, primary energy consumption (PEC), and carbon dioxide emissions (CDE). Cost spark spread, PEC spark spread, and CDE spark spread are examined as performance indicators for the D-CCHP system. D-CCHP system performance correlates well with spark spreads, with higher spark spreads signifying greater savings through implementation of a D-CCHP system. A new parameter, thermal difference, is introduced to investigate the relative performance of a D-CCHP system compared to a dual PGU combined heat and power system (D-CHP). Thermal difference, together with spark spread, can explain the variation in savings of a D-CCHP system over a D-CHP system for each location. The effect of carbon credits on operational cost savings with respect to the reference case is shown for selected locations.     

Micro-combined cooling,heating and power systems hybrid electric-thermal load following operation

Micro-combined cooling, heating and power (mCCHP), typically designated as less than 30 kW electric, is a technology that generates electricity at or near the place where it is used. The waste heat from the electricity generation can be used for space cooling, space heating, or water heating. The operation of mCCHP systems, while obviously dependent upon the seasonal atmospheric conditions, which determine the building thermal and power demand, is ultimately controlled by the operation strategy. Two of the most common operation strategies are to run the prime mover in accordance to either electrical or thermal demand. In this study, a mCCHP system operating following a hybrid electric-thermal load (FHL) is proposed and investigated. This operation strategy is evaluated and compared with mCCHP systems operating following the electric load (FEL) and operating following the thermal load (FTL). This evaluation and comparison is based on site energy consumption (SEC), primary energy consumption (PEC), operational cost, and carbon dioxide emission reduction (CDE). Results show that mCCHP systems operated following the hybrid electric-thermal load have better performance than mCCHP-FEL and mCCHP-FTL. mCCHP-FHL showed higher reductions of PEC, operational cost, and carbon dioxide emissions than the ones obtained for the other two operation strategies for the evaluated case.     

Cost-effective operating strategy for residential micro-combined heat and power

It is commonly assumed that dispatch of micro-combined heat and power (micro-CHP) should be heat driven, where the unit turns on when a heat load is present, and turns off or modulates when there is little or no heat demand. However, this heat led operating strategy—typical of large-scale CHP applications—may not be economically justified as scale decreases. This article investigates cost-effective operating strategies for three micro-CHP technologies; Stirling engine, gas engine, and solid oxide fuel cell (SOFC), under reasonable estimates of energy prices. The cost of meeting a typical UK residential energy demand is calculated for hypothetical heat led and electricity led operating strategies, and compared with that of an optimal strategy. Using central estimates of price parameters, and with some thermal energy storage present in the system, it is shown that the least cost operating strategy for the three technologies is to follow heat and electricity load during winter months, rather than using either heat demand or electricity demand as the only dispatch signal. Least cost operating strategy varies between technologies in summer months. In terms of environmental outcomes, the least cost operating strategy does not always result in the lowest carbon dioxide emissions. The results obtained are sensitive to electricity buy-back rate.     

A review on energy,economical, and environmental benefits of the use of CHP systems for small commercial buildings for the North American climate

The use of combined heating and power (CHP) systems to produce both electricity and heat is increasing rapidly due to their high potential of reducing primary energy consumption (PEC), cost, and emissions in domestic, commercial, and industrial applications. In addition to producing both electricity and heat, CHP systems can be coupled with vapor compression systems to provide cooling. This paper analyzes a natural gas engine CHP system together with a vapor compression system for different American climate zones. Performance is measured in terms of operational costs, PEC, and carbon dioxide emissions as a percent of a reference building. The objective of this paper is to compare the performance of a CHP system operating 24 h a day with a system that only operates during typical office hours. Furthermore, the system is optimized based on reducing PEC, minimizing costs, and reducing emissions. In addition, the benefits of CHP systems based on the Energy Star program and the Leadership in Energy and Environmental Design (LEED) program are presented. Results show that, in general, it is more beneficial to operate the CHP system during typical office hours than to operate the system 24 h a day. Also, the CHP system performance strongly depends on the location where it is installed. In addition to reductions in cost, primary energy, and emissions, CHP systems can help achieve the Energy Star label for commercial office buildings and help obtain LEED points that go toward achieving LEED certification status. Copyright © 2009 John Wiley & Sons, Ltd.     

MW cogenerated proton exchange membrane fuel cell combined heat and power system design for eco-neighborhoods in North China

Buildings account for most of the greenhouse gas (GHG) emissions causing global warming. The development of eco-neighborhood can improve the energy efficiency of buildings and reduce GHG emissions. A combined heat and power (CHP) system based on proton exchange membrane fuel cells (PEMFCs) is designed to supply electricity and thermal for eco-neighborhood in North China with low GHG emissions. Effects of different inlet parameters, such as PEMFC inlet pressure and current density, on multi-stack CHP system performance are discussed. Coupled with a dynamic load scenario, the adaptability of the designed PEMFC-CHP system is studied through PI control with an electricity-led strategy and a thermal-led strategy. Both strategies can effectively reduce GHG emissions and the eco-neighborhood with PEMFC-CHP system is more environmental friendly compared to conventional energy supply. The electricity-led strategy can satisfy the energy consumption of the eco-neighborhood but with thermal waste. The energy consumption for most of the time during a year can be satisfied by the PEMFC-CHP system under the thermal-led strategy, but the electricity gap exists as the thermal demand is lower. Under the electricity-led strategy, the GHG emission reduction of the eco-neighborhood under electricity-led strategy and thermal-led strategy are around 7000 ton and 5000 ton per year, respectively.     

Methodology to estimate the economic,emissions, and energy benefits from combined heat and power systems based on system component efficiencies

This paper presents a methodology to estimate the economic, emissions, and energy benefits that could be obtained from a base loaded CHP system using screening parameters and system component efficiencies. On the basis of the location of the system and the facility power to heat ratio, the power that must be supplied by a base loaded CHP system in order to potentially achieve cost, emissions, or primary energy savings can be estimated. A base loaded CHP system is analyzed in nine US cities in different climate zones, which differ in both the local electricity generation fuel mix and local electricity prices. Its potential to produce economic, emissions, and energy savings is quantified on the basis of the minimum fraction of the useful heat to the heat recovered by the CHP system (φ_min). The values for φ_min are determined for each location in terms of cost, emissions, and energy. Results indicate that in terms of cost, four of the nine evaluated cities (Houston, San Francisco, Boulder, and Duluth) do not need to use any of the heat recovered by the CHP system to potentially generate cost savings. On the other hand, in cities such as Seattle, around 86% of the recovered heat needs to be used to potentially provide cost savings. In terms of emissions, only Chicago, Boulder, and Duluth are able to reduce emissions without using any of the recovered heat. In terms of primary energy consumption, only Chicago and Duluth do not require the use of any of the recovered heat to yield primary energy savings. For the rest of the evaluated cities, some of the recovered heat must be used in order to reduce the primary energy consumption with respect to the reference case. In addition, the effect of the efficiency of the power generation unit and the facility power to heat ratio on the potential of the CHP system to reduce cost, emissions, and primary energy is investigated, and a graphical method is presented for examining the trade‐offs between power to heat ratio, base loading fraction, percentage of recovered heat used, and minimum ratios for cost, emissions, and primary energy. Copyright © 2014 John Wiley & Sons, Ltd.     

Energy and economic evaluation of cooling,heating, and power systems based on primary energy

Cooling, Heating, and Power (CHP) systems have the potential to make better use of fuels than other technologies because of their ability to increase the overall thermal energy efficiency. Feasibility of CHP systems is generally driven by economic savings. In addition, economic evaluation of CHP systems is based on site energy use and cost, which could lead to misleading conclusions about energy savings. Since energy savings from CHP systems only occurs in primary energy, the objective of this investigation is to demonstrate that feasibility of CHP systems should be performed based on primary energy savings followed by economic considerations. This paper also evaluates the effect of the power generation unit (PGU) efficiency over the primary energy reduction when a CHP system is utilized. The advantages of operating CHP systems under a primary energy operational strategy, such as the proposed Building Primary Energy Ratio (BPER) strategy, are also discussed. Results show that for some cases economic savings are attained without the corresponding primary energy savings. However, the use of the BPER operational strategy guarantees better energy performance regardless of economic savings. Regarding to the PGU efficiency, an increase of the efficiency reduces the primary energy use more than proportionally. For example, increasing the PGU efficiency from 0.25 to 0.35 (increase of 40%) can reduce the primary energy use from 5.4% to 16% (increase of 200%).     

European perspective on absorption cooling in a combined heat and power system – A case study of energy utility and industries in Sweden

Mankind is facing an escalating threat of global warming and there is increasing evidence that this is due to human activity and increased emissions of carbon dioxide. Converting from vapour compression chillers to absorption chillers in a combined heat and power (CHP) system is a measure towards sustainability as electricity consumption is replaced with electricity generation. This electricity produced in Swedish CHP-system will substitute marginally produced electricity and as result lower global emissions of carbon dioxide. The use of absorption chillers is limited in Sweden but the conditions are in fact most favourable. Rising demand of cooling and increasing electricity prices in combination with a surplus of heat during the summer in CHP system makes heat driven cooling extremely interesting in Sweden. In this paper we analyse the most cost-effective technology for cooling by comparing vapour compression chillers with heat driven absorption cooling for a local energy utility with a district cooling network and for industries in a Swedish municipality with CHP. Whilst this case is necessarily local in scope, the results have global relevance showing that when considering higher European electricity prices, and when natural gas is introduced, absorption cooling is the most cost-effective solution for both industries and for the energy supplier. This will result in a resource effective energy system with a possibility to reduce global emissions of CO₂ with 80%, a 300% lower system cost, and a 170% reduction of the cost of producing cooling due to revenues from electricity production. The results also show that, with these prerequisites, a decrease in COP of the absorption chillers will not have a negative impact on the cost-effectiveness of the system, due to increased electricity production.     

Techno-economic analysis of using corn stover to supply heat and power to a corn ethanol plant – Part 2: Cost of heat and power generation systems

This paper presents a techno-economic analysis of corn stover fired process heating (PH) and the combined heat and power (CHP) generation systems for a typical corn ethanol plant (ethanol production capacity of 170 dam³). Discounted cash flow method was used to estimate both the capital and operating costs of each system and compared with the existing natural gas fired heating system. Environmental impact assessment of using corn stover, coal and natural gas in the heat and/or power generation systems was also evaluated. Coal fired process heating (PH) system had the lowest annual operating cost due to the low fuel cost, but had the highest environmental and human toxicity impacts. The proposed combined heat and power (CHP) generation system required about 137 Gg of corn stover to generate 9.5 MW of electricity and 52.3 MW of process heat with an overall CHP efficiency of 83.3%. Stover fired CHP system would generate an annual savings of 3.6 M$ with an payback period of 6 y. Economics of the coal fired CHP system was very attractive compared to the stover fired CHP system due to lower fuel cost. But the greenhouse gas emissions per Mg of fuel for the coal fired CHP system was 32 times higher than that of stover fired CHP system. Corn stover fired heat and power generation system for a corn ethanol plant can improve the net energy balance and add environmental benefits to the corn to ethanol biorefinery.     

Techno-economic analysis of a hybrid energy system for CCHP and hydrogen production based on solar energy

A typical problem in Northeast China is that a large amount of surplus electricity has arisen owing to the serious photovoltaic power curtailment phenomenon. To effectively utilize the excess photovoltaic power, a hybrid energy system is proposed that uses surplus electricity to produce hydrogen in this paper. It combines solar energy, hydrogen production system, and Combined Cooling Heating and Power (CCHP) system to realize cooling, heating, power, and hydrogen generation. The system supplies energy for three public buildings in Dalian City, Liaoning Province, China, and the system configuration with the lowest unit energy cost (0.0615$/kWh) was obtained via optimization. Two comparison strategies were used to evaluate the hybrid energy system in terms of unit energy cost, annual total cost, fossil energy consumption, and carbon dioxide emissions. Subsequently, the annual total energy supply, typical daily loads, and cost of the optimized system were analyzed. In conclusion, the system is feasible for small area public buildings, and provides a solution to solve the phenomenon of photovoltaic power curtailment.     

A high proportion of new energy access–combined cooling,heating, and power system planning method

Distributed generation (DG) technologies are environmentally friendly and have low operating costs, and thus, distributed generators are widely used for the energy supplies of buildings. Solar energy used for on‐demand heating of buildings is also a mature technology that is environmentally friendly and inexpensive and has a short recycling life cycle. This paper proposes a high proportion of new energy access–combined cooling, heating, and power (HPNE‐CCHP) system composed of a distributed generator and solar energy heat pump system. To obtain the optimum capacity of the HPNE‐CCHP system, nondominated sorting genetic algorithm‐II (NSGA‐II) was used to optimize the system by considering the life cycle cost (LCC) and life cycle pollutant emissions (LCPE) as the objective functions. A mixed integer economic scheduling model (MIESM) was proposed to make the HPNE‐CCHP system operate more economically. Finally, an HPNE‐CCHP system was constructed for a building in Northern China. The simulation results show that an HPNE‐CCHP system with a moderate proportion of new energy is more economical and environmentally friendly than a traditional CCHP system. Building occupants, depending on their desired spending, can select the best capacity configuration on the Pareto frontier. Although pollutant emissions will be reduced as the proportion of new energy increases, this type of configuration is expensive.     

Multi-Objective Particle Swarm Optimization(MOPSO) for a Distributed Energy System Integrated with Energy Storage

Distributed energy systems are considered as a promising technology for sustainable development and have become a popular research topic in the areas of building energy systems. This work presents a case study of optimizing an integrated distributed energy system consisting of combined heat and power(CHP), photovoltaics(PV), and electric and/or thermal energy storage for a hospital and large hotel buildings located in Texas and California. First, simulation models for all subsystems, which are developed individually, are integrated together according to a control strategy designed to satisfy both the electric and thermal energy requirements of a building. Subsequently, a multi-objective particle swarm optimization(MOPSO) is employed to obtain an optimal design of each subsystem. The objectives of the optimization are to minimize the simple payback period(PBP) and maximize the reduction of carbon dioxide emissions(RCDE). Finally, the energy performance for the selected building types and locations are analyzed after the optimization. Results indicate that the proposed optimization method could be applied to determine an optimal design of distributed energy systems, which reaches a trade-off between the economic and environmental performance for different buildings. With the presented distributed energy system, a peak shaving in electricity of about 300 kW and a reduction in boiler fuel consumption of 610 kW could be attained for the hospital building located in California for a winter day. For the summer and transition seasons, electricity peak shaving of 800 kW and 600 kW could be achieved, respectively.     

Estimating the power and number of microturbines in small-scale combined heat and power systems

Utilizing the combined heat and power (CHP) systems to produce both electricity and heat is growing rapidly due to their high efficiency and low emissions in domestic, commercial, and industrial applications. In the first two categories among available drivers, due to the compact size and low weight, microturbines are attractive choice. In this paper, by using an energy–economic analysis the type and number of the required microturbines for the specific electricity and heat load curves during a year were selected. For performing this task an objective function annual profit (AP) was introduced and maximized. The operation strategy and the payback period of the chosen system was also determined in this study.     

Integrated energy balance analysis of a stand‐alone wind power system for various typical Aegean Sea regions

The wind power industry is nowadays a mature energy production sector disposing to market commercial wind converters from 50 W up to 5 MW. In the present work the possibility of using stand‐alone electricity production systems based on a small wind turbine in order to meet the electricity requirements of remote consumers is analysed for selected Aegean Sea regions possessing representative wind potential types. The proposed configuration results from an extensive long‐term meteorological data analysis on a no‐load rejection condition basis during the entire time period examined. Accordingly, an integrated energy balance analysis is carried out for the whole time period investigated, including also the system battery depth‐of‐discharge distribution versus time. Finally, the predicted optimum system configuration is compared to other existing technoeconomic alternatives on a simplified total production cost basis. The results support the viability of similar solutions, especially for areas of high or medium wind potential. Copyright © 2002 John Wiley & Sons, Ltd.     

Application of combined heat-and-power and absorption cooling in a supermarket

In recent years, it has become standard practice to consider Combined Heat-and-Power (CHP) systems for commercial buildings. CHP schemes are used, because they are an efficient means of power generation. Unlike conventional power stations, they produce electricity locally and thus minimise the distribution losses, however, they also utilise the waste heat from the generation process. In applications where there is a combined heating and electricity requirement, a very efficient means of energy production is achieved compared to the conventional methods of providing heating and electricity. With new initiatives from the UK government on reduced energy-use, energy-efficient systems such as CHP have been considered for new applications. This paper summarises the results of an investigation into the viability of CHP systems in supermarkets. The viability of conventional CHP has been theoretically investigated using a mathematical model of a typical supermarket. This has demonstrated that a conventional CHP system may be practically applied. It has also been shown that compared to the traditional supermarket design, the proposed CHP system will use slightly less primary energy and the running costs will be significantly reduced. An attractive payback period of approximately 4 years has been calculated. Despite these advantages a considerable quantity of heat is rejected to atmosphere with this system and this is because the configuration utilises the heat mainly for space heating which is only required for part of the year. To increase the utilisation time, a novel CHP/absorption system has been investigated. This configuration provides a continuous demand for the waste heat, which is used to drive an absorption chiller that refrigerates propylene glycol to −10°C for cooling the chilled-food cabinets. The results show this concept to be theoretically practical. The system has also been shown to be extremely efficient, with primary energy savings of approximately 20%, when compared to traditional supermarket designs and this would result in significant revenue cost savings as well as environmental benefits. Based upon these savings a payback period for this system of approximately 5 years has been demonstrated.     

Stockholm CHP potential—An opportunity for CO2 reductions?

The potential for combined heat and power (CHP) generation in Stockholm is large and a total heat demand of about 10 TWh/year can be met in a renewed large district heating system. This model of the Stockholm district heating system shows that CHP generation can increase from 8% in 2004 to 15.5% of the total electricity generation in Sweden. Increased electricity costs in recent years have awakened an interest to invest in new electricity generation. Since renewable alternatives are favoured by green certificates, bio-fuelled CHP is most profitable at low electricity prices. Since heat demand in the district heating network sets the limit for possible electricity generation, a CHP alternative with a high electricity to heat ratio will be more profitable at when electricity prices are high. The efficient energy use in CHP has the potential to contribute to reductions in carbon dioxide emissions in Europe, when they are required and the European electricity market is working perfectly. The potential in Stockholm exceeds Sweden's undertakings under the Kyoto protocol and national reduction goals.     

Primary energy savings through thermal storage in district heating networks

District heating is an efficient way to provide heat to residential, tertiary and industrial users. Heat is often produced by CHP (combined heat and power) plants, usually designed to provide the base thermal load (40-50% of the maximum load) while the rest is provided by boilers. The use of storage tanks would permit to increase the annual operating hours of CHP: heat can be produced when the request is low (for instance during the night), stored and then used when the request is high. The use of boilers results partially reduced and the thermal load diagram is flattered. Depending on the type of CHP plant this may also affect the electricity generation. All these considerations are crucial in the free electricity market.In this paper, a multi-scale model of storage tanks is proposed. This model is particularly suitable to analyze the operation of storage systems during the heating season and to predict their effects on the primary energy consumption and cash flows. The analysis is conducted considering the Turin district heating system as case study. Results show that primary energy consumption can be reduced up to 12%, while total costs can be reduced up to about 5%.