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Ground-source heat pumps (GSHP) are receiving increased interest in the construction industry as the demand for renewable energy sources grows. GSHP's are already a well established technology in the North America and parts of Europe, with the UK slowly following suit (Omer 2008). The reason for the increased interest in GSHP's is their potential to reduce primary energy consumption, thus reducing emissions of green house gasses. This reduction of green house gasses into our global environment has the potential to be the solution to countering global warming and global climate change, securing the wellbeing of future generations.
How It Works and Aesthetic Appearance
GSHP's operate on the principle that heat can be moved from a warmer temperature to a cooler temperature, and using a mechanical device combined with this principle, both heating and cooling can be achieved. A refrigerator is a prime example of these principles put in to affect. The refrigerator uses a heat pump to extract the heat from the food inside, and disperses it out the back of the unit. A GSHP works using the same principle, but uses the temperature captured by the earth from direct radiation from the sun. It can achieve heating and cooling because below the frost line, the ground is a constant temperature, on average 8°C in the UK (Invisible Heating Systems 2007), which is warmer in the middle of winter, and cooler in the middle of summer than the outside air temperature. The process of elevating low temperature heat to a high temperature heat and transferring it indoors involves a complex cycle of evaporation, compression, condensation and expansion (Ultralligence Corp 2011). The heat transfer from the ground to a building can be used for space heating and, in some cases, to pre-heat domestic hot water.
GSHP's comprise of three important elements;
The ground loop - which comprises of lengths of pipe buried in the ground, either; vertically in a borehole, straight horizontal or spiral horizontal in a trench. The piping is usual a closed system or open system, and contains a mixture of water and antifreeze which absorbs heat from the ground as it is pumped round the circuit.
A heat pump - which comprises of three main parts:
The evaporator - that takes the heat from the water in the ground loop.
The compressor - moves the refrigerant round the heat pump and compresses the gaseous refrigerant to the temperature needed for the heat distribution circuit.
The condenser - gives up heat to a hot water tank, which feeds the distribution system.
Heat distribution system - consisting of under floor heating or radiators for space heating and in some cases water storage for hot water supply (Omer 2008).
Figure 1 - GeoExchange System (Heating Mode) (Source: Ultralligence Corp 2011).
Figure 2 - GeoExchange System (Cooling Mode) (Source: Ultralligence Corp 2011).
As the GSHP's element is buried under ground, there are no aesthetic compromisers.
Efficiency and Output
GSHP's operate most efficiently in climates like North America and Europe (MacKay 2009) were the climate is mild to moderate and the ground temperature stays at a constant, between 10-13°C (Omer 2008). This constant ground temperature is the key to the heat pumps success, as in winter the air temperature above the ground is below this figure so the heat exchange can provide adequate heating. Were as in summer the air temperature above ground is higher, so adequate cooling can be achieved using the heat exchange principles.
Heating efficiency is expressed in coefficient of performance (CoP), the higher the CoP the more efficient the heating system is. CoP is a reflection of the percentage of efficiency, for example it is agreed among authors that a standard condensing boiler run's at around 90% efficiency depending on make and model, so would have a CoP of 0.9. Typically GSHP's have a CoP of 3-4, which reflects as a percentage as 300-400% efficient (MacKay 2009). This huge efficiency is due to the amount of electricity it takes to power a GSHP and the amount of heat power that is transferred through the heat exchange. Typically an electric heat pump will take 100kWh of power to turn 200kWh of freely available waste heat from the ground into 300kWh of useful heat (Omer 2008). Also as the heat pump use's less primary energy than a conventional heating system, it produces fewer emissions of harmful greenhouse gasses (GHG) that destroy our environment like; carbon dioxide (CO), sulphur dioxide (SOâ‚‚) and nitrogen oxides (NOx). This is shown on figure 3 below. However there is conflicting evidence to how much COâ‚‚ GSHP's produce compared with conventional heating systems. The Energy Saving Trust in 2004 have worked out, assuming an average COâ‚‚ emission factor for electricity of 0.414 kg/kWh, that using a GSHP with a CoP of 3.5 would result in 0.12 kgCOâ‚‚ for every kWh of used heat provided. They then compare this with a condensing gas boiler, assuming a COâ‚‚ emission factor for gas of 0.194 kg/kWh, operating at an efficiency of 85% (CoP 0.85) would result in 0.23 kgCOâ‚‚ for every kWh of used heat supplied. This means that the COâ‚‚ emissions from a gas condensing boiler would be double those from a GSHP (Energy Saving Trust 2004). As stated above COâ‚‚ is a GHG and when comparing GSHP with conventional heating and cooling systems that use fossil fuels, another conflicting source say's, GSHP's can reduce GHG emissions by 66% or more (Omer 2008). There are many different calculations and variables (such as climate, type of system used and cost/efficiency of electricity) that affect the efficiency ratings of GSHP's, but as the technology develops, efficiency ratings improve. The Japanese government legislated a decade-long efficiency drive to improve the performance of air-conditioning units, and have so far managed to reach CoP ratings of 4.9.
Table comparing efficiancys of tech.bmp
Figure 3 - Comparison of different heating systems. (Source: Omer 2008).
GSHP's also provide cooling in summer which is measured in energy efficiency ratio (EER). EER is the cooling capacity of the unit, divided by its electrical input (in watts), at standard conditions of 25°C entering water for closed-loop models and 15°C entering water for open-loop systems (Federal Energy Management Program 2011). As with the CoP, the higher the EER the more efficient the system is. Typically an open-system has a cooling EER rating of between 11 and 17 and a closed-loop system has an EER rating between 10.5 and 20.
Cost and Payback Period
The cost of installation of a GSHP is far more expensive than some natural gas, oil or electrical boilers and heating units (Omer 2008). But compared with a system that provides both heating and cooling GSHP's are quite competitive, and when taking into account the energy running cost from the grid, GSHP's appear to be the more attractive option (Energy Saving Trust 2004).
There are a huge number of variables to contend with when calculating the costs and payback period of installing and implementing a GSHP, a few of these are;
the size of the project
the climate conditions
the heating and cooling loads required
different company options
the efficiency of the selected option in terms of Cop and EER
local energy costs
increasing energy costs as fossil fuel reserves decrease
lifestyle habits (Omer 2008).
Another variable that is less obvious, and is not commented on by authors, is the technological advances that are achieved. As more emphasis is put on renewable energy production, more money is invested by governments into the improvement of the technology. As new more efficient systems are released and sold, the price of the old system is reduced. This fast moving industry pricing is reflected in some of the old literature. The 2008 journal written by A.M. Omer titled 'Ground-source heat pumps systems and applications' states that 'a closed-loop ground-source system may cost as much $20,000' (Omer 2008, p. 367) (£12,067). Where as the Energy Agency quoted £8000 for a similar system for residential use in 2011 (Energy Agency 2011). Energy installations are often priced by how many kW of energy you get per pound, and are represented like this, £/kW. A quote from the Energy Saving Trust in 2004 say's that a typical horizontal GSHP system would cost £1000 per kW, so if you wanted an 8 kW system installed, it would cost around £8000 (this price is excluding a heat distribution system).
A case study called Fort Polk's was conducted in Louisiana, America for GSHP's to replace the space conditioning systems for 4000 military family homes. The 4000 houses were situated on 5.6 million square feet of land and the GSHP's were replacing air-source heat pumps (or in some case's central air/gas furnace combinations). In total 6600 tonnes (23205.6 kW) of cooling was installed to supply the 4000 houses along with some other retrofit efficiency measures like; low-flow shower heads, compact fluorescent lamps and attic insulation. Including all this measure the project cost in total $19 million.
Once the project was completed an independent evaluation revealed that the project's total saving in electricity were 25.6 million kWh, which represents 33%, for a typical year. The maintenance costs were also reduced by 77% which lead to a total saving on energy and maintenance of approximately $3 million per year. This project was financed by an energy service company under an energy savings performance contract (ESPC) which meant that part of the savings were passed on to them (Department of Energy 2001). Figure 4 shows the reduced daily energy use after implementing the energy saving strategy. Each data point represents the electrical usage of 200 homes (one electrical feeder) on a given day (Department of Energy 2001).
This case study shows how effective GSHP's can be on a large scale and that after just 6 years, the annual energy savings of $3 million each year, can pay for the whole project.
Figure 4 - Graph to show the difference in energy use after implementation of GSHP. (Source: Department of Energy 2001).
Photovoltaic Solar Panels
Photovoltaic (PV) solar panels convert sunlight into electricity (MacKay 2009) without emitting any harmful pollutants (Meral and Dincer 2011). PV panels have received a huge amount of interest for creating electricity from both governments and the construction industry. There has been significant investment into PV over the last decade with countries like; Spain, Germany, USA, Japan and China leading the way with the amount of gigawatts (GW) produced. As more interest and investment has been put into this technology, further advances have been achieved in the efficiency and the output of PV panels. However there is a lot of conflicting literature surrounding PV as solar radiation is intermittent.
How It Works and Aesthetic Appearance
A typical PV system uses panels of solar cells, made from semi-conducting materials that react with sunlight to produce electricity (Midwest Green Energy 2007). This electricity is the transferred to DC (direct current) for home application or to be sold to the grid. A solar cell consists of two thin layers of semi-conducting materials, such as silicon (Si), that have been treated with chemical substances to form an electric field, positive on one side and negative on the other. When sun light hits the solar cell, electrons are knocked loose from the atoms in the silicon (or semiconducting material). The electrons that have been knocked loose are then captured in the form of an electrical current by attaching electrical conductors to both the negative and the positive sides of the cell (NASA 2002). These solar cells can be electrically connected to each other and mounted in a support structure or frame which is called a photovoltaic module. Multiple modules can also be wired together to form an array, which is shown in figure 5.
Figure 5 - Photovoltaic array make-up. (Source: NASA 2002).
There are four different types of photovoltaic technology; Mono-crystalline, Poly-crystalline, Amorphous and Hybrid. Each technology has different characteristics and performs different in certain light conditions (Horizon Renewables 2010). The performance of PV is measured in kilowatt hours per kilowatt peak (kWh/kWp) which represents electrical units (kWh) and maximum output the panel can produce (kWp) (Horizon Renewables 2010). However the rate of output at peak does not necessarily reflect the overall performance of the technology, as the amount of solar radiation is the major factor when selecting which PV technology is suitable. Figure 6 shows how the four different technologies perform as the light condition improve.
Figure 6 - Graph illustrating comparative outputs of typical PV technologies based on the amount of solar radiation. (Source: Horizon Renewables 2010).
Figure 6 clearly shows that the hybrid technology is the best all around performer and produces more power in less favourable light conditions. L.El Chaar et al. is of the same opinion that hybrid PV panels are the superior technology, and states that the hybrid panel, made up of amorphous-Si and multi-crystalline-Si, results in higher efficiencies of about 8-9% more than amorphous Si.
Aesthetically, PV solar panels can be an eyesore for some residents. Figure 7 shows an example of poor design and bad workmanship that has resulted in, what has been called an eyesore. Figure 8 shows an example of how, when fitted properly, PV panels can look attractive. It is however, personal opinion weather you like the look of solar panels or not, and weather you do or do not like them, they are a physical reminder of our ever increasing battle with energy demands.
Figure 7 - An example of a poor PV installation. (Source: Housing Energy Advisor 2011).
Figure 8 - An example of an aesthetically pleasing PV installation. (Source: Energy & Environment Ltd 2005).
Efficiency and Output
As there are four different types of PV panels, the efficiency and outputs will be reviewed separately, as they all have different characteristics.
Mono-crystalline PV (MCPV) cells are cut from a single crystal of silicon, which is grown from highly pure molten silicon, and are 'generally considered to be the workhorses of the PV market due to their high efficiency and dependability' (Horizon Renewables 2010). This type of cell in solar electrical production is the most commonly used and constitutes about 80% of the market (L.El Chaar et al.). The maximum efficiency under standard test conditions (STC) has reached around 23%, with the highest ever recorded efficiency reading of 24.7% (L.El Chaar et al.). However module efficiency always tends to be higher than in actual practice because of the variables affecting the panels operation like; angle of panel, direction of panel in comparison to the sun, wind and shading from clouds or other buildings. The actual efficiency of these cells, in practice, is agreed among authors and is typically 13-17% (Evo Energy 2011) or 15% (Meral and Dincer 2011) efficient. The output of an installation of MCPV depends on the size, site of the installation and the variables that affect that particular site. Assuming a reasonable tilt, orientation and system efficacy, the following output can be used as a rule of thumb for a crystalline array, 1m² will provide a useful output of 90-110 kWh per year (GreenSpec 2010).
The performance of MCPV technology is 800 kWh/kWp (Horizon Renewables 2010).
Poly-crystalline PV (PCPV) cells are also produced from molten silicon, but instead are manufactured using a casting process. The silicon is heated and cooled very carefully, set in a mould, then cut into wafer think slices of 0.3mm (Evo Energy 2011). PCPV modules are slightly cheaper and slightly less efficient than MCPV modules (L.El Chaar et al.). There is little debate among authors of the efficiency rating for PCPV, with a typical module efficiency of 11-15% (Evo energy 2011) and 12% (Meral and Dincer 2011). However Mitsubishi in 2010 broke the world record for Solar Cell Conversion Efficiency when they released a PCPV cell that delivers 19.3% efficiency (Mitsubishi 2010). The record was previously held by them at 19.1% in 2009, which shows how quickly technology is advancing and how much investment of time and money is been put into improving renewable technology.
PCPV modules don't run very efficiently in lower light conditions as shown in figure 6, so wouldn't be suitable for a climate with lower radiation levels or continuous cloud cover.
The performance of PCPV technology is 750 kWh/kWp (Horizon Renewables 2010).
Amorphous PV cells, also known as Thin Film (Horizon Renewables 2010) are non-crystalline silicon (Evo Energy 2011). The Thin Film panels are created by depositing the silicon atoms on to glass in a thin homogenous layer (Meral and Dincer 2011). The lay of semi-conductor material is only 0.5-2.0um thick, where 1um is 0.001mm (Evo Energy 2011). As this layer is so thin and less materials are been used, it has a cheaper manufacturing cost than the other three types of PV cell. Thin Film PV is said to hold the key to reducing the cost of PV array by lowering the materials and manufacturing, without jeopardizing the cells lifetime (Chaar et al. 2011). Thin Film PV cells are a relatively new technology that currently holds 15% of the market (Meral and Dincer 2011). It is in the last 10 years that production of Thin Film PV has taken off, now with just as much been shipped out of the U.S as there is other PV technologies, this is shown in figure 9 (Chaar et al. 2011).
Figure 9 - Showing the increase in Thin Film shipments from the U.S from 2001 to 2008. (Source: Chaar et al. 2011).
Meral and Dincer state that the efficiency of Thin Film technology is 6% and is its biggest disadvantage (Meral and Dincer 2011). Evo Energy somewhat agree with Meral and Dincer and state the efficiency of Thin Film PV is 6-8% (Evo Energy). Another disadvantage of Thin Film technology is outlined by Horizon Renewables, they say that Thin Film PV modules require more space than the other three PV modules for the same comparative output. For a 4 kW module a Thin Film would require 50m² where as a MCPV module and a PCPV module would require 25m² and 30m² respectively (Horizon Renewables 2010), this is shown on figure 11. As a result of this large sized module configuration Thin Film PV panels are not suitable for residential building use, but do have other applications like; curved roofs, bus shelters (figure 10) and solar portable devices like calculators (Horizon Renewables 2010).
Figure 10 - Flexible amorphous PV panels. (Source: Horizon Renewables 2010).
The performance of amorphous Thin Film technology is 850 kWh/kWp (Horizon Renewables 2010).
Hybrid PV cells are made up of two different types of PV technology, MCPV and an ultra-thin amorphous silicon PV layer (Evo Energy 2011). A Hybrid PV module provides the best all around performance, but at a premium price (Horizon Renewables 2010). Usually the size of technology and the cost go hand in hand, with technology getting smaller and more efficient, the price get higher. This is true also for PV cells as the Hybrid module produces the most output per m², which is demonstrated in figure 11. The small size and the large output of the Hybrid configuration mean it is ideal for residential application where there is limited roof space (Horizon Renewables 2010).
Figure 11 - Output per m² for all four PV systems reviewed. (Source: Horizon Renewables 2010).
The efficiency of the Hybrid system is in excess of 18% (Evo Energy 2011) and the performance is 900 kWh/kWp (Horizon Renewables 2010).
Cost and Payback Period
As with all renewable energy sources, the cost and payback for a system is unique to the project instalment and the site conditions. Therefore typical system installations will be reviewed with various energy outputs and various system efficiencies.
Evo Energy states that an initial investment of £9000 would install a system of around 1.5 kWp and as a rule of thumb a PV system cost around £2000-£4000 per kWp (Evo Energy 2011).
To work out the payback period for a typical installation requires all the relevant data like; cost of whole system, output of system, how much energy that is used, how much energy can be put back into the grid, the cost for both of these, the annual benefit, life time of the system installed. If 12, 180 we modules were installed on to a roof, this would amount to a total system size of 2.16 kWp. A system this size would cost £10,200 and give a projected yield of around 1846 kWh, which would qualify for the 0kW-4kW Clean Energy Cashback Rate of 41.3p per kWh (ANC 2010). Assuming that 70% of the energy generated at this residency is used on site and 30% sold back to the grid and estimated annual benefit can be worked out at £933.60. This annual benefit combined with the initial system cost would result in a payback period of 10.92 years and a total return on your investment of 9.15% presuming a total life expectancy of 25 years (ANC 2010). These figures provided by ANC, even though they are estimates and projection, represent typical figure for a residency and a PV system, and are quite accurate when compared with other estimates. An estimate for a 4 kW system from Horizon Renewables also has a payback period similar to ANC's projection, this is shown in figure 12.
Figure 12 - Typical payback period for a roof mounted 4 kW PV system. (Source: Horizon Renewables 2010).
Wind energy is the fastest growing renewable energy source. By the end of 2010 the worldwide wind capacity reached 196630 Megawatts (MW), with China leading the way with 44.73 MW and the UK with 5.2 MW installed (WWEA 2011). Wind energy has received huge interest and investment in the last 15 years and is said that 'wind power holds the most promise to make a significant impact on reducing carbon output' (GreenSpec 2010).
How It Works and Aesthetic Appearance
Wind turbines work by converting kinetic energy in the wind into electrical energy for distribution or on-site use. This is achieved when the wind blows the blades and causes them to rotate. The rotation of the blade causes a shaft to rotate inside the nacelle (the box on top of the turbine), which in turn goes into a gearbox and increases the rotation speed enough to power the generator. The generator uses magnetic fields to convert the rotational energy into electrical energy in just the same way that normal power stations operate. This electrical energy then goes into a transformer which converts the electricity coming out of the generator to the appropriate voltage to be distributed (Renewable UK 2010). To ensure the wind turbine is facing the right direction the nacelle is fitted with a wind vane which measures wind direction and communicates with the yaw drive to orient the turbine properly with respect to the wind (EERE 2010). The nacelle is also fitted with an anemometer to measure the wind speed, this is so if the wind conditions are to strong and there is a chance the equipment could get damaged, they can switch it off. An image of a wind turbine nacelle is shown below in figure 13.
Figure 13 - Inside the Wind Turbine. (Source: EERE 2010).
The aesthetics appearance of wind turbines generates mixed views. Some people recognise the need for them and appreciate that sacrifices have to be made to produce energy. But some people believe that wind turbines are scaring our landscape. Aesthetics is a very individual and opinionated subject, which means that there will always be someone who does like the look of something. As more and more pressure is been applied to governments about carbon reduction and greener energy, more of the landscape will have to be compromised if the lavish 21st century lifestyles are to continue.
Efficiency and Output
As with most energy sources, the efficiency and output can be measure by looking at the variables of a particular site and particular piece of plant. Wind energy has received the most scrutiny out of all the renewables for output and with many pieces of conflicting literature it is hard to choose what to believe.
Most importantly is that the wind speed must be determined at the proposed site. Wind speed is measure in meters per second (m/s) and will be different at every site. Below, figure 14, is a typical power curve over a year for a 2.5 kW wind turbine, and it can be seen that an increase in the average wind speed by 2 m/s, from 4 m/s to 6 m/s, can triple the energy output annually from 2000 kWh to 6000 kWh.
Figure 14 - A typical power curve for a 2.5 kW downwind turbine. (Source: Horizon Renewables 2010).
Another source states that a 1 m/s increase in wind speed can reduce electricity generation costs by around 25%, this can be seen on figure 15.
Figure 15 - The effect of wind speed on the generation cost of wind power. (Source: Sustainable Development Commission 2005).
Typical wind turbines are designed to start running at wind speeds of around 3 to 5 m/s. In Cambridge in 2006 the wind speed was measured over a year, and the daily average wind speed reached 6 m/s in only 30 days out of the year (MacKay 2009). This shows that it doesn't matter the size or power of your investment, if the wind isn't there, then the power can't be produced. In D.J.C Mackay's report about sustainable energy in 2009, he quotes Watson et al. 2002, which say's 'a minimum annual mean wind speed of 7.0m/s is currently thought to be necessary for commercial viability of wind power. About 33% of UK land area has such speeds' (MacKay 2009, p. 281). This statement about an average annual wind speed of 7.0 m/s means that if we are to make wind energy viable then there needs to be careful thought and consideration about were wind turbines are placed, and not just invest in a wind turbine to portray a sustainable ethos.
There are many negative articles in the news and press about low efficiency wind turbines and how wind is an intermittent source of energy, but these negative comments are challenged by Wind Energy Planning with some facts and figure, about electrical output, posted on their website. A modern day wind turbine has a maximum capacity of around 2 MW and will generate around 30% of its maximum theoretical capacity. There are 8760 hours in a year, which would result in 5256 MWh generated per turbine per year. The article then goes on to say that 5256 MWh is enough green electricity to power 1100 homes for a year, with an average electricity consumption of 4700 kWh per house, which is based on the Digest of UK Energy Statistics (Wind Energy Planning 2008). These figure are promising for wind energy and if wind turbines of 2 MW could be placed in the areas with average annual wind speeds of 7 m/s, outlined by Watson et al. as 33% of the UK, then wind electricity generation could silence some of its critiques.
Cost and Payback Period
To see if wind turbines are feasible, and a payback period can be worked out, specific condition need to be outlined. Peacock et al. carried out a feasibility test on four different wind turbines, at two urban locations with different average wind speeds at the Heriot Watt University. The four turbines that were tested were of sizes 0.4 kW, 0.6 kW, 1.5 kW and 2.5 kW and the average wind speeds of the two sites were 2.0 m/s (low) and 4.9 m/s (high), with the data been collected from two years, 2000 and 2001. The lifespan of a wind turbine is approximately 20-25 years and the costs of the systems were £5000 per kW installed. From the data two tables were drawn up, figure 16 and figure 17, to show the payback period and emissions savings.
Figure 16 - Payback for high wind speed site. (Source: Peacock et al. 2008).
Figure 17 - Payback for low wind speed site. (Source: Peacock et al. 2008).
In an urban environment at low wind speeds payback is not achievable as there is a negative cash flow, figure17. The only turbine that could guarantee payback, figure 16, was the 0.6 kW modal, in high wind speeds with a grant of 30% from the government. To obtain a payback from all four of the wind turbines tested, reductions of 60% would have to be made in terms of cost from the manufacture (Peacock et al. 2008). This may be achievable in the near future as wind turbines are mass produced and the manufacturing process becomes cheaper. This test did show that at high wind speeds there was a significant COâ‚‚ emission reduction, which suggests that the technology has an important role to play in the decarbonising of the UK domestic sector.
However, Horizontal Renewables say that using one of their 15 kW wind turbines, producing 25000 kWh annually, with an average wind speed of 5.5 m/s and a turbine life expectancy of 25 year, a payback period of 7-8 years is achievable and a total income of £8675 can be acquired, figure 18, (Horizontal Renewables 2010). This is an extremely promising figure for anyone with an average annual wind speed of 5.5 m/s.
Figure 18 - Payback period for a 15 kW wind turbine with an average annual wind speed of 5.5 m/s. (Source: Horizon Renewables 2010).
A typical small wind turbine system costs are around £2500-£5000 per kW capacity installed however grants are in place that offer £1000 per kW installed, with a maximum of £5000 available (Renewable UK 2010). For a large system, of a MW scale, the cost is around £800 per kW capacity with no grants available as the maximum size installation to qualify for a grant is 5 kW (Renewable UK 2010).
Solar thermal is currently the largest and most established micro-generation industry. With approximately 78500 installations, an estimated total cost of £357696065 and an average cost per installation of £4558, in 2005 (Allen et al. 2008). There was a total installed capacity of 154 GW of solar hot water systems in 2007, with China being the world leader with 70 GW installed and Israel and Cypress getting the most use out of solar hot water systems with over 90% of homes using them (Shukla et al. 2009).
How It Works and Aesthetic Appearance
There are two main types of solar hot water collectors; flat plate collectors and evacuated-tube collectors, with the main common component of both being an absorber plate to collect the sun's radiation (GreenSpec 2010). The plate then releases this energy captured from the sun through radiation and convection. The heat is then transferred to a heat transfer fluid which can be stored in a tank to feed the hot water system (GreenSpec 2010).
Flat plate collectors use flow tubes to carry the heat transfer fluid to a tank, or a boiler if the temperature of the water needs topping up. The flat plate collector is housed in aluminium or galvanized steel with insulation and a cover to reduce heat loss (GreenSpec 2010). This design is the simpler of the two and is shown on figure 19.
flat plate collector components
Figure 19 - A flat plate collector. (Source: GreenSpec 2010).
Evacuated-tube collectors absorb solar energy in a different and more complex way, but still using the same principle of a heat transfer fluid. The system works by having a tube within a tube, with the heat transfer fluid in the inner tube and a special fluid, which begins to vaporize at a low temperature, in the outer tube. As the sun heats up the fluid in the outer tube it vaporizers which in turn heats up the inner tube. The hot fluid in the inner tube is then carried to a hot water cylinder or a boiler to be toped up. For this system to be efficient the pipes must be angled at a specific degree as to allow the process of vaporizing and condensing to function correctly, figure 20 (GreenSpec 2010).
solar heat pipe components
Figure 20 - Evacuated-tube collectors. (Source: GreenSpec 2010).
Solar thermal panels look very similar to PV panels, so aesthetically they cause no serious discomfort to their surroundings. But as with PV panels solar thermal panels are subjective, some will like the look of them and their green credentials and others will see them as an eye sore. Workmanship also plays a role in achieving an aesthetically pleasing solar system, which can be seen in figure 7 and figure 8.
Efficiency and Output
Solar hot water system efficiency, just like the other renewable energy sources reviewed, depends on the amount of variables that affect a particular site. The company Velux currently produce solar thermal systems and outline the following variables that could affect a typical system;
Pitch of roof
Efficiency of auxiliary heating
Size of collector (Velux 2006).
To give potential clients an idea of how much energy a solar thermal system from Velux could save, they have outline some figures to correspond with the variables outlined above. They say that a typical Velux solar hot water system for a four person household with; 5m² of solar collectors, a 280 litre tank, due south orientation, an 80% efficient gas boiler in Manchester could provide a total benefit of 2874 kWh/yr (Velux 2006). This is a promising number, but when compared with other literature it seems far too great a figure. In Allen et al. journal titled 'Prospects for and barriers to domestic micro-generation: A United Kingdom perspective' a side-by-side test was carried out on eight available solar thermal systems. The annual outputs of the eight systems were collected, and a mean of 1145 kWh/yr was found with a range of 954-1339 kWh/yr (Allen et al. 2008). These figures are far less than that quoted by Velux. Allen et al. also provides a figure for the amount of hot water that a solar thermal system can provide, which is 33% (Allen et al. 2008), Velux outline another larger figure and state that their system can provide 70% (Velux 2006) of your annual hot water requirements The difference in output and efficiency from these two pieces of literature is vast and could be very confusing for a home owner looking to invest in a solar hot water system.
To show how much affect the orientation of the building and the pitch of the roof can have on the efficiency of a solar collector, Velux provide a table for each region, figure 21 shows the Manchester Region.
Figure 21 - Effects that orientation and pitch have on the performance factor. (Source: Velux 2006).
Cost and Payback Period
The cost of a full solar thermal hot water system for domestic application is around £2000-3000 for a flat plate collector and £3500-4500 for an evacuated tube system (Allen et al. 2008). GreenSpec agree with these figures and say for a typical domestic installation it would cost £2000-3000 for a flat plate collector and £3000-5000 for an evacuated tube system (GreenSpec 2010). A typical solar hot water system is expected to last 20-25 years and has a payback period of 5-8 years which allows for considerable saving to be made in the remaining life time of the system (Enerfina 2011).
Comparison of Different Technologies
Four technologies have been identified, researched and reviewed, based on the literature available for each. The areas of research for each renewable energy source (RES) was; aesthetics, efficiency and output, and cost and payback. This section will now combine the research for the four technologies in the three separate areas of research to compare the technologies against each other.
This subject is a very personal one as people have different valuations of how important aesthetics are and what defines something that is aesthetically pleasing. The overriding factor for all four RES's is space, all of the systems outlined need space, either for a water cylinder, roof panels, a ground loop, electrical plant or a wind turbine, some needing more than one. If space is adequate then the attention of a potential client will turn to how the installed system looks. As GSHP's are under the soil they have no negative effects on a dwelling, were as solar panels (thermal and PV) and wind turbines have a physical presence. As stated earlier this physical presence is down to personal opinion and should be given great though.
Efficiency and Output
To compare the four different technologies fairly, they will be compared against the average electricity usage of a dwelling in the UK which is 4700 kWh a year (Wind Energy Planning 2008).
A GSHP is the most efficient system and can convert 1 unit of electrical energy into 3 units of heating. This means a GSHP is 300% efficient and can provide all the required heating and cooling for a dwelling year round. PV cells are around 10-18% efficient, in turning solar radiation in to electricity, depending on which system is applied. A typical PV system in the UK can provide 50% of annual electrical bills. Solar thermal panels can provide 50-70% of hot water requirements in the UK. Wind turbines run at around 30% efficiency and a 2.5 kW system could cover all annual electricity bills with some energy left to pay into the grid. So depending on what is required from a RES, electricity, space heating or hot water, will depend on which suits your needs best. A GSHP is by far the most efficient and consistent out of the four systems as the ground temperature in the UK stays at around 11°C, therefore providing a constant heat transfer without the intermittency that restrict the other technologies.
Cost and Payback Period
The cost that will be outlined will be for an application on a dwelling with no grants considerd.