Combustion of fossil fuels
Energy consumed across the globe is primarily achieved by combustion of fossil fuels. This has negative effects on the environment and on global economics. In order to meet the demands of the Kyoto Protocol, and secure ourselves against impending energy vulnerability; renewable sources of energy are required. While some renewable sources are best employed for “macro-generation”, notably hydro-generation; many renewable energy sources are well-suited to the domestic “micro-generation” scale. Though the common domestic renewable energy systems that have been installed in recent times are - in the majority of cases - not proving to be as economically beneficial as expected; they are nevertheless serving a purpose to stimulate interest in domestic renewable energy systems amongst the public; this should increase demand for renewable energy systems, which in turn will lower costs due to mass production.
Of the many forms of renewable energy systems that have emerged, since the introduction of hydro-power at the beginning of the last century; not all are suitable for use as domestic energy solutions. While some have, fundamentally, not changed much since they were first introduced; others are constantly being improved, and developed. In this report the author will focus on those renewable energy systems which are applicable to the domestic renewable energy scene, and moreover those which have been developing rapidly in the last decade. This report will look at bio-energy and geothermal systems, but the main focus will be on wind and solar renewable energy systems; both in terms of electricity generation, and heat energy production.
2. Domestic renewable energy systems
There are a myriad of options available with regards domestic renewable energy systems. Some though are more efficient, more economical, and hold more potential; than others. While hydro-power stations are a well-matured renewable energy system, there are currently no economically feasible modularised micro-hydro systems. For hydro-power to be feasible either a large volume flow rate, or a high head of water, is required; thusly the potential to apply such a renewable energy system on a domestic level is very low, mostly due to high capital investment required, and the relative lack of suitable sites. No major developments towards affordable micro-hydro have occurred in the past decade. Similarly, renewable energy systems such as ocean, wave, and, tidal are not applicable for domestic use. In the following sections then we will look at renewable energy systems that supply heat to homes, followed by a discussion of recent developments in two particular renewable energy systems which are showing much promise.
2.1 Geothermal Energy
Geothermal energy involves tapping into naturally occurring pockets of hot water or steam within the Earth's crust; this method has famously been used in Iceland on a commercial level for quite some time. This type of commercial geothermal energy system is largely confined to regions of young volcanism. However low temperature sources suitable for direct domestic use can be found in most countries, and there has been much development of this renewable energy system in recent times. These developments have led to the application of ground source heat pumps using the earth as a heat source.
Heat pumps are used to create high temperature heat from a low temperature heat source. Working in the opposite direction it can be used as a cooling device. The low temperature input comes from the Sun. In ground-coupled heat pump systems, the surface can be seen as a cheap solar collector and the ground beneath it as a heat storage system. Geothermal heat pumps take advantage of the constant temperature of the earth several metres below the surface, which remains very stable, especially in Ireland due to the climate. The heat pump utilises this constant heat as a heat source in winter and as a heat sink in summer.
There are several types of geothermal heat pumps, generally using water with antifreeze as the fluid. There are a variety of geothermal systems available for residential applications, including water to water heat pumps, water to air heat pumps and hybrid heat pumps. The most common of these used in Ireland is the geothermal or ground heat pump and is generally used for domestic heating. The geothermal heat pump system has three main components: a heat pump, buried piping adjacent to the house and a heat distribution system. The pipes are buried vertically or in a horizontal trench near the building. The purpose of the piping is to transfer the heat to and from the ground. The heat pump is the unit that replaces the boiler in a conventional heating system.
The use of geothermal energy in Ireland is limited to geothermal heat pumps, used for space heating and cooling as well as water heating. The initial capital cost of installing geothermal heat pumps is still relatively expensive in comparison to conventional systems, with a typical payback period of 8-10 years. Currently no electricity is generated from geothermal sources in Ireland. Generating electricity from geothermal energy such as the hot rock technologies employed in Iceland is extremely expensive due to high construction costs.
Geothermal heat pumps are an efficient technology for space and water heating in Ireland. The heat pump typically produces four units of heat for every unit of electricity used. The barrier to geothermal renewable energy systems domestically was that an external ground area, equal to the floor area of the house to be heated was required ; however recent developments have led to heat pumps that extract straight from the open air; making domestic installation cheaper; and opens up geothermal to the urban domestic market. An example of such a geothermal heat pump that extracts low-temperature input heat from the air is used in domestic ventilation systems, where heat is extracted from the stale air being extracted to heat incoming fresh air.
Bio-energy includes energy production (heat and electrical) from biomass, and bio-gas. Biomass is a term that includes wood, kitchen waste, harvest residue, and sewage. While solid biomass such as wood logs, and compressed straw, are well known and used; the potential of biomass is seriously developed only when it is converted into liquid or gas; and it is in this area that major developments have been taking place, with regards bio-energy. Bio-liquids include ethanol, which can be obtained by fermenting crops such as apples, potatoes, corn and sugarcane. This is mostly being used as bio-fuel for cars. Currently the best use being made of biomass is as an alternative to traditional fossil fuels for boilers, and indeed solid fuel fires in homes. Wood chips from willow, and a new solid fuel consisting of processed Miscanthus grass compressed into briquettes are the latest developments in bio-energy. These are produced from fast-rotation crops which are harvested on three-year and annual cycles respectively. They can be used to replace coal and oil in the home, to provide space heating, and hot water.
Bio-gas is produced by passing steam over a burning solid biomass, with the resultant production of a hybrid gas consisting mostly of hydrogen, carbon monoxide, and methane. The latest technology in gasifiers can produce a gas which is 90 per cent hydrogen and carbon monoxide, a highly reactive mixture capable of running a turbine. Another bio-gas is somewhat impure methane that is produced by the diverse mixture of biomass contained in landfill sites. This is not currently being pursued as a domestic renewable energy system, but is undergoing major development on a municipal level; while bio-gas methane from agricultural waste is continuing to develop. For the domestic scene anaerobic digesters are doing a good job of waste-disposal, producing fertiliser along with methane. However generally just the resultant fertiliser is of use, as the quantity of methane produced is not suffice for the household to invest in equipment to clean, store and use this gas. To avoid emission of dioxins bio-gas fired furnaces need to operate at 850° C, which understandably is only realistic on a commercial scale. It should also be noted that bio-energy is not a particularly efficient way of harnessing solar energy, capturing only one per cent of the energy that is available to it. The long-term implications of the widespread use of biomass as a feasible renewable energy system need to be recognised. Should global demand lead to vast swathes of land being put under biomass energy crops, soils will quickly be sapped of their nutrients, these nutrients need to be replenished somehow; this author then does not believe biomass is the answer to our energy predicaments, neither commercially, nor domestically. For this reason then we will concentrate on the two most promising and far more efficient, renewable energy systems that use solar energy: wind and direct solar (photovoltaic/solar thermal).
3. DEVELOPMENTS IN DOMESTIC WIND TURBINES
Beyond doubt wind energy is the highest profile renewable energy source today, thousands of commercial wind turbines have been installed, with California and Denmark leading the way. Here in Ireland, with wind turbines popping up along our coastlines and upon our mountain ranges, wind power is capturing people's imagination more than any other renewable energy system; and it has been found that people are willing to install this technology domestically.
Where does the wind come from? Similar to almost all other renewable energy systems it ultimately comes from the Sun. Winds develop when solar radiation reaches the Earth's highly varied surface creating temperature, density, and pressure differences. Tropical Regions have a net gain of heat due to solar radiation, while Polar Regions are subject to a net loss. This means that the Earth's atmosphere has to circulate to transport heat from the tropics towards the poles, wind is a result of convection then. The Earth's rotation means Coriolis' forces further contribute to planetary-scale circulation patterns in the atmosphere. Topographical features and local temperature gradients also alter wind energy distribution.
The performance of a wind turbine is determined by the wind regime in the location, and by its power curve. The power available in the wind is proportional to the wind speed cubed, as well as the square of the blade length (i.e. radius of area being swept out). (Please see Appendix 1 for more on these power laws) These cube and square laws mean the output is extremely sensitive to wind speed, and subjected to very large forces during periods of unusually high winds. In this section then we will look at how the latest developments in wind turbines are addressing this issue.
3.1 Horizontal Axis Wind Turbine
Onshore commercial designs have converged on the horizontal axis propeller-type wind turbine. There are two types 1) stall-regulated, with fixed blades shaped to limit output at high winds, and 2) pitch-regulated, where blade angle is adjusted to control output, this type is capable of capturing more energy at low wind speeds. Both types drive an asynchronous generator through a step-up gear box.
Horizontal axis wind turbines have 3 “lift” blades (rather than “drag” blades), which point into the wind stream, using direction sensors and large hydraulic “yaw” motors. The blades are all attached to the hub, which rotates at 20 - 60 rpm; this in turn rotates a shaft connected to gears that step-up the rotational energy speeds about 50 times up to 1200 - 1500 rpm, to run the generator; which feeds DC into an inverter that converts the DC to AC, which can be used in the home. In smaller domestic turbines the yawing is passive where the fin behind the turbine causes it to point in the direction of the wind stream.
3.1.2 Developments in Horizontal Axis Wind Turbines
The Skystream® turbine is manufactured in the US and is an all-inclusive wind generator (with controls and inverter built-in) which is designed to be very quiet and to connect to the grid. The Skystream® turbine is a downwind machine with a 3.7 m rotor diameter, and is probably not suited to inner urban homes. The manufacturer also recommends mounting it quite high in the air, at least 6 m higher than anything within 100 m of it. The power curve shows a cut in wind speed of about 3 m/s and a rated 12 m/s (2400 W). At 5 m/s it will generate around 200 W. significantly greater power can potentially be generated then if your home is in a rural location.
The Swift® turbine (Fig.1) is a 5-blade, 1.5 kW turbine manufactured in the UK. It has a circular rim around the blade tips which dramatically reduces noise. Like the Turby®, the Swift® cuts in at around 4 m/s, however it peaks at a lower 12 m/s. Again, the output at 5 m/s is approximately 100 W.
Motorwave, a company in Hong Kong, have invented a wind system which consists of an array of small, cheap plastic rotors. These can be purchased online in kits of either 8 or 20 turbines. The idea is that the individual turbines are built up as an array. Not including delivery, 8 turbines cost €100 and 20 turbines cost €150. Obviously these turbines are built to be used in large arrays and the output at 5 m/s is too small to predict in the 8 and 20 turbine arrangements. It would be more useful to produce a power curve for a larger array of the turbines. This wind turbine system is a significant development; it is modularised and affordable, allowing domestic users to continuously build up their array as they have money available to do so.
3.2 Vertical Axis Domestic Wind Turbines
While horizontal axis wind turbines have become the norm, vertical axis wind turbines may be more practical for domestic purposes, particularly if infiltration into urban domestic market -where space is an issue - is desired. There are two types of vertical axis wind turbine: Darrieus and Savonius. Darrieus turbines are lift-based and generally have a few blades (typically three) joined to the central axis at their top and bottom. Savonius turbines, like anemometers, are drag-based, this means Savonius turbines can never have a tip speed ratio (the ratio of blade tip speed to wind speed) greater than one, and the reduced rotational speed results in quieter operation. Of the two types of vertical axis wind turbines, Darrieus are significantly more efficient and it is this style which is being developed for domestic use. However they still have a lower aerodynamic efficiency compared to a lift-driven horizontal axis wind turbine, because the airfoils periodically stall during each revolution.
Since vertical axis wind turbines typically have fewer moving parts and a lower tip speed ratio than horizontal axis wind turbines they have the advantage of being significantly quieter. This is important when considering a turbine for an urban environment. Not having to yaw and face into the wind, vertical axis wind turbines are also less sensitive to changing wind directions and therefore to turbulence. This is a very important advantage for the urban environment. A disadvantage of the vertical axis wind turbine is that they do not generally self-start. That is, they require power from the grid in order to start when adequate wind is blowing. However recent developments have led to a specialised designs allowing the rotor to divert the air; thus avoiding this problem. Horizontal axis wind turbines are still the most efficient in a clear wind stream and almost all of the domestic turbines currently available on the domestic market are horizontal axis machines; however we will now look at some vertical axis wind turbine for the domestic market which are beginning to emerge.
3.2.1 Developments in Vertical Axis Wind Turbines
A company called Altaus® have developed a vertical axis turbine for the domestic market (Fig.2). This turbine has been available since January 2008. As can be seen from Fig.2; this turbine is ideally suited for an urban dwelling. The Turby® (Fig.3) is a reasonably well-known Dutch vertical axis wind turbine designed to be mounted on top of flat roofs. It has a 2 m diameter and is 3 m tall, and is rated at 2500 W at 14 m/s. Several Turbys have been installed in Europe. The power curve for the Turby shows a cut-in speed of 4 m/s and a peak output at 14 m/s, followed by a shutdown of the machine at higher wind speeds. At 5m/s the turbine output is around 100 W. The NGUp WindWall® is a very different turbine; it is similar to a vertical axis wind turbine but is installed horizontally. It is designed to be mounted on the edge of buildings and capture updrafts.
3.3 Developments in Technology & Performance
Turbines on average only generate 30 per cent of the time, due to seasonal and daily wind variations. Currently potential sites with mean wind speeds of at least 7 m/s at the rotor axis are necessary; in order for any significant economic benefit to be gained. Clearly then if wind turbines are to become common-place in domestic dwellings, improvements in their technology, and their performance are required.
Problems with wind turbines are primarily related to noise. This is a critical issue for small turbines especially in the urban context, as they will be placed very close to houses Noise is generally related to tip speed, which is why slower-moving vertical axis wind turbines tend to be quieter than horizontal axis wind turbines. Noise emission has prevented designers from increasing the tip speed of the rotor blades, which could increase the rotational speed of the drive train shaft, and thus reduce cost of gear-boxed generators. While direct drive units are being developed to eliminate gearbox noise; it is the air flow over the fast-moving blade tips that can cause the high pitched whine commonly associated with small turbines.
One of the most notoriously noisy turbines was the Air 403 model, which preceded the current Air X. 54 dBA noise levels were measured, which exceeded the limit of 45 dBA and many Air 403 model wind turbines had to be shut down permanently. Control improvements in the Air X, which stall the blades when rotor speed exceeds set limits, reduced the occurrence of flutter induced noise.  Acoustic research has provided configurations to make blades considerably more silent, reducing the distance needed between wind turbines and houses.  Recently developed turbines such as the Swift® in the UK have modified the traditional horizontal axis wind turbine design to include a ring around the circumference of the blade tips (Fig.1). This ring prevents some vibration of the blade tips and is claimed to dramatically reduce noise. According to Swift® technical information, “acoustic suppression aerodynamics, notably the patented diffuser, removes the noisy, turbulent vortices at the blade tip”. The Swift's technical specifications list the acoustic emissions as less than 35 dBA for all wind speeds. The UK's Department of Trade and Industry (DTI) wrote a report called ‘The Assessment and Rating of noise from wind farms'. The report states that as a general rule, noise emitted from any turbine should not exceed 5 dBA above background noise, with a fixed limit of 43 dBA recommended for night time.
Turbulence is a problem with yawing systems as the turbine is constantly rotating around the vertical axis causing wear and tear on that bearing. Horizontal axis wind turbines also have problems with over speeding when running in an unloaded state resulting from grid failure or battery bank disconnections. A free spinning turbine causes extra stress on components and can result in self-destruction. The latest domestic horizontal axis wind turbines are now equipped with an over-speed mechanism, such as ‘furling'. Furling is when a turbine bends out of the wind during very high wind speeds and thus protects itself from the full force of the gusts.
The inverter turns the DC output of the wind turbine into useable grid-compliant 230V AC. It is also an important safety device, and approved inverters will shut off the grid supply in the event that the grid fails or is switched off for maintenance. As it is vital that the wind turbine is safe and that the power is conditioned to match grid power, inverters require testing and approval before they are allowed to be connected to the grid. There is no single standard which is used for this approval, as some inverters have high frequency transformers, some have standard frequency transformers, and the very latest inverters operate without a transformer. Most approved inverters on the market are designed for PV panels; however they are, in principle, suitable for any small generator. In practise though, minor adjustment of a software set-point makes them more suited to wind.
The only inverter designed specifically for domestic wind turbines is the WindyBoy ®, made by the manufacturers of the popular and already approved SunnyBoy® from Germany. The WindyBoy® has not yet been approved, however according to the manufacturer's specification the SunnyBoy® and the WindyBoy® are identical and can each be used interchangeably, with the exception of a single software set-point. It is required to reduce the delay period in the software to make the inverter more suitable for the intermittent nature of wind power.
3.3.4 Further Problems & Developments
The rotation of the blades is said to cause interference with radio and TV signals; as well as birds. Studies have found electromagnetic interference and bird strike are unlikely to be significant problems with small-scale domestic turbines. It is claimed that the blades of wind turbines ‘chop up' television signals causing a ghosting effect on screen. According to the American Wind Energy Association this has occasionally happened where a wind farm is in the line of sight between a television transmission tower and the residential reception antenna. This was perhaps 20-30 years ago and caused by large turbines with metal blades. Modern domestic wind turbine blades are far too small to cause this effect. Additionally blades are now made from fibreglass, wood and plastic which are all materials that are transparent to electromagnetic signals. As such, the advances made with these materials for manufacturing blades means that there is little threat of electromagnetic interference from small wind turbines. It is unlikely that micro-wind turbines will cause a significant increase in bird strike, beyond those already arising from birds flying into existing buildings, windows and other obstacles.
A further problem is the quality of the power generated by wind turbines. Phase-compensating capacitors are now being incorporated to ensure that the induction generators do not take reactive power from the distribution network. Wind turbines should not run unloaded. In an unloaded situation a turbine may spin too fast and eventually self-destruct. If the turbine is generating power but there is nowhere for that power to go, it must be ‘dumped'. Such a load dump is necessary in the case of grid failure, or when the power company shuts the grid down for maintenance. Also, any power generated in the delay time before the inverter connects the turbine to the grid goes to the load dump. Load dump is also necessary in times of extreme gusts, as only a certain amount of current can be drawn out of the generator before it overheats. New technology allows for this dumped energy to be harnessed for something useful, such as heating water.
Wind-power systems for single households have been piloted in a few countries and performance has been good, by factoring in these latest developments discussed above, wind turbines can certainly be considered as a feasible option, to reduce a domestic dwelling's dependence on grid-electricity. However reports from these pilot schemes have shown that in summer the winds drop, leading to many households upgrading their systems with solar photovoltaic systems to complement their wind resource.
4. Developments in SOLAR renewable energy systems
Solar renewable energy systems include photovoltaic solar panels, and solar thermal panels. The roofs of many domestic dwellings have become adorned with these solar panels in recent times. The reason for their new-found popularity is due to the many improvements in both their technology and performance over the past decade.
As with wind turbines, the power available from solar-generation varies with time, and weather; however by combining the two an effective renewable energy system for the domestic dwelling is realised.  It is worthwhile then looking at recent developments in solar renewable energy systems.
4.1 Photovoltaic Solar Panels
Until recently the efficiency of photovoltaic solar panels has been stagnant at an average 15 per cent. Energy losses were being caused by reflection, internal resistance, and recombination within the photovoltaic cells. Thin-film cells are also being developed using amorphous silicon, along with copper indium diselenide, and cadmium telluride; with the aim of reducing costs, as semiconductors made with pure silicon wafers are costly due to the rarity of silicon. By employing new thin-film cells photovoltaic panels are now comparable in price with the most expensive cladding used on high-spec domestic dwellings. Thin-film technology then offers the best long-term prospects for very low production costs and a reduced energy pay-back time of less than one year. However the latest technology now uses multi-junction photovoltaic cells and efficiencies of over 30 per cent are being reported. It has also been found that higher efficiencies can be obtained by stacking cells with different optical properties.
The amount of solar radiation intercepted by Earth is more than three orders of magnitude higher than annual global energy use (table 5.18). But for several reasons the actual potential of solar energy is somewhat lower. The amount of solar energy available at a given point is subject to daily and seasonal variations. So, while the maximum solar flux at the surface is about 1 kW per square meter, the annual average for a given point can be as low as 0.1-0.3 kW per square meter, depending on location. For large-scale application of solar energy—more than 5-10 percent of the capacity of an integrated electricity system—the variability of insolation necessitates energy storage or backup systems to achieve a reliable energy supply. The availability of solar energy also depends on latitude. Areas near the equator receive more solar radiation than sub-polar regions. But geographic variation can be significantly reduced by using collectors capable of following the position of the Sun. Polar regions show a notable increase in irradiance due to light reflection from snow.
The efficiency of an ideal photovoltaic cell is about 30 percent at most (for a single cell under natural sunlight). Higher efficiencies can be achieved by stacking cells with different optical properties in a tandem device, by using concentrator cells, or by combining these two. Solar cells for concentrator systems are mounted in a one-dimensional or two-dimensional optical concentrator.