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Combined Heat and Power

Category: Systems of interest September 30th, 2010 by mbc

Combined Heat and Power (CHP) systems generate electrical power and heat simultaneously.

There are three main technologies utilised to provide CHP systems – External combustion, Internal combustion and Fuel Cells.

External combustion
Central to the generation of electricity is the Stirling engine. A Stirling engine operates by being heated externally and so is described as an external combustion engine (ECE).

Fuel Cells
A fuel cell is an electrochemical cell that converts an input fuel into a number of outputs – an electrical current, heat and waste water or carbon dioxide. The primary function of a fuel cell is usually generating electricity but they run hot and so provide heat as a ‘bonus’. A wide range of fuels, both fossil based (gas, coal, oil) and renewable (biomass) can be utilised.

Internal combustion
Commonly used in automotive applications the fuel burning internal combustion engine (ICE) that we are all familiar with is applicable to CHP applications although the noise, emissions and need for servicing make the other options more generally suitable.

The Green Angle
Greenpeace claim that:

 ‘On average, our large, centralised power stations throw away two thirds of the energy they generate.’


In contrast the overall efficiency of CHP systems can be in excess of 80% at the point of generation.

With CHP power & heat generation is moved closer to the consumer. Being close to the consumer has two main benefits:

  1. With its dual function – heat and electrical power – the CHP facility avoids the inefficiency of it’s single purpose cousins such as the traditional coal electric power station. In such traditional power plants, generated heat is often treated as a waste product and merely vented or otherwise disposed of.
  2. Power does not need to be transferred over distance to remote consumers and so losses during transmission and distribution of electricity through the national and local distribution networks are avoided.

The Combined Heat and Power Association website has more detail on the advantages of CHP over traditional power plants.

Large CHP installations are sized to provide for the needs of large consumers of heat and power such as industrial sites or large hospitals. Mini installations can be sufficient for the needs of a group of dwellings. Micro CHP is applicable to the individual householder.

CHP techology can be further evolved to incorporate cooling. Through cooling generated heat using absorption chillers a CHP plant can add the ability to provide cooling and become a Combined Cooling, Heat and Power (CCHP) plant.

Micro CHP ~ Now?
The first widely available micro CHP unit in the UK is the Baxi Ecogen micro-CHP unit. This is basically a replacement for a traditional gas boiler that contains a sterling engine and so is able to generate electricity as well as heat water. It can generate 3.5kW to 6kW of heat that can be boosted to 10kW or 24kW if necessary – thus allowing a relatively standard heating range of between 3.5 and 24kW. The Ecogen is capable of producing up to 1 kWh of electricity. It can be grid connected, with excess electricity sold back to the grid operator. In the UK these micro CHP units can be included in the Government’s Feed-in Tariff Scheme providing owners with an additional payment on top of the standard price for any power they sell back to the grid. See my recent article on Feed-in Tariffs for more details.

Worcester (a part of the Bosch group) have the Greenstar CDi DualGen in development. It has a similar footprint and specification to the Baxi Ecogen unit. It utilises a stirling engine that can generate up to 7kW of heat (that can be boosted higher) and 1kWh of electricity. The Greenstar unit is due for full release in 2012 following a europe wide field trial.

So, is now (September 2010) the time to consider CHP as a viable technology?

I think the answer to the question depends on your specific project. Let’s start with this advice from the Worcester-Bosch website:

‘Micro CHP is most suited to older buildings that are poorly insulated for example, with sash windows and no cavity wall insulation.’

That quotation causes a couple of issues. Firstly, for the sake of sustainability and common sense the fabric of the building itself should be addressed. Insulation installed and leaky windows renovated or replaced. Then secondly, once the building has been sorted out, what are the heating & energy requirements – is a CHP system appropriate and can it be made to pay? That’s a pretty straight-forward analysis based on:


  • The cost of installing the CHP system.
  • It’s cost of running in terms of how much fuel it will consume.
  • Cost of servicing.

  • How much it will save you financially in comparison to other forms of heating.
  • How much will be generated electricity save from your electricity bill.
  • Feed-in tariffs confuse things somewhat, suffice to say that the electricity generated will be have a value to you beyond the current 13 pence or so that you’ll pay per kWh on a standard tariff.

Also, bear in mind that the systems I’ve been discussing here are gas powered (the Ecogen has an LPG version), so you’ll need a mains connection or the ability to juggle LPG bottles.

To me this seems an immature technology, one perhaps for early adopters who aren’t too sensitive to the raw economics of these systems in comparison to the heating only alternatives such as modern condensing gas boilers, heat pumps and wood-burners. These systems also seem more suitable for replacing old gas boilers than for installing in ‘green-field’ sites such conversions or new builds with no mains gas.

Further References

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Heat pumps

Category: Systems of interest October 9th, 2008 by mbc

Many people visiting this site are looking for information about heat pumps, being one who believes in giving the people what they want I thought an up-to-date summary was in order…

Heat pumps take heat from a donor heat transfer medium (earth / air / water) and condense it for the purpose of heating water or air.

The basis of heat pump technology is something we are all familiar with as it is similar to that used in refrigerators or air conditioning units ~ but in reverse. The heated water generated by a heat pump is often used for space heating but can also be used as the basis for general hot water provision given an additional boost from a complementary boiler system to raise water temperature.

Heat pumps consume electricity to power the compressor & pump that circulate fluid and gas around the system; this is a relatively efficient use of energy typically producing between two and four units of heat for each unit of electricity consumed. The ratio of energy consumed to heat generated is know as the coefficient of performance (COP). There is a theoretical limit of around 14:1.

What types of heat pump are there?
The air source heat pump (ASHP) draws thermal energy from the air.

The ground source heat pump (GSHP) gathers heat from a ground loop. This is constructed from lengths of pipe filled with a blend of water and antifreeze in a closed loop. This loop is buried in the earth either horizontally in trenches or vertically in a bore hole.

The water source heat pump (WSHP) uses water as the heat transfer medium.

How do they work?

  • A liquid refrigerant is forced through an expansion valve & in doing so it loses pressure and evaporates.
  • In evaporating, the liquid refrigerant removes heat from the input loop. The exact nature of the input loop will vary depending on the type of heat pump as discussed earlier ~ in general terms it brings heat energy to the evaporator coil from the donor medium.
  • Cooled water in the input loop is recirculated and reheated by the transfer medium.
  • The evaporated refrigerant passes through the compressor that increases pressure and causes the refrigerant vapour to condense at an increased temperature.
  • As the refrigerant vapour condenses heat energy is released, this heat raises the temperature of the condenser coil which in turn heats water.
  • This water is then used for space heating.
heat pump parts and process
the basic structure of a heat pump

Where should I use a heat pump?
They are best coupled with well insulated, energy efficient buildings with an under floor heating system for heat distribution. Traditional radiators can be used, but will need to be larger in total surface area than those found as part of a conventional system due to the lower water heating temperatures, such a system will also be less efficient than an underfloor alternative. Air source heat pumps are often used in conjunction with ducted air heating systems.

You will need plenty of outdoor space for a horizontally installed GSHP ground loop. Alternatively, a vertical bore hole can be sunk but will cost more.

Bear in mind that the payback period will be longer if the heat pump is replacing gas rather than other heating options such as solid fuel or electricity. However, with soaring gas prices the payback period is shortening.

The higher the temperature that water is heated to, the lower the COP of the pump will be. Bear this in mind in designing your overall hot water system, providing additional inputs to raise water temperatures to domestic hot water (washing) levels.

How much do they cost?
Costs have remained relatively static over the last year, a 6kW ASHP costs around £3,500 with a larger 12kW pump coming in at about £6,000. You will them need to pay for installation and excavation, this will potentially double costs, so allow £6,000 to £12,000 for a domestic installation. This excludes the cost of the distribution system such as an under floor heating.

Future developments, such as photovoltaic cells with a shorter payback time or more exotic combinations for example, bio-fuel driven Micro Combined Heat and Power (micro-CHP) installations providing input electricity, offer greater levels of sustainability and self-sufficiency in heat pump utilisation. This doesn’t stop heat pumps from offering a green heating solution today, with guaranteed CO2 savings over traditional heating systems and a relatively fast payback where mains gas is not available.

Other Resources
Grants may be available for UK installations see
Proceed with caution where gas is available according to

Previous Heat Pump Articles on MBC

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Category: Systems of interest April 21st, 2008 by mbc

Photo = light & voltaic = electricity

Photovoltaics is a technology that utilises light to generate electricity. As such it is an essential tool in the development of more sustainable methods of electricity generation. Simplistically, electricity is generated by the photons from sunlight colliding with electrons within the solar cell.

Solar cells are solid state devices that produce direct current electricity from light. They are arranged into interconnected groups to form a module. In turn photovoltaic (PV) modules are connected together into photovoltaic arrays. A module is big enough to power a single device with larger applications such as a family home requiring an array. PV arrays can be built into the fabric of a building, in its roof or walls, or developed as a stand-alone system as we see connected to street lights or on caravans.

PV cells use both direct light and indirect or diffuse light and so are effective even in temperate climates and operate under grey overcast skies, not just on bright sunny days.

As in most cases they constructed largely from silicon, the manufacture of PV modules has relatively green credentials, although the need for batteries for storage in off grid situations can somewhat sour this.

Most UK implementations of PV will be grid connected PV systems. In these systems there is no need for battery storage. The PV system is connected to the local electricity network (grid) and any electricity not consumed locally can be sold to the electricity supply company. Where the local PV system is unable to provide all electricity demanded, for example at night, then electricity is bought from the grid. The ‘grid’ acts as the storage system.

One of the leaders in the new thin film technology that allows cells to be printed on a aluminium film at low cost is based in Wales just like MBC!
The company is G24 Innovations who have a fabrication plant in Cardiff.

An inverter will be required to convert the low voltage (12 volt) DC electricity generated by PV to high voltage (230 volt) alternating current (AC) consumed by most UK appliances.

How much?
A typical domestic system will need between 1500 and 2000 Watts peak (Wp)
Typical modules have power output of 75 to 120 Wp.
Therefore, 10 to 20+ modules will be required.

I have ‘tirelessly’ searched the internet for illustrative costs from various sites and articles of various ages I’ve come up with the following prices each from an individual source:

£4,000 to £9,000 per kWp installed.

£8,000 and £15,000 on a typical domestic installation of 1.5 kW.

…this works out at £12 000 – £14 000 for a 2 kWp system for a house.

To provide a PV power supply capable of meeting the demand from a typical domestic energy efficient house costs in the region of £20,000.

…costs can be around £5,000- £8,000 per kWp installed with most domestic systems usually between 1.5 and 3 kWp.

Which gives an average of somewhere around £6,000 per kW so £10,000 for a typical domestic installation of 1.5 kW. As this will save you several hundred pounds a year on electricity costs the financial payback is long. The overall cost-benefit will only tip into the positive if you personally value the ecological benefits highly.

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Solar Water Heating

Category: Systems of interest March 13th, 2008 by mbc

Solar water heating systems use energy from the sun to heat water for use in the home. This water can be utilised for washing and general domestic uses as well as for heating purposes.

There are three main components:

1. Heat Collectors or Panels. The two main types are flat plate and evacuated tube both are normally fitted onto roofs but can be ground mounted. Evacuated tube systems use metal strip collectors in vacuum tubes and are the smaller but more expensive option. Flat plate systems use a dark coloured plate in an insulated container collect heat.
2. Heat transfer system. Transfers the collected heat to the water.
3. Hot water cylinder. This stores the heated water and supplies it onward to those systems that consume it.

In the UK, appropriately sized systems can provide up to 90 per cent of the hot water needs of a typical home.

Depending on specifics such systems should cost upwards of £1,000 in a new build domestic property with higher initial costs for a retro-fit. From recent research it should be possible to install system for £4,000-£5000 into the average home. Payback may be up to twenty years, but with current fuel price escalation this is likely to drop rapidly.

Designing a system to meet your needs will require consideration of supplementary heating systems, the site and its orientation to the sun (you will need some south-facing aspect), the heating & hot-water requirements of the buildings inhabitants and budget. As we in the UK are not overly endowed with sun during the winter months you will also need a supplementary heating system to provide hot water when the sun can’t – log, pellet, gas and multi-fuel burners and boilers can fulfil this role if configured correctly within the overall system.

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Pellet Stoves – a current evaluation

Category: Systems of interest March 5th, 2008 by mbc

I feel like I’ve somewhat misrepresented pellet stoves in the past.

To recap, pellet stoves are wood burners with a difference, they burn pellets made from pressed waste wood, that are feed from a hopper into a combustion chamber all of which is often controlled electronically.

I’m sure no-one has really been reading who’ll take offence, but I feel that I should redress the balance. When writing about biomass heating, I stated that:

biomass systems are often bulky and so can require extensive storage space for both fuel and machinery

…with no statement of assumptions and seemingly no exceptions.

After further research, I’ve found some nifty looking pellet stoves such a those from Rika, that offer full biomass credentials whilst remaining compact in size and offering levels of control not available with more crudely fuelled wood or multi-fuel burners. I especially like the Rika option that allows remote control of the system by mobile telephone. Read More

So, to redress the balance: Biomass can be compact as well as clean and green!

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