After having carried out all of these measures, the only solution is to then generate electricity from renewable sources. The key issue with this is that it relies on expensive equipment which has a limited shelf life and deteriorates over time, and so the only way that it can be made to stack up financially is through government subsidy (the RHI), without which these products would never achieve a payback before they needed to be replaced. However, government funding has been set up to encourage the implimentation of these strategies, so there is certainly a financial incentive.
The options outlined below describe the strategies available. Solar panels and wind turbines provide electricity from a renewable source and heat pumps make more efficient use of this electricity once it has been generated.
There are also strategies which do not have the clear and direct impact on the costs of running our homes which are a stronger motivation for a lot of people, but they may be considered by homeowners who are concerned about reducing their impact on the environment. Some of them have complications with their implimentation (which are highlighted below), but they will always have a positive impact on the sustainability of the dwelling.
Our cities are increasingly covered in concrete, tarmac, and other hard surfaces, meaning that the rainwater which used to naturally soaked into the soil now runs straight into the drains all at once, creating a storm-water surge in rivers downstream. Reducing the rainwater run-off from a housing development is an increasingly onerous task as the Utility companies place higher requirements on the builders to ensure that the systems which are already over-loaded are not stretched to breaking point. Every new housing development has large tanks which hold the rainwater back so that it does not flow at once into the sewer system. Although not required by regulations, you can do your bit to reduce the impact of this by taking efforts to reduce the impact of rainwater running off your site by holding it on the site – Green Roofs & Sustainable Urban Drainage Systems (SUD’s) or recycling it – Rain-Water Harvesting (RWH).
Solar panels (Thermal and PV)
Pros
- 12-23 year payback
- Relatively expensive
Cons
- Design life of 25 years
- Reduction in performance of 1% per year
These can take the form of photo-voltaics (PV) which produce electricity, or thermal panels which pre-heat the water supply. They can be installed on most roofs, but work best facing south, or between south-east and south-west. They can be fixed to the outside of you roof (with the tiles or slates below), or built into the roof (i.e. flush with the tiles) or you can use solar tiles for the roof. The second two are more expensive than the simplest solution which is to put it on the outside, and makes repair or replacement a lot simpler 25 years down the line.
A thermal system will cost £3-5,000 for a 4sqm system, and a PV system will cost £5-8,000 for a 4kW system. A solar thermal system will pay back £330/year and a PV system will pay back £280/year and so they achieve payback in 12 & 23 years consecutively. Whilst the cost of these systems has come down a lot in recent years, there has also been a drop in the government subsidy offered to them (the RHI) and so they still take some time to achieve payback, bearing in mind that it is only after this that you are getting free energy, you will be waiting a long time for a return on your investment.
These systems also have a design life of 25 years so they will need to be replaced at some point, and they gradually lose performance with their efficiency dropping by around 1% per year (which simple payback calculations do not take into account).
Green Roofs
Pros
- Attractive feature
- Natural habitat
- Reduces drainage requirement
Cons
- Expensive to install
- Structure needs reinforcement
- No financial incentive
These can be intensive or extensive. Extensive roofs have less than 6 inches of soil and so weigh less, and will have reduced watering and maintenance requirements, but due to the depth of soil the planting options are limited to water retaining plants such as sedum. Intensive roofs hold more water and so weigh more, but can be planted with a much wider range of plants. These make an attractive roof to look out over, especially if they are predominantly seen from above, and they need to be relatively flat to retain the soil on them.
They can form a part of a sustainable drainage system by reducing the rainwater run-off from buildings. Storm water surges are an increasing issue, and large projects have to install large tanks to reduce rainwater run-off, and it is getting more onerous every year with the statistics (which prove global warming). The soil retains the water which falls on it so that it doesn’t go straight into the rainwater pipes and into the drains, which will have a beneficial impact on your local sewer network. And it also creates a small habitat for wildlife, especially insects and butterflies which struggle to find space in our cities which are increasingly concreted over. In this way a green roof makes particular sense in an inner city environment where the issues of habitat and rainwater run-off are more critical and especially if they can be combined with an accessible roof garden which is enjoyed by people.
However, they are expensive to build, adding to the water-proofing needed and the structural requirements of the roof. You will need a specialist waterproofing system to make sure that there are no leaks, and your Structural Engineer will likely need to increase the size of his structure to accommodate the weight of the system (particularly if you go for a more attractive intensive roof). There is also no chance of being rewarded financially for your choice because there is no funding or subsidy for this.
Wind turbines
Pros
- Simple, bolt-on technology
- Small ones can be wall-mounted
- Large ones can produce significant amounts of electricity
Cons
- Expensive to install
- Can be noisy and unpopular with neighbours
- Low energy generation in most urban sites
Wind turbines come in varied sizes from Micro – around 1kW to Small – 2.5kW and Medium – 6kW. The Micro-type can be mounted on the side of a house, although the performance of these is significantly reduced because of the turbulence caused by the roof itself. In terms of domestic-sized turbines the diameter of the blades is roughly equivalent to the rating of the turbine, i.e. Micro will have 1 metre diameter rotors and Medium will have 5.5m wide rotors. The electrical current is produced in DC form, so it needs to be converted to AC with an inverter.
The cost is around £2,000 for a Micro turbine, £15,000 for a small turbine and £30,000 for a medium turbine. The rating of the turbine (measured in kW) is not a measure of expected electricity production, but tested production at a consistent wind speed of 12 m/s. Bearing in mind that this is 30 mph and the average wind speed in the UK is 5.6 m/s and you can see that the rating is substantially more than will be achieved unless you have a very windy site!
Which magazine recently undertook a test of a basic 1kW turbine and found that it used more energy than it generated, because the inverter uses power constantly whether the turbine is running or not! They can also be noisy, generating a sound from the rotors through the air as well as vibrations, which can be deemed a nuisance by passers-by and neighbours. Also, a planning requirement for wind turbines is that they are located the height of the wind turbine + 10% from any site boundary, which in itself is likely to makes them impossible for most urban sites.
However, if there is space for a large turbine, and your site is suitable, they can generate significant amounts of energy.
Rainwater harvesting (RWH)
Pros
- Effectively reduce water consumption
- Reduce run-off into public sewer network
- Can be used on any site
Cons
- No financial incentive
These involve collecting the rainwater from the roof of your house in storage tanks and recycling it for purposes other than drinking, such as watering the garden, flushing toilets & supplying washing machines. The tanks are generally underground from where the water is pumped back up to the level of the bathrooms or kitchen. There are variations which store the water in tanks in the roof eaves, but the size of these is limited by the strength of the ceiling rafters and so the capacity is significantly reduced.
They typically cost £2-4,000, but there is little to no way to get repaid for your investment. In England, water consumption is not usually metered, but if it is you can save some money. In Scotland, however the Water rates are included with the Council Tax so there is no way to avoid them. This may make sense in a location where the water supply is not good or consistent, i.e. it may rely on a stream rather than mains water, in which case minimising your water consumption may be more of a priority.
A simple and economic system which is not quite as glamorous is to install a rainwater butt for watering the garden to get you around a hosepipe ban.
Greywater recycling
Pros
- Effectively reduce water consumption
- Reduce run-off into public sewer network
- Can be used on any site
Cons
- No financial incentive
These systems treat and store the water from showers and sinks and recycles it for toilet flushing and gardens. They need smaller tanks than RWH systems and so cost a little less at £2-3,000. They can also be integrated into a combined rainwater / greywater harvesting system.
One drawback is that you have to assume that there are no serious contaminants in the water which need more effective treatment, such as blood or urine. This may not be possible to guarantee, especially with babies or toddlers.
Like RWH systems, they only really make sense where access to water is limited or metered.
Heat pumps
Pros
- Can provide heat very efficiently
- Make a decent payback with the RHI
Cons
- Still rely on electricity
- Require well-insulated house to be effective
There are a number of types available, but they all fundamentally work in the same way. Electricity is used with a refrigerant which passes through metal coils, and it extracts heat from a material, and provides it to another. In the case of a fridge, it removes the heat from the small, insulated space of the fridge and moves it outside. An air conditioning unit is exactly the same but on a larger scale. Fundamentally they are a great idea because they allow us to create heat more efficiently.
Heat pumps are more effective if the coils pass through a material which is a better conductor of heat, and ideally warmer than air temperature. The ground is consistently warmer than the air, so this creates a better heat pump, and water is a better conductor, and has the ability to flow so the heat can be extracted from it more effectively. Because of the greater efficiency, a ground source heat pump will generate 3-4 times the heat the equivalent electricity would generate, and a water source heat pump is even better, generating 5-6 times the heat.
The operating temperature of a heat pump is generally quite low, heating water to around 30 degrees. A central heating system runs on water at around 80 degrees, so a conventional boiler would still need to do quite a lot of work on the pre-heated water to make it sufficient to heat a house. It is much more efficient if a heat pump can be combined with an underfloor heating system which runs on water at around 30 degrees, and so the boiler does not need to do nearly as much work.
The problem with heat pumps which means they are still allocated into the camp of Eco-Bling is that they still use electricity to generate the heat, which is less efficient than other methods of heat generation. Gas is generally double the price per kWh than Electricity, so an Air Source Heat Pump will not generally give any savings, and a Ground Source and Water Source are not as efficient as they first appear (i.e. 2-4 times better than gas). They can however, still be part of an effective energy saving strategy, as long as the fundamental issue of heat loss has been targeted first.
Air source heat pump (ASHP)
This is a simple, wall-mounted or floor-standing unit on the outside of a house, which looks a lot like an air-conditioning unit. These are the cheapest heat pump to install because of their simplicity, so costing £7-11,000, taking into account a fuel saving of £85/year and an RHI of around £1,200/year, they should achieve payback in 7 years, after which the government is paying you £100/month, which is nice!
Ground Source Heat pumps (GSHP)
It is generally a system of pipes which are laid in a series of loops like an underfloor heating system, under your garden. These are around a metre deep and a metre apart, and should be sized to make sure that the heat they extract from the ground is sufficient for the heating season, and is re-stocked by the sun during the summer. If space in the garden is at a premium, these can be laid vertically in boreholes, although this is a more expensive solution. They cost £13-20,000, and give a potential saving of £200/year compared to a modern gas combi boiler, which assuming a medium cost, to achieve payback in 82 years. So it’s still a big investment for relatively low savings. However, the RHI on these is quite favourable and when taking this into account, it would achieve payback in 4.5 years which is much more healthy.
Water source heat pump (WSHP)
These are a series of loops much like a ground source heat pump, but laid in water and weighted down to prevent them floating to the surface. Again, the system should be sized to make sure that the heat being extracted from the water is sufficient for the heating season, and it can be re-stocked by the sun during the summer. There are cases of water source heat pumps causing bodies of water to freeze solid because they extract so much heat from the water! The body of water therefore needs to be a lot larger than a pond, more like a lake, which does unfortunately mean that this technology is only useful on a minority of sites. They are still relatively specialist, so there is not enough cost and savings data on which to base predictions.
Geothermal energy
This is a variant on a heat pump, but rather than burying the pipes around a metre deep in your garden, they are put into a borehole which travels a lot deeper into the earth and so makes more efficient use of the earth’s heat. They are still quite specialist, and less popular than ground source heat pumps, the main reasons being that they are less accessible if something goes wrong and the pretty substantial outlay to start with.
Off-grid drainage
If you cannot or do not want to connect into the public sewer network, you can store the sewage from your house on site. The main choice is whether you go with a Septic Tank or a domestic Sewage Treatment System. The Septic tank is cheaper, but stores all the waste from the house for extraction at regular intervals by lorry. A Sewage Treatment System treats the effluent so the the liquid is allowed to run-off whilst the solids are stored for collection. Collections are less frequent (and so cheaper) with a Sewage Treatment System. However, in most sites it makes financial and hygienic sense to connect into the local sewer network.
Reed beds
Reed beds can be used to treat the effluent run-off from a house, but this will create a lot of unwanted smells. They also need to be at least 1-2.5 sqm per person which makes them quite large. Both of these reasons make them impractical for most urban gardens! They can be combined with a sewage treatment system which takes out the worst (and smelliest) of the noxious substances first.
Eco-Minimalism
So, having explained how each of these is not the main priority in terms of an Eco-House, I don’t want to make out that there is no point in trying to be Eco, or trying to be better for the environment, but what should you do to make your house into an Eco-House? The answer is to focus your money on the fabric: the insulation of the roof, walls and floor, achieving a high level of air tightness and designing all the details right so that heat loss is dramatically reduced. Installing a heat pump on a traditional house is like a fridge with a door which does not seal. It will never achieve the right temperature and it will take a lot more electricity than necessary. Once your house is as air-tight and well-insulated as that fridge, then the heat pump will start to work. So the focus has to be on the basics.
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