Combined Heat and Power (CHP)
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Large Scale CHP
Micro Turbines
SMALL SCALE COMBINED HEAT AND POWER (CHP) SYSTEMS
'Small scale' or 'micro' CHP may be used to describe engine driven machines which are integrated into host systems linking with the National Grid Company network and heat producing plant, usually boilers. For many years the use of smaller packages incorporating alternators driven by reciprocating spark ignition gas engines have been available. Electricity outputs in the range up to 500 kW make these units economically attractive for base loads in many industrial and commercial market applications provided that the waste heat, recovered as hot water, can be used. The market potential was given a considerable boost by the Energy Act of 1983 which permitted full interfacing between site generators and the main grid supply.
Whilst there are novel engine options available and under development, for example the rotary engine, the reciprocating internal combustion engine is the only current shaft efficient prime mover available for small scale CHP. This type of gas engine is available from a number of suppliers in the USA, central Europe and the UK. The majority are spark ignited and developed from automotive or industrial designs.
Typical heat outputs for micro-CHP units are up to 2 to 2.5 times the nominal electricity output. The maximum operating water temperatures and pressures at which CHP Systems can be used are generally similar to those for low pressure hot water services at approximately 70-85 0C and 3-7 bar.
The
Generator
The generators used in packaged micro-CHP Systems have alternating outputs at 415V being either synchronous or asynchronous.
There are three principal ways of operating a micro-CHP system:
- Standalone operation, where the particular site electrical load is permanently isolated from the grid and connected only to the CHP unit.
- Parallel operation in which the CHP unit generates electricity to meet the electrical load but is supplemented by the grid should demand be greater than the system output. When the load is smaller than the output, power can be exported to the grid.
- Standby operation, where the unit supplies part of the load if there is a failure of the grid.
The asynchronous generator is mains excited and therefore cannot operate as a stand alone unit nor provide standby power in the event of a grid failure.
The Control System
Micro-CHP units may be set to respond to heat demand or alternatively
may respond to electrical requirements.
In the first mode the control requirements of the CHP system are similar to those of a conventional boiler installation. The CHP set should either be configured to preheat return water to the boiler or be operated as a lead boiler in parallel with the site boiler plant.
For control in relation to variations in the electrical load an electronic speed governor is linked to the line current to provide automatic proportional control typically from half to full electrical output. Modulation of electrical output should typically only be necessary for a few hours each day and for the majority of the period of operation the unit will be at full load.
Installation Considerations
Gas engines converted to natural gas can be obtained from several manufacturers. There are also companies who will design, build, install and commission complete small scale CHP projects. The engine must be installed safely, with suitable controls, and match the speed and torque requirements of the driven plant. Similarly, the heat recovery equipment must ensure adequate jacket water cooling at all times, and ideally have a matching use for the available heat. If a gas meter is to be installed this must not be of the turbine type as these are inaccurate when subjected to a pulsing gas flow.
The optimum size of a system depends on several factors:
- The load profile for both electricity and heat.
- Engine sizes available.
- Payback period required.
- Installation and maintenance costs.
Heat demand profiles can generally be estimated from total heating fuel usage, while monitoring for approximately a one week period may be necessary to establish electricity demand profiles.
Fuel tariffs are available from the fuel utility industries. While gas tariffs are uniform throughout the UK regional variations occur in tariffs for electricity and careful analysis of electricity cost is generally required.
Self excited generator packages can be used to provide standby power in the event of a mains failure and this can increase the cost effectiveness of the investment in terms of standby generating capacity which would otherwise be purely an overhead. However, to justify the investment cost most units installed for this purpose will be sized for near base load duty thereby limiting the usefulness of the plant to the provision of essential services only.
Sequential running of a multi-unit installation is an option for buildings with a high heat and electrical demand such as might occur in a hospital or swimming pool/leisure centre complex. In these situations one or two CHP units may be operated to satisfy a continuous daytime base load whilst a third unit may only operate during the winter period or at times when an excess heat demand exists and there is still a need for some additional electricity. Multiple unit installations have the advantage that some savings can be made against maximum demand surcharges as well as unit electricity costs. It should be possible to ensure that at least one unit is operating in order to reduce the maximum demand which is monitored continuously in ½ hour periods throughout the year, although effectively only incurring a cost penalty during the winter months.
It is important that CHP is correctly integrated into the existing boiler system to gain access to the highest possible heat load. This is particularly relevant if the hot water and space heating circuits are separated. The best option is for the CHP to be the leading 'heat' machine with the boiler as a top up facility. A suitable arrangement would be for the CHP set to preheat return water prior to the boiler. Alternatively the CHP could be operated in parallel with the boiler plant. For maximum running time the CHP set would be sized to produce most of the building's summer hot water requirement. During the winter the CHP continues to provide the domestic hot water while the boiler satisfies the space heating load. If the prevailing electricity tariff is sufficiently high to justify its purchase then a second CHP unit might be used to meet some of this winter space heating.
Engineering Recommendations
Gas installations are covered by the Gas Act 1986, the Building Regulations (Building Standards in Scotland) and by the Gas Safety (Installation and Use) Regulations 1984.
mearsecroft.co.uk. are able to assist customers and contractors on the requirements for the installation of gas engines. Some main points are worthy of note:
- A gas control train comprising valves, pressure switches, a pressure regulator, and a flexible pipe element is required.
- An air supply providing 4.2 Sm3 /kWh (shaft power) for combustion is required whilst a further 76 Sm3 /kW h may be needed for ventilation.
- An exhaust pipe discharging into open air where there is neither risk of exhaust gases coming into contact with persons nor of entering buildings or other plant through windows, air intakes etc, is required.
- General considerations include isolation of mechanical vibration and acoustic attenuation.
With the advent of the UK Energy Act in 1983 the then Electricity Council issued Engineering Recommendation G 59 which gives the conditions to be met when connection to the grid is required.
The regulations stipulate that the CHP unit must be isolated from the grid under the following conditions:
- If the difference in the declared supply voltage and the generator exceeds +/- 10%.
- If the difference in frequency between generator and 50 Hz falls outside the range of +1% to -4%.
- In the event of failure of any one phase in the distribution grid.
- In the event of a loss of the Electricity Company's supply.
If the starter motor of the CHP unit draws its power from the grid other regulations cover the voltage variations (P 13/1) and harmonics (G 5/3) introduced on the grid. The actual interference caused on start-up depends on the grid as well as the CHP unit, but general experience indicates that compliance with G 5/3 and P 13/1 is not a problem with small scale CHP.
Maintenance
and Reliability of Gas Engines
Maintenance involves a change of oil and spark plugs, and adjustment of valve clearances at 500 hour intervals in the worst case, although 1 000 hours is considered to be generally achievable. At about 10 000 operating hours the cylinder head will require removal for a top-end overhaul. The ultimate life of the engine before a major overhaul is necessary is probably in excess of 20 000 operating hours.
A maintenance contract or schedule that includes regular combustion tests is recommended; as well as maintaining high operating efficiencies such tests should reduce CO emission levels.
Reliability
Manufacturers of CHP sets offer maintenance contracts for servicing which in some cases include the replacement of components after a predetermined number of running hours. Problems have been encountered on several sites, for instance, the heat recovered has been less than the design value in some cases and mild steel exhaust gas exchangers used in chlorinated swimming pool water circuits are susceptible to metal corrosion. Unscheduled shutdowns have occurred because of problems with engine ignition systems, battery discharging (where fitted), failure of engine protection devices, and failure of micro-processor control components. Most of these problems have been quickly remedied resulting in improved system specification such as more efficient heat exchangers, modified control circuits and electronic engine ignition systems. The occurrence of these problems does, however, emphasise the need for good technical support to customers in the early phases of exploitation.
Economics
Annual cost savings available to a customer as a consequence of installing CHP depend on the capital costs, maintenance charges, prevailing gas and electricity tariffs and the total hours of operation. Capital costs are approximately £400-600 per electrical kW output. Costs for a particular site may deviate significantly from these average values if extensive modifications to the site services are needed to utilise the recovered heat. Maintenance costs vary with type and size of package and the extent of customer participation in servicing schedules and are in the range 0.Sp to 1 Op per kW h of electrical output.
Electrical import charges can be computed from the Electricity Company tariffs, and heating costs determined assuming an operating thermal efficiency of the existing site boilers and an average 55% efficiency of heat recovery from the engine. A daytime unit cost of electricity imported from the grid is of the order of 3.5p/kW h but a calculation based on this value will not include possible savings due to the CHP reducing monthly maximum demand and supply availability charges imposed because of demand during the winter months. Over 90% of electricity sold to the industrial and larger commercial markets is sold under a 'Maximum Demand' tariff although Electricity Companies are gradually introducing 'Unit Based' time of day tariffs. The latter have a supply availability charge but the monthly maximum demand surcharge is replaced by a higher unit cost during the winter daytime periods. To determine the effect of either tariff (within the same Electricity Company) on the savings produced by a CHP unit, it is important to establish the existing heat and electrical load profiles of the site which, in the first instance, can be assessed from previous monthly fuel bills.
In order to provide a good economic performance it is essential that the recovered heat is fully utilised, and therefore the daily weekly and annual patterns of heat load should be established. The savings are only generated during run time and hence continuous operation, i.e. more than 6 000 hours per annum will be most attractive whilst 4 500 hours or less is unlikely to prove economic. It follows that because winter space heating loads typically do not exceed 4 500 hours they are not an appropriate exclusive use for the recovered heat. In addition, space heating loads are often too variable to support base load electricity generation. If an appreciable hot water load can be established for (say) 5000 hours per annum then CHP would provide attractive annual running cost savings when compared to conventional supply systems.
The installation of a CHP set often coincides with the introduction of other energy saving measures on a particular site. The effect of all these measures must be properly considered, otherwise insufficient load may remain for the CHP unit to operate for the minimum 4 500 daytime hours necessary to give savings compatible with a three year payback on original capital investment. It is worth noting that some revenue can always be achieved by exporting excess electricity generation but dumping excess heat is not recommended practice. Special meters are necessary if the export of electricity is being considered and their installation incurs additional expense. It is therefore generally more cost effective to design a system to use all the generated heat and electricity on site rather than to export it.
Applications
and Market Potential
The most favourable circumstances for demonstrating the technology and economic viability of CHP within the commercial sector occur in buildings which have a simultaneous need for both heating and electrical power for a large part of the day and for extended periods during the year. Major consumers of both forms of energy include offices and similar premises. Part of this load is met by electricity which also provides additional power for lighting and miscellaneous appliances. Unfortunately the heating requirement for offices is often limited to about 10 hours per day and only for a six to seven month winter period. In this situation CHP may not be economically viable in terms of providing adequate annual savings on fuel costs to offset the capital investment for a CHP package. However, opportunities may exist for a modified CHP scheme incorporating an absorption chiller, which could provide heat during the winter and cooling during the summer. Office complexes with computer suites are particularly suited to this latter technology because of the continuous demand for air conditioning and electrical power.
Medium to large hotels are attractive applications for CHP since a space heating load exists for up to 18 hours/day and there is a high hot water demand for residential and catering needs. The heat to power ratio for a hotel in the winter is typically 3:1 which is well suited to the output of a CHP set which could therefore be sized to satisfy most of the building electrical demand. Hotels with swimming pools are even better applications since the pool water may be heated with low grade heat recovered from a condensing heat exchanger on the gas engine exhaust. This additional heat recovery can improve the CHP overall thermal efficiency to approximately 90%.
The public sector offers the greatest opportunity for exploiting CHP since this area includes hospitals, grouped residential accommodation, university and college campuses, prison and detention centres, swimming pools and leisure centres. These premises require both heat and power for extended periods and at a ratio suited to CHP technology. The potential for CHP must therefore be considerable. A hospital requires space heating for most of the day and substantial quantities of water for up to 20 hours per day. On average the winter heat to power ratio is about 3.5:1 indicating that CHP could be usefully employed in this market sector. A large leaching hospital with student accommodation can have an electrical demand of up to 1 MW which makes it suitable for large scale CHP equipment using bigger engines or gas turbines. However the high capita outlay for these systems is not readily available and investment has at present been restricted to the smaller packages with electrical outputs in the range 40 kW to 85 kW. Several hospitals have been supplied with this size of unit.
Grouped residential accommodation suitable for elderly people operates at relatively high space heating temperatures for up to 18 hours per day with hot water and electricity demands for 14/16 hours per day. The space heating load diminishes during the summer time so that 4 500 hours may be the maximum annual running time for a CHP set. Whilst this reduced operating time may lead to an extended payback period against the installed capital cost, this cost is partly offset by the CHP eliminating the need for a standby generator, the latter being an essential requirement for this type of accommodation. Small packages in the range around the 35 kW would be satisfactory in this market area.
The education sector has a high space heating demand. Universities and colleges with residential accommodation have suitable heat and electrical loads which may extend for a sufficient period during the year to justify the installation of several small CHP sets. Schools are less viable except when a swimming pool is available for using the CHP heat recovery during the summer months. Several schools operate 15 kW CHP sets on this basis and scope for further installations must exist.
Swimming pools and leisure centres probably provide the most suitable conditions for micro CHP with a continuous demand for space heating and hot water and with a fairly constant electrical load for lighting and for driving numerous pumps and fans, resulting in a day time heat to power ratio of between 4 to 5:1. A typical pool of some 400 m2 area with conventional heat recovery devices such as variable air ventilation and a run around coil already fitted, would still have a sufficient heat and electrical load to justify at least one 40 kW CHP set. A typical installation has four such units generating 160kW of electricity with 448 kW of recovered heat; three gas boilers provide additional heat when required.
Whilst there will be other applications and circumstances in which small scale CHP sets can be cost effective the most immediate market within the commercial sector therefore includes:
- Hotels
- Hospitals
- Grouped residential accommodation
- University and college campus and some schools
- Prison and detention centres
- Swimming pools and leisure centres.
Future
Developments in Micro CHP
For most customers the viability of a CHP set is judged on the savings in total fuel bills and the ability to demonstrate a short payback period on the total capital investment. Assuming that the ratio between electricity and gas prices remains at a constant value, then the most effective method of increasing hourly cost savings is to reduce the CHP maintenance costs which at the present time can be as much as 30% of the fuel savings. Improved systems reliability and the development of servicing capability throughout the UK are important objectives in the future development of micro-CHP sets.
A further priority is to reduce the capital cost of the package such that payback periods of less than 3 years are the norm rather than the exception. These shorter periods are particularly difficult to achieve if the CHP heat recovery is compared against the higher efficiencies of heat generation now attained with modern gas fired boilers. Reduced capital costs may be realised as numbers being installed increase and manufacturers are able to reduce component costs. Increasing the shaft efficiency of the prime mover may also assist in expanding the market for CHP technology. Assuming that the overall efficiency of the package is maintained, then a higher ratio of electricity generation to heat recovery makes it easier to match the CHP set to the total site electrical demand.
LARGE
SCALE COMBINED HEAT AND POWER (CHP)
Large scale CHP is used here to refer to installations with an electrical generation capacity of 500kW or more. In general the choice of prime mover for large scale CHP will depend on the heat/power ratio of the site and the base load requirement. Except where there is constantly more heat required than power (i.e. heat/power ratios above 3 or 4:1), reciprocating engines, with their high efficiency of shaft power generation and therefore high ratio value of energy produced to energy consumed, would normally be preferred as prime movers to gas turbines or steam turbines. Gas turbines come into their own at heat/power ratios about 4:1, and increasingly so as the ability to after-fire their exhaust gases at very high efficiency to meet heat/power ratios of up to about 10 or 12:1 is utilised. Pass-out steam sets must also, of course, be considered where high heat/power ratios are required, but, except where this ratio is constantly high, usually above 15:1, steam turbines have not the flexibility of gas turbines. Gas turbines can follow fluctuating load profiles with far greater economy of operation.
Gas turbines operate at shaft power efficiencies which vary between about 25% for very simple designs to 30% for more complex plant involving intercooling between several stages together with regeneration. From the total energy point of view open cycle gas turbines have the advantage that virtually all the thermal energy in the fuel which is not converted into shaft power is available for recovery. Typical exhaust gas temperatures are in the rang&550 0C to 850 0C but it must be remembered that if regenerators or recuperators are used the final exhaust gas temperature will be considerably reduced, possibly down to 270 0C. Although this leads to higher turbine efficiencies the quantity of heat recoverable for use in processes will be significantly lower.
There are several methods of using the waste heat from gas turbines. mearsecroft.co.uk. gives guidance on the installation of industrial gas turbines, associated gas compressors, etc.
Direct
Drying
When natural gas is used as the primary fuel the exhaust gases are clean and can be used directly for process drying requirements. The most common of these are:
- Brick, tile, ceramic and glass manufacture.
- Leather and allied trades.
- Agricultural based establishments e.g. vegetable drying, fish, meal and meatmeal factories.
Wasteheat
Boilers and the Provision of Process Steam
The waste heat boiler can be used as a waste heat recovery unit on gas turbines. Hot water waste heat boilers can recover up to 75% of the available heat in the turbine exhaust and can thereby produce an overall efficiency of 80%.
With a high-pressure steam waste heat boiler up to 65% of the available heat can be recovered giving an overall efficiency of 72%.
The use of supplementary firing increases the steam output with very little additional expenditure and releases the end user process from its dependence on the performance of the gas turbine.
The
Combined Steam/Gas Cycle
The boiler serving a conventional steam turbine can use large quantities of preheated air from the gas turbine exhaust through making use of the oxygen present for supplementary firing. The operation of the gas turbine will be marginally affected by the back pressure that is imposed upon it. In addition steam turbines cannot accept quick starts and stops like a gas turbine. As the steam turbine machinery takes longer to put into operation than the gas turbine plant it is necessary to have some kind of disconnection device to permit the gas turbine to run on its own until the steam turbine is ready to run With such a system it is possible to obtain an overall useful power efficiency of more than 40%.
The type oS steam turbine used in conjunction with the gas turbine depends on the duty, and the various steam turbine modes have been discussed in 13.2.3. Where electricity generation is of prime importance a condensing steam turbine would be used. With a need for further process heat in the form of steam a back pressure turbine would be required in which the steam exhausts at a relatively high back pressure. Alternatively a passout steam turbine can meet this second requirement: steam is extracted between turbine stages at the required conditions.
Reciprocating
Engine Systems
Reciprocating engines have a number of significant advantages over the gas turbine when a substantial mechanical output is required. In general their shaft efficiency is greater than that of the turbine and their cost is less. However the ancillary systems required for reciprocating engines are more complex and the units have a low power to weight ratio. In addition a major difference is that engine heat recovery includes low grade heat as hot water as well as exhaust gases which can provide high grade heat as steam. This hot water may not be required by the site thus reducing the energy savings for the system.
In the UK most of the large reciprocating engine based total energy systems have been installed in sewage works, hospitals and factories, although there are a number used in private power stations serving chemical works.
Many reciprocating engines are available as a package direct from the manufacturer with heat recovery units utilising the engine cooling water and lubricating oil heat. The engine exhaust may then be coupled to waste heat boilers or hot water heaters which also act as exhaust silencers. Units for exhaust waste heat recovery are available for engines ranging in output from 100 kW to 15 MW.
As an alternative to conventional water jacket cooling systems on engines an ebullient system can be used. However the engines capable of utilising ebullient cooling are more expensive than those employing sensible heat removal methods. Additional steam can be provided by utilising a waste heat boiler.
In assessing the economics of heat recovery from reciprocating prime movers maintenance and reliability costs must be carefully estimated. Complete maintenance costs are composed of three basic items:
- Routine maintenance and service cost including make-up oil but excluding the necessary labour cost.
- The cost for performing necessary top end and engine overhauls including the required labour cost.
- The labour costs necessary for routine maintenance and oil additives.
The first two items
will vary with the conditions under which the engine is required to operate.
The third item is the most variable and will depend on plant location
and unit labour costs. Maintenance costs should be based on past experience
in the area being considered.
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