Green Energy

Green EnergyWhy use Green Energy?
Most of us still take our energy supply for granted. Heat and electricity are readily available at the flick of a switch. We don’t need to think about where our power supply comes from or the environmental impact of its generation.

However the effects of climate change and dwindling oil supplies are becoming more apparent. A rising awareness of the impact of non-renewable energy consumption has contributed to a series of international climate change summits. The resulting Kyoto Protocol commits governments to reducing greenhouse gas emissions and placed expediency on developing alternative energy sources.

New organisations, such as Action Renewables and the Energy Saving Trust, have been set up to provide information and offer grants in support of green energy. Businesses and households can now make previously unavailable choices in energy consumption. Although not without feasibility issues, green energy is an increasingly viable alternative to fossil fuels for everyone


Biomass refers to non-fossil organic matter that can be used to produce energy. Wood, crops and grasses, as well as animal and food waste are prime examples. Biomass can be converted to different forms of energy – including heat, power, combined heat and power (CHP) or liquid biofuels – through processes such as direct combustion, anaerobic digestion and fermentation.

On a domestic level, biomass refers primarily to wood fuel. Wood is a great natural source of energy that can be managed sustainably and has served humans well for thousands of years. Modern wood burning technology is much more efficient than the days of open fires and hearths, and is able to convert up to 90% of the energy in wood into heat. Open fires only manage to use around 20% of this energy.
Large-scale facilities, such as apartment blocks or factories, use large wood-fired boilers to produce heat on a grand scale. For example, a medium density fibreboard (MDF) plant in County Tipperary uses a 19 megawatt wood-burning furnace to burn its waste wood and supply heat to the facility. In a domestic setting, smaller-scale wood-burning stoves are more common.

Wood fuel on a domestic level is available as wood pellets or logs. Most modern equipment is designed to burn pellets, which is derived from wood waste, such as sawdust and small timber pieces, left over from industrial processes. Pellets are easier to transport and handle than logs, take up less storage space, and can be fed automatically into the boiler or stove.

Pellet stoves generally achieve efficiencies of around 80%. They work as standalone systems or can be fitted with back boilers to provide hot water and heating. Pellet boilers can also be fed automatically and are plumbed into the LPHW (Low Pressure Hot Water) circuit of a house to provide heat to radiators and the hot water cylinder. Pellet boilers can be up to 90% efficient.

Logs can be sourced sustainably from properly managed forests and short rotation coppice (e.g. willow). They demand a lot of storage space and are more readily available in rural locations than in the midst of towns and cities. Log stoves rely on manual feeding, which can be more troublesome and messy.

Like pellet stoves, log stoves can produce standalone heat or be fitted with a back boiler to provide hot water and heating. Modern log stoves are clean burning, producing less smoke than in days gone by. They’re not quite as efficient as pellet stoves, achieving efficiencies in the region of 70%. Log boilers work like pellet boilers to produce hot water and radiator heating.

The cost of a log or pellet boiler depends upon the size and type of system. A 15kW system aimed at heating a typical three-bed domestic property can cost between £4,000 and £6,000. There’s also the associated cost of the fuel. If the energy being replaced is LPG (Liquefied Petroleum Gas) or electricity then there are definite savings to be made. The economic benefit is diluted when comparing wood to gas.

Biomass for burning

Miscanthus and willow are the most popular crops that are used for burning to create hot water for heating.
Miscanthus plants last for 20 years. The crop is harvested in November each year when it is dry. A large part of its attractiveness is its easy management, from planting to harvesting all of which can be done with conventional farm equipment, and requiring virtually no inputs in terms of chemicals. There are some complications with uncontrolled burning of the dried miscanthus such as burning in wood chip burners, and it is recommended that it be processed into briquettes or pellets where it has lime added and it burns very cleanly with a low-ash content.

Willow is a native tree and many of its varieties are particularly well suited to growing in the cool, wet maritime climate typically found in Ireland. It is easy to establish and grows extremely rapidly. Willow can be coppiced, meaning that in the winter it can be cut back to ground level and the ‘stool’ left in the ground will resprout the following Spring. Willow does need intensive preparation and management, including weed control. It is harvested on a three-year cycle giving a willow plantation a life of approximately 30 years. Yields average out at about 10 to 12 dry tonnes per hectare per year. The willow is harvested with a forage harvester to produce wood chip. This chip is about 50% moisture at the time of harvest and needs to be dried to reduce moisture to less than 20% for storage and utilisation. It is also possible to harvest willow as billets or even whole rods. If left in the field these will dry naturally to approximately 30%. However, a second handling procedure will be needed to produce wood chip. Site selection is crucial and some parameters have to be observed. The machinery for harvesting is heavy and uses a lot of fuel.

Combined Heat and Power

Combined heat and power (CHP), also known as co-generation, is an old technology that’s witnessing something of a renaissance in the UK and Ireland due to climate change legislation. It operates by using the heat produced by generating electricity, which is normally discarded into the atmosphere, and turning it into useable electricity or heat.

Until now, CHP has typically been used in large scale industrial applications such as hotels, hospitals and industrial and commercial buildings where a constant source of heat and power is needed. It’s estimated that for every 1MW of CHP installed, CO2 emissions are reduced by 1,000 tonnes per annum.

Micro CHP is an emerging technology that’s essentially a miniature version of its industrial cousin, which has been designed to take the place of conventional boilers in homes and small buildings. Micro CHP systems look and behave like boilers but they’re mostly floor standing (although there are wall-mounted ones in development) and sound a little like fridges. As well as providing conventional heating and hot water, they can provide some electricity thereby reducing energy bills and CO2 emissions.

There’s scant information regarding their cost and viability at the moment, because there aren’t many off-the-shelf units commercially available. A report by the Energy Saving Trust titled ‘Potential for microgeneration study and analysis’, which concludes that microgeneration technologies have the potential to deliver 30- 40% of the UK’s electricity needs by 2050, shows that CHP has the biggest potential of the renewable technologies for meeting household energy demands.

Although micro CHP is not a tried and tested technology on a domestic basis, there appears to be great potential for achieving greater efficiencies and carbon savings over conventional gas boilers. CHP has been used successfully for years in larger industrial and institutional buildings as a means of maximising efficiencies and cutting costs. It’s advisable to keep a close eye on reports about CHP as they’re published. If the technology is proven, this will stimulate demand and a growth in suppliers.


The Warm Homes Scheme is funded by the Department for Social Development to make your home warmer, healthier and more energy efficient.  The Warm Homes Scheme is for people who receive certain qualifying benefits and own or rent their home from a private landlord in Northern Ireland.  Visit nidirect website for more information.

Ground Source Heat Pumps

Ground source heat pumps (GSHP) use the latent heat energy that’s stored in the ground to produce air and water heating, and air cooling. It might be hard to believe on a frosty morning, but the earth’s surface acts as a huge, stable heat sink for solar radiation. In the UK, ground temperatures remain constant at around 10-12ºC throughout the year. Ground source heat pumps take advantage of this by transferring this heat to buildings in the winter and taking heat out of buildings in the summer.

These systems have been used successfully for years in commercial, residential and institutional buildings in countries such as the US and parts of Europe. While they undoubtedly deliver reductions in CO2 emissions, the actual quantities, and the cost savings, are largely dependent on what kind of conventional energy it is replacing and what kind of distribution system is used.

A ground source heat system has three main components: the ground loop, the heat pump and the heat distribution system. The ground loop is comprised of a series of pipes buried in the ground, either vertically in a bore-hole or in a horizontal trench. The pipes form part of a closed system and are filled with a mix of water and anti-freeze.

Just like in a fridge, the heat pump works by evaporating and condensing a refrigerant which moves heat from one place to another. There are three parts within the pump: the evaporator, which absorbs the heat; the compressor, which moves the refrigerant around the pump and compresses it to the necessary temperature; and the condenser, which passes the heat to a hot water tank.

The last part of the system is the heat distribution system. This can take the form of under-floor heating, radiators, or sometimes a hot water system. Ground-source heat pumps are best suited to under-floor heating or low temperature radiators because they can only raise the temperature to around 40-50°C. Therefore, they tend to be better suited to new builds that can plan for this, rather than retro-fitted to work with conventional boilers or immersion heaters that require temperatures of around 60-80°C.

GSHP can be suitable for most building types, but in new builds a developer can work out an overall plan that maximizes efficiencies by including under-floor heating and low temperature radiators, ensuring good insulation, and considering cooling requirements. Ground source heating is particularly suitable for areas not on the gas grid, although rising gas prices may start to make them more financially attractive in grid-served locations.

Harvesting Energy from Agriculture

The vast majority of biomass is used for burning to create heat and hot water. It is becoming more common to use it for generating electricity – especially biogas and biodiesel and bio-oils to run combined heat and power units.

The key drivers for interest in biomass based fuels include: recognition of the damage caused by fossil fuels (pollution and climate change); energy security (unstable supply, peak oil); Farm Diversification; Increased structural support in the form of EU grants and national grant schemes; and market demand from domestic customers as well as larger scale users like Government and business.

Heat Exchanges

Air source heat pumps (ASHPs) work in a similar way to ground source heat pumps, but as the name suggests they use heat from the air as their energy source instead of latent ground heat. ASHPs can be used to bring heat into buildings for space and water heating in the winter, and extract it out of buildings in the summer to provide air cooling.

Energy savings, and a reduction in CO2 emissions, are potentially significant, although they tend to work best in energy efficient buildings using low temperature distribution systems such as under floor heating. While ASHPs form the largest market share of all the heat pump options, most of these installations are used in their reversed mode to provide cooling.

There are two types of air source heat pump systems: air-to-air, which is most common, and air-to-water. Air-to-air systems take heat energy from the outside air and pump it to the air inside using fan-assisted units. This is reversed in winter to provide cooling. Air-to-water systems are more advanced and can be used in homes with hydronic heat distribution systems such as radiators or under floor heating.

Systems range from 3kW-100kW and can be used in residential, commercial and institutional buildings. As well as moving heat from outside to inside and vice versa, it can also be moved around buildings as needed, further increasing efficiencies.
An air source heat pump has three main parts: the evaporator coil, which absorbs heat from the outside air; the compressor, which pumps the refrigerant through the heat pump and compresses it to the temperature needed for the heat distribution circuit; and the heat exchanger that transfers the heat from the refrigerant to either the air or water.

Additionally, the fan and compressor in an ASHP system can emit a lot of noise and vibrations. The pump should be located as far as possible from bedrooms and neighbouring properties. There’s also a lack of sufficient numbers of qualified and trained installers due to the industry being in its infancy in the UK and Ireland.

The performance of air source heat pumps is measured by the coefficient of performance (CoP) and falls anywhere within the 2 to 4 range. This means that for every unit of energy put into the system, you get 2 to 4 units out. Determining an exact figure is difficult due to variable factors such as the outside air temperature and the heat distribution system in use.

On the whole, ASHPs tend to have lower efficiencies than ground source heat pumps, due to the seasonal differences in air temperature. When the air temperature decreases, it’s more difficult to extract heat from the cooler air. Using a distribution system, such as under floor heating, which requires low temperatures can help to increase efficiencies.

These efficiencies equate to reduced CO2 emissions and possible reductions of heat energy bills of around 50%, which should be enough to warrant the initial investment. A typical cost of a 5kW domestic system is about £6,000-£8,000, excluding the heat distribution system. With efficiencies of around 2.5 CoP, this would result in a payback period of between 8 and 15 years, with the service life of an ASHP being around 15 to 20 years.

Installation and integration

PV cells use only low voltages and currents, so they tend to be grouped into rectangular, weatherproof modules. These modules are often grouped into arrays to cover a surface large enough to produce the energy demanded of them. The cells can be moulded into solar slates or tiles for integration into roofs, or bonded onto glass or metal sheets and fixed onto brackets just above the roof.

It’s possible to have standalone PV systems, but they require large expensive storage batteries. Typically, PV installations for buildings will be connected to the grid by means of an AC inverter. This enables the system to put any excess energy into the grid, and to receive top-ups when supply is low.

Optimally, PV systems should be installed on a roof or wall facing south, with an optimal inclination of around 30-40º. The surface should be free from any potential shadow obstructions such as trees or buildings. A typical domestic system would provide around 1.5kWp* of power – that’s around a third to a half of the average household’s electricity requirements.
A typical , domestic PV system operating will, in its expected lifetime of 25 to 30 years, save the world from around 6900kg of damaging CO2.

At the same time, a PV system can increase the value and overall energy efficiency rating of properties, reducing electricity bills along the way. When sited correctly, PV cells are relatively unobtrusive. They’re silent, have a low visual impact, and are low-maintenance. They’re largely self-cleaning, and, with no moving parts, they only demand the occasional checking of wiring and components.

Unfortunately, as with many new technologies, and other renewable energy solutions, PV installation is expensive. For domestic use, it currently costs around £6,000 to install a 1kWp system. This would generate around 16,000 kWh of electricity over its lifetime, delivering savings of around £1,300.

Photovoltaic cells

Slim and rather sophisticated looking, photovoltaic (PV) cells are made from a semi-conducting material, typically silicon, that uses sunlight to create an electric field and stimulate the flow of electricity. This electricity can then be used to power lights and other appliances, for either domestic or other uses.

PV cells only need daylight so even sun-deprived parts of the globe, such as the UK and Ireland, can benefit from their use. Amazingly, in our less-than-Saharan part of the world, between 900 and 1100 kWh of solar energy falls annually on each square metre of un-shaded surface.

Solar Water Heating

Of all the renewable energy technologies, solar water heating has arguably proven itself to be one of the most reliable and cost-effective. It’s already used by millions of households across Europe, and while the UK and Ireland have been slower on the uptake, government grants are helping to raise the number of adopters.

Solar water heating systems are standalone systems that collect solar radiation, which is converted to heat. This heat is then used to pre-heat water in a water cylinder, either directly or indirectly by means of a circulating fluid. They can use indirect sunlight and so will work even in the north of the British Isles and Ireland. On the downside, they are less effective in winter months when hot water needs are likely to be greater. A backup heat source is needed to make up for this shortfall. Unlike photovoltaic cells, they don’t produce electricity and cannot be connected to the grid.

The amount of roof space needed to install solar water heater collectors is typically only around three to four m2 for an average household. This is slightly less than the space required for photovoltaic cells. The solar collectors can be mounted so that they’re flush with the roof and look just like a skylight. Positioning guidelines require a south facing roof where possible, but southeast or southwest will also work. A 30-45º pitch is ideal, although the panels can normally be orientated to face the sun if the roof is flat. There should also be minimal obstructions from other buildings or trees.

Like photovoltaic cells, solar water heating harnesses the free and clean energy of the sun, which reduces the use of fossil fuels and damaging carbon emissions. A domestic system will provide around 1,600-2,000kWh of clean energy a year, which equates to a reduction of CO2 released into the atmosphere by around 400-1,000kg.

Solar water heating is a well-developed technology which means there’s a vast range of equipment and a good number of installers to choose from. Maintenance is also minimal, with nothing much more than a cursory system check needed every three to five years.

Solar water heating systems can, cost effectively, meet a large proportion of a household’s energy needs. They use a good, clean energy source, have proven to be reliable and can reduce energy bills significantly. This is a renewable energy technology worth looking at more closely.