District Heating and Cooling Energy Equilibrium Model
Since many energy policies, e.g. strategies for utilizing new energy technologies, may have long-term economic impacts, many energy-related economic models have been developed to aid in energy planning and decision-making. In one class of energy-related economic models, the effects of energy policies are modelled as shifts in energy market equilibrium positions. Such models employ mathematical programming, and are often referred to as energy equilibrium models.
An energy equilibrium model of a competitive energy market examines the interaction between energy supplies and demands, and determines the optimal levels of production (supply) and consumption (demand) that satisfy the equilibrium property that the prices consumers pay for each commodity should equal the marginal costs of production. In the energy equilibrium model, supply is represented by a cost-minimizing linear submodel and demand by a smooth vector-valued function of prices. Several algorithms exist for the solution of the models, including the well-known Project Independence Evaluation System (PIES) algorithm of Ahn and Hogan.
In multi-period energy equilibrium, environmental impacts can be important factors, and an analyst may wish to introduce environmental measures into the model system. These environmental measures can be used to evaluate environmental impacts over the time horizon of the model for a given energy policy, or can be constrained by applying upper bounds to limit environmental impact. In the latter case, the model can provide the optimal solution of the model system while accounting for environmental impact control, e.g. CO2 emission control.
Cogeneration-based district energy (DE) systems use central cogeneration plants with heating and/or cooling networks to provide electrical and heating and/or cooling services to communities. Such systems often have many advantages over conventional separate systems for electricity, heating and cooling, including increased efficiency, reduced environmental emissions and more economic, safe and reliable operation. Although the number of cogeneration-based DE systems is relatively small at present, the utilization of such systems is growing. One of the main impediments to their wider application is lack of experience with, and understanding of, the behaviour of integrated forms of such systems, which can often be complex and confusing. A larger base of knowledge exists for each of the component technologies when applied independently. The authors feel that the utilization of district energy and cogeneration could be greatly increased if better optimization and analysis tools were made available. The present work is intended to address this need. More specifically, we employ in this paper an energy equilibrium model to study conventional heating, cooling and electricity-generation systems and cogeneration-based DE systems in order to compare the potential economic and environmental benefits of utilizing cogeneration-based DE systems for several scenarios, and to develop the optimal configuration of the model system, considering such factors as economic and environmental impacts.
The energy equilibrium model is set up, formulated and solved within the software called the Waterloo Energy Modelling System (WATEMS), which employs sequential nonlinear programming to calculate a spatial intertemporal equilibrium of energy supply and demand.
Section 2 of this paper discusses cogeneration and district energy, while Section 3 presents the energy equilibrium model and its mathematical formulation. Section 4 explores methodologies for analysis and evaluation. In Section 5, an illustrative case study is presented. Section 6 presents the conclusions.
A cogeneration process involves the simultaneous production of electricity and heat (usually in the form of steam and/or hot water). The main advantage of cogeneration is that less input energy is consumed than would be required to produce the same thermal and electrical products in separate processes. Additional benefits of cogeneration often are reduced environmental emissions (due to reduced energy consumption and the use of modern technologies in large, central installations), and more economic and safe operation. The additional safety associated with cogeneration in part is due to the fact that only one fuel-fired combustion plant is required, compared to the two such plants needed for separate heating and electricity-generation systems. Cogeneration has been used, particularly by industry, for approximately a century, and there are presently over 4000 cogeneration projects. Cogenerated heat can satisfy air- and water-heating demands in the residential, commercial as well as institutional sectors (using on-site cogeneration, or central cogeneration with district heating, and industrial heating needs (e.g. drying, boiling). Cogenerated heat can also provide space cooling via heat-driven absorption chillers.
Cogeneration systems are similar to thermal electricity-generation systems. In most thermal electricity-generation systems, an energy resource (normally a fossil or nuclear fuel but sometimes a renewable energy resource) is converted to heat, of which a portion (normally 20 to 45%) is converted to electricity, and the remainder rejected to the environment as waste heat. In cogeneration systems, depending upon the needs of the customers, part of the generated heat is used for electricity production, part is delivered as product, and waste-heat output is reduced. Cogeneration energy efficiencies (based on both electricity and heat) of over 80% are achievable.
District energy systems (which can include both district heating and district cooling systems) use central heating and/or cooling facilities to provide heating and/or cooling services for communities. In a district-cooling system, a chilled fluid, normally treated water, is supplied from a central chiller plant and transported by pipeline to users of the cooling capacity, then returned for recooling. The chilling plant can utilize electrical chillers or heat-driven absorption chillers. In district heating systems, a similar heating loop with a central heat supply is utilized. The advantages of district energy systems over conventional heating and cooling systems include improved efficiency, reliability and safety, reduced environmental impact, and for many situations better economics. The increased safety associated with DE systems exists because the heating and cooling plants, where problems are likely to originate in the event of accidents, are located at a different site than the user of the heating and cooling, where DE is used. The increased reliability of DE systems is attributable to the fact that many DE users are normally connected through a network having more than one heating and cooling provider; thus in the event of a breakdown at one heating/cooling location, an alternate can usually be started up to avoid service disruptions. District energy systems can be particularly beneficial when integrated with cogeneration systems.
Many integrated systems for cogeneration and district energy are possible. Several selected applications are discussed below:
• A district-heating and cooling system operated by Energy Networks Incorporated since 1962 currently serves over 70% of the buildings in downtown Hartford, Connecticut. A natural gas-fired cogeneration plant, completed in 1990, produces the hot and cold water required for the system, as well as electricity.
• The feasibility of district heating and cooling was assessed for downtown Des Moines, IA, considering a mothballed 210 MW (electric) coal-fired power plant as the source of heat. The system was predicted to break even economically in 20 years, and have a lifetime of 40 years. District cooling, using electrically-driven centrifugal chillers, was estimated to provide cooling at competitive prices. The use of absorption chillers driven by hot water from the district heating system was estimated to be slightly more expensive.
• Edmonton Power proposed for downtown Edmonton, AB a major cogeneration-based district heating and cooling project having (i) an initial supply capacity of 230 MW (thermal) for heating and 100 MW (thermal) for cooling, with the potential to expand to about 400 MW (thermal) for heating over the next 10 years; (ii) the capacity to displace about 15 MW of electric power used for electric chillers through district cooling; and (iii) the potential to increase the efficiency of the Rossdale power plant, which would cogenerate to provide the steam for district heating and cooling, from about 30 to 70%. Electrical chillers were to be used originally, and absorption chillers in the future.
Tags: heating and cooling
