The Azimuth Project
Combined heat and power plant

Contents

Idea

Energy International‘s website carries this description of CHP:

Combined Heat and Power or Cogeneration is the use of some form of heat engine to simultaneously generate both electricity and useful heat. All thermal power plants emit a certain amount of heat during electricity generation. This can be released into the natural environment through cooling towers, flue gas, or by other means.

Block diagram

By contrast, CHP captures some or all of the by-product heat for heating purposes. To produce heat from a boiler and consume electricity from the grid in the conventional way in the UK has an overall efficiency of C. 56% whereas the efficiency of a CHP scheme could be over 80%.

Technical developments

: “There are numerous different ‘types’ of CHP and variations on the theme. CHP systems are often classified by size with typical definitions being: Large Scale >5MWe, Small Scale

Large scale

  • Gas Turbines – these range from very large power station type turbines such as the GE 9FE at 261MWe through purpose designed industrial turbines, modified aero derivatives to micro turbines.“

Very large gas turbines have quite good efficiencies of around 35% smaller turbines have lower efficiencies and at the bottom end of the scale micro turbines tend to incorporate regenerators to boost their efficiency to acceptable levels.

Gas turbines have some particular characteristics which can be exploited in a CHP scenario. Most of their waste heat is produced at high temperature enabling steam to be generated either for process use or in combined cycle mode (through a steam turbine) to boost electrical efficiency or both. They have relatively low maintenance costs. They are lighter and more compact than their reciprocating engine counterparts.

  • Steam Turbines – probably the oldest type of prime mover used in CHP mode. Unless used on a very large scale these tend to have low efficiencies and high capital costs. Steam to drive these may be raised in a boiler fired by many different type of fuel including biomass and geothermal.

  • Reciprocating Engines – these can range in size from the a Wartsila at 18MWe to the Dachs at 5kWe. Both the Otto and Diesel cycle can be used with the former being much more prominent. Reciprocating engines can operate with efficiencies of >40% and can run on many different gaseous and liquid fuels e.g. Natural gas, bio methane, bio diesel. Reciprocating engines produce two waste heat streams, high temperature exhaust gas and low temperature cooling water.

  • Organic Rankine Cycle - The ORC shares many characteristics with the steam turbine however they do not use water as their working fluid instead they use a variety of organic compound working fluids, hence the name. Whilst the idea of ORC is not new this technology does seem to becoming of age. Generally ORC uses recovered heat which is used to boil the working fluid. The vapour is passed through a turbine and then condensed. Typical examples of ORC are: biomass, process waste heat, bio-gas engine exhaust. ORC capital costs are relatively high and the efficiency low but when using ‘free’ heat the economics can be attractive.“

Economic and environmental analysis

: “There are three fundamental objectives to using CHP: To reduce carbon emissions, avoid fossil fuel depletion and to reduce energy costs. Emissions reduction and fossil fuel husbandry are obviously highly desirable. In a capitalist society, though it is very often return on investment that is the biggest factor in investment decisions.

: CHP systems only make savings when they are running and then only when they are operating at a high level of efficiency. Key then to obtaining a good economic return on a CHP project is to find an application where this can be achieved. In the vast majority of scenarios a site’s electrical demand (kWh) will be greater than and more constant (kW) than it’s thermal demand. In the majority of cases therefore the challenge is to match the thermal output of the CHP system to the thermal demand of the building or process.

: To achieve attractive economic returns e.g. < 4 year payback it is normally necessary for the CHP unit to operate for at least 6,000hrs per annum.

: Most UK buildings have very low thermal demand in the spring, summer and autumn often rendering them unsuitable to CHP. Where hot water service loads are significant CHP can be attractive good examples of this are hotels, hospitals and community housing. Another classic, none industrial application is swimming pools where the need to maintain pool water and pool hall air temperatures at 28-30 deg. C provides a year round thermal load.

: Many industrial processes require steam or have been designed to use steam for justifable technical reasons. Reciprocating engines cannot deliver all of their thermal output in the form of steam. Gas turbines can provide all of their output as steam but at sizes < 5MWe they are generally expensive. This dilemma has tended to limit the amount of CHP capacity that has been installed in small / medium size process industries and makes application engineering more challenging. CHP is commonly used in the pharmaceutical,chemical, paper and board and food and drink industries where large quantities of steam are required for sustained periods of time.“

Cost and complexity

The online calculator suggests a typical installed cost range of £500 - £1,200 per kWe.

“As with most things in life as the size inceases the specific cost reduces. Typical cost increasing complexities in CHP installations include:The need to generate steam from reciprocating engine exhausts. Interfacing with industrial processes. Noise constraints - the need for specialised acoustic treatment.Plant room weight constraints.”

Online calculator

This was written in Javascript by Dave Whitman and is one of a number of energy calculator tools for professionals, children and schools hosted on Energy Benchmarking‘s website.

“The calculator is for natural gas CHP, replacing a natural gas boiler, but this is probably the most common scenario in the UK, certainly for medium and large scale CHP.”

Questions

  • How much could universal adoption of CHP in the UK reduce energy consumption and greenhouse gas emissions?
  • What are the most promising new technologies in this field?

References

Source

Information and graphic supplied by Dave Whitman of Energy International Ltd.