Combined Heat and Power Systems Introduction: Cogeneration and fuel cells Cogeneration projects include the following types of energy system:
In this project for small scale systems we consider provision of electrical power and heat (CHP) by a simple cycle using waste heat recovery from the electricity generator. Export of electricity to a national grid is not considered. The fundamental characteristic of fuel cells is their high electrical efficiency which can also be realised at low power. High electrical efficiency generally results in lower heat outputs and lower heat to electrical power ratios than combustion engine systems. Thus fuel cell systems should be regarded primarily as efficient electricity generators. However, the possibility of locating fuel cell systems at the point of use enables the waste heat to be utilised more effectively. Trigeneration includes provision of electrical power, heating, and cooling; the later being provided using mechanical output from the electricity generator prime mover to drive a heat pump compressor directly. Alternatively, excess heat emitted from the prime mover may be used to drive absorption refrigerators. Neither of these methods are applicable with small scale fuel cell systems but the high electrical efficiency of fuel cells does increase the efficiency of using electric motors and conventional heat pumps. |
Index of technical reviews |
Small scale combined heat and power projects
Small scale CHP schemes have been defined as those supplying less than 1 MW of electrical power:
Currently more than 80% of CHP schemes are large scale of greater than 10 MWe but small scale CHP is anticipated to be a major growth area including:
Commercial building and community applications in the 200 kWe to 1 MWe range are estimated to be 42% of the potential CHP market and use one third of the total energy in the US.
Early markets are likely to include:
Fuel cell systems are especially beneficial for locations in sensitive areas as they avoid air quality problems, are quiet, and may be installed unobtrusively inside a building.
Commercial pressures for reduced costs; and market, financial, and legislative pressures for improved environmental performance should encourage penetration of these markets and longer term future markets. However, technical and financial innovation will be required for the latter. Partnerships between the various stakeholders and professional disciplines are prerequisites, and maximum benefit will be achieved by integrating project management and sustainable development to meet human and organisations needs.
These comments apply especially to fuel cell systems due to their imminent introduction to the market, the expected reduction in costs with technical innovation and high production quantities, and their advantages in terms of environmental performance, efficiency, reliability, and flexibility of installation.
The stages of a CHP project include:
Small scale packaged CHP systems may be designed, installed, and serviced under a single contract. The energy service provider funds the provision and installation of the system, and recovers the investment from the energy savings over the agreed operating life of the installation.
This project provides methods for completion of the preliminary assessment of a fuel cell based CHP system, and a basis for the detailed feasibility study.
Specification of application requirements
Local energy applications may include process or community applications in developed or developing countries, urban or rural communities. Community applications involve many factors and will often have more complex loads than industrial processes. Building systems may be for new buildings or for building refurbishment, the latter providing a far greater market.
The application requirements are identified including:
Indicative values for typical applications are given below:
| Requirements /application |
Power range kWe |
Fuel | Grade of heat | Heat to power ratio |
| Domestic dwelling | 0.5 -5kW | Natural gas Propane |
Space heating 80C | 2:1 |
| District or community | 30kW-8MW |
Natural gas Gas oil Biogas Liquid biofuels |
Medium hot water
90-120C Steam heating 170-215C |
2:1 |
| Commercial or public building | 30-1000kW | Natural gas Gas oil Biogas |
Space heating 80C Medium hot water 90-120C |
2:1 |
| Hospitals | 30kW-5MW |
Natural gas | Space heating 80C Medium hot water 90-120C Steam heating 170-215C |
2:1 |
| Laboratory | 30-1000kW | Natural gas | Space heating 80C | 2:1 |
| Waste water treatment | 30-500kW | Natural gas Gas oil Biogas |
Anaerobic
digestion 30-40C Pathogen kill >70C |
|
| Industrial process | >30kW | Natural gas Process by- products |
Steam heat 170-215C Hot air drying 300C |
3:1 to >10:1 |
General requirements for typical applications
Before specifying the CHP system the scope for energy conservation should be considered. The grades of heat used should be minimised wherever possible. Heat may be distributed using water, oil, or steam circuits. The latter requires high grade heat but may be required for existing heat distribution networks or process applications, and is required for absorption cooling.
A possible development of this project would be to design an overall assessment method to aid initial design and assessment of fuel cell based CHP systems.
Preliminary assessment
A preliminary technical and economic assessment is undertaken to establish the suitability of a CHP system for the application, and assess it's economic viability. The scope includes:
Feasibility study
In the feasibility study detailed investigation of the nature and development of the site; the suitability of CHP systems for the site infrastructure; and the fuel, electricity, and mains gas supplies are undertaken. Alternative supply infrastructure options and alternative prime mover options are considered. The CHP system is sized against current and future energy needs, and annual and daily load profiles. Local and possibly global environmental impacts are assessed. Long term running costs are assessed, and a sensitivity analysis undertaken, typically for a ten year period. The following are undertaken:
Important parameters which determine the viability and sizing of the plant are given below.
| Electrical power, heat demand, or cooling loads | Source |
| Base load | Existing load profile, floor area, or production units |
| Peak load | Existing load profile, load plant capacities and use |
| Peak load : base load ratio | From above |
| Operating hours per year | Existing load profile, building use, hours of production |
| Load factor | Existing load profile, or from above |
| Start - stop operation | Existing load profile, building use, hours of production |
Energy demand parameters to be included in a design requirement specification
Major technical, economic, and environmental factors are
associated with the availability, quality, and value of the fuel
used:
The fuel availability may limit the size of the CHP system or dual fuel systems may be used. Fuel cells may use a variety of fuels depending on the type of fuel cell and fuel reforming process.
A critical consideration for small scale CHP systems is the dispersion of the locations where heat is required, which will determine the requirements for a district heating grid. The location of fuel processing and prime mover heat sources relative to the heat demand is critical due to:
Fuel cell systems offer significant advantages due to their scale-ability and flexibility in siting. Multiple fuel cell CHP units can be sited wherever the heat is required without increasing the cost/kW of the fuel cell systems significantly, but reducing the costs of distributing the heat.
Project approval
Project approvals are sought before, during, and after the feasibility study and may include:
CHP system design and optimisation
The potential advantages of distributed electricity generation and local CHP systems compared to centralised electricity generation are:
Electricity is the most valuable form of energy but electricity generation uses the most primary fuels and the most costly plant. Therefore, maximising the electricity generation efficiency is most important. This is achieved by selecting the most efficient type and size of prime mover for the fuel available and the electricity load profile. Fuel cells offer maximum efficiency using a variety of fuels over a wide load range, including high efficiency at partial loads.
The electrical generation efficiency of conventional combustion engine systems may be lower than that of centralised generation and transmission, and in this case the CHP system is justified and sized on the basis of matching the large quantity of waste heat available to the heat demand.
Hence the choice of CHP prime mover is determined in conventional systems by the:
The most economic type of prime mover will generally have an equal or lower output heat to power ratio than the site heat to power ratio.
Maximising waste heat recovery is important for all types of system, but heat may be generated efficiently and at low cost locally by direct combustion if there is a shortfall of waste heat. Maximum heat recovery is achieved by locating heat sources such as high temperature fuel processing and prime movers at the required location of the heat supply. However, this may not be possible due to:
Fuel cell systems offer major improvements in all these areas compared with combustion engine prime movers.
In larger systems with suitable prime movers, supplementary firing may be justified, which makes use of high temperature exhaust oxygen for combustion. This is a very efficient means of providing additional high grade heat, the efficiency of combustion being 88% compared to 80% for a conventional natural gas boiler. This may be applicable to large scale high temperature fuel cell systems.
Composite boilers comprise an unfired waste heat boiler and a conventionally fired boiler, feeding a common heat exchanger to transfer heat to the water or steam distribution medium. These are of lower capital cost than supplementary firing and are more applicable to small scale systems.
CHP systems sizing
CHP systems for buildings have usually been sized to suit heat loads due to the low efficiency of the prime movers and higher output heat to power ratio than the site heat to power ratio, but this may not be the best method in the future.
The minimum base load is usually used to determine the capacity of the CHP system which reduces the size and hence the cost of the plant, and allows continuous full load operation. Since neither an excess of heat or electricity are ever generated, high overall efficiencies and financial rates of return are maintained.
However, these methods do not make best use, or maximise the environmental benefits, of high efficiency prime movers which can generate electricity more efficiently and at lower cost than large centralised plants with transmission losses. In this case the system size should be based on the electricity demand above base load.
The following recommendations are a development of traditional practice to suit the higher efficiencies of emerging CHP prime mover technologies, such as fuel cells, and would be used in an assessment method.
Supplementary burners are provided in addition to all types of prime mover, to provide make-up heat or to provide heat when on-site electricity generation is not required, or is unavailable due to prime mover maintenance. It is preferable to connect these in parallel with the prime movers.
When the system is sized above base load the operating hours when the system produces an effective output can be plotted against the system capacity for a given load profile and type of prime mover. This enables a decision to be made on the best compromise between maximising the long term benefits and minimising the capital cost. This approach is especially applicable to fuel cell systems because of their high efficiency at partial loads, but high capital costs. The minimum required operating hours for economic viability range from 11 hours/day or 4000 hours/year to 17 hours/day or 6000 hours/year.
However, the peak heat and power demands must coincide time wise for the CHP system to make use of the waste heat, if the heat output then exceeds the base load.
Emerging technologies
The technical characteristics of alternative prime movers for small scale CHP systems are given below.
| Prime mover | Power range kWe |
Fuel | Grade of heat | Heat to power ratio | Full load electrical efficiency | Combined heat & power efficiency | Part load electrical generating efficiency | Start-stop operation |
| Diesel engines | >15 | Gas oil Biodiesel |
80C/400C |
0.5:1-1.5:1 | 40% |
70% |
30% at half load | Yes |
| Gas engines | >15 | Natural gas Biogas |
80C/400C |
1:1-1.7:1 | 35% |
80% |
30% at half load |
Yes |
| Micro turbines | 20-200 | Natural gas Biogas Alcohols Petroleums |
300C-500C | 1.7:1 | 30% |
80% |
Low below 70% full load | Yes |
| Stirling engines | 1-30 | Natural gas Biofuels Hydrogen Alcohols Petroleums Solar |
80C-200C | 2:1 | 30% |
80% |
30% at half load | Yes |
| Fuel cells Low temp |
PEM any PAFC >100 |
Hydrogen Methane Methanol Ethanol |
PEM 80C PAFC 80-120C |
PEM 1.5:1 PAFC 1:1-0.5:1 |
PEM 35%+ PAFC 40% |
80% |
Inverter reduced below 40% full load | PEM Yes PAFC No |
| Fuel cells High temp |
>1 | Hydrogen Methane Methanol Ethanol Petroleums |
650C-1000C | <1:1 | 50-60% | 85% |
Inverter reduced below 40% full load | No |
Alternative prime movers for small scale CHP
Technical attributes of fuel cell systems
More details of fuel cells are given on our fuel cells page and in other technical reviews. A summary of the main technical, environmental, and economic attributes for small scale CHP systems is given here.
As with the other CHP prime movers, fuel cells can make use of cheap natural gas or biogas as a fuel. However, their high efficiency means that they can utilise biofuels which have limited availability more effectively. They can also use liquid biofuels.
Comparison
of prime mover power applications and efficiencies
Courtesy American Electric Power
As stated above, fuel cells have high electrical generation efficiencies extending down to low partial loads. The efficiency of the fuel cell itself is actually higher at low loads but the overall efficiency is limited by the power conditioner at loads of less than 40% full load. This gives high flexibility to meet varying electricity demands, the main constraint being the high capital cost of the fuel cell which is exacerbated if it operated at a low load factor.
The low output heat to power ratio means that more of the heat demand is met by direct combustion of the fuel. This provides flexibility to meet varying heat demands and uses low cost equipment. The same burner can be used to supply make-up heat and preheat the fuel at start up of the fuel cell.
Improvements in the electrical generation efficiency of PEM fuel cells may be expected with the extensive development for automobile applications. Because the high temperature fuel cells are more efficient at generating electricity than low temperature fuel cells, they produce a smaller quantity of heat even though it is at a higher grade. Thus there is a trade off between the quantity and grade of heat which applies between different fuel cell types and can apply to different cooling regimes for the same type of fuel cell.
Maximum heat recovery can be achieved with fuel cell systems by locating high temperature fuel reforming and the heat of the fuel cell reaction at the required location of the heat demand. Because the installed costs and running costs of fuel cell systems will be almost independent of size, a large number of small units may be installed exactly where the heat is required. This will increase the effective heat recovery and reduce costs of heat transfer and loss reduction measures significantly.
Fuel cell systems are quiet with very low emissions (see our environment page); are fully modular or scaleable, and are compact. They are therefore unobtrusive which gives flexibility for siting inside buildings or externally.
Installation and operation requirements
Installation requirements for a CHP system depend on the site but will include:
Fuel cell systems are compact, quiet, and give low emissions. The installation requirements will be similar to those of a boiler with the addition of the electrical connection.
The installation of biofuel systems will require a digester, fermentation vessel, or gasification plant; and fuel cleanup or processing plant for fuel cell systems, which will be a significant cost.
The main operational requirement is an economic and secure supply of fuel, and a reliable CHP system. The security of electricity supply required, and the reliability and availability of the prime movers, will determine the necessity for standby power, whether from the network or a standby generators.
Other important requirements are:
Although fuel cells are a new technology, they have no moving parts, are very reliable, and should require minimal maintenance. The main maintenance requirements will be scheduled inspections, and maintenance of the fuel processors (reformers and desulphurisers for biofuels), and other ancillaries. The most likely cause of faults will be thermal cycling, or failure of the fuel processing equipment allowing impure fuel to poison the fuel cell catalysts. Servicing personnel may need to be qualified to work on hydrogen systems.
CHP schemes are typically designed for a life of 25 years. The life of fuel cells may be limited by the materials specific to the fuel cell type or thermal cycling. It is therefore important to select a suitable type of fuel cell for the duty cycle, and allow for any periodic replacement costs.
Environmental Impact Assessment
Sustainable energy needs are considered in a separate review. Integration of sustainable development with project management, and global environmental aspects for local energy systems are considered on our Environment page.
The main local considerations are emissions to atmosphere, noise, safety, and visual impact. Fuel cells provide excellent performance under all these aspects, as described on our Environment page.
Economic assessment
The economic assessment process is considered in detail in our review on Fuel Cell Economics.
Installed capital costs include:
| Typical
installed costs for small scale CHP systems are shown
here. The installed cost/kWe of gas or diesel engine systems increase significantly at low power. The figures typically show a total installed cost of 2x the "system" cost which is 1.5x the "packaged" cost. Provision of the site infrastructure such as the heat distribution system and an engine room is likely to be expensive in terms of design time and construction. The total capital cost of combustion engine CHP systems will therefore be more than 3x the equipment package cost. Total design costs for local energy projects are likely to be one third to one half of the total project costs, so if design time is included the factor will be increased to well above 4x the equipment package cost. |
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Stirling engines are of potentially lower cost than gas or diesel engines, and the micro turbines currently available have equipment costs of the order of twice the cost of gas engines. There is insufficient experience of installing these systems to give the installed costs. Neither of these technologies give as high electricity generating efficiency, or as low heat to power ratios as the best diesel or gas engines and fuel cells.
The Phosphoric Acid Fuel Cell (PAFC) system, which has been commercially available since 1996, has been installed at a number of landfill gas and waste water treatment demonstration sites for £5000/kWe, of which the system cost was over £4,500/kWe. This cost is expected to reduce to approximately £1,000/kWe by 2001. PEM type fuel cells are expected to enter the market in 2001 at a similar price, but the cost is expected to reduce to £500/kWe by 2010 due to development and mass production for the automotive market. Assuming a total installed cost of 1.5x the system cost, gives the costs shown. It can be seen that PEM fuel cell systems should be competitive on simple installed cost terms for applications of 10kWe or less. In ten years time they might be competitive across the whole range of small scale CHP applications.
It has been shown that the total project costs are usually above 4x the CHP equipment package costs. The flexibility offered by fuel cell systems to reduce the requirements for site infrastructure works such as heat distribution systems and specially built engine rooms should enable design time and site construction costs to be significantly reduced. Hence the total capital cost may be reduced even if the fuel cell package cost is significantly higher than alternative prime mover packages.
Operating costs include:
Revenue from the heat and power supplied, or savings from reduced fuel and electricity costs must cover repayment of the initial investment and subsequent operating costs over the life of the system. The system should also be competitive with alternative systems in terms of rate of return or pay back period.
The operating costs of fuel cell systems should be much less than combustion engine systems due to their high efficiency and operating flexibility, and minimal maintenance. Further maintenance savings are possible due to simpler heat distribution systems and reduced building maintenance.
References
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