Swithenbanks solar online FAQ's
PRACTICAL RENEWABLE ENERGY |
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How to design and implement your own renewable energy systems in the real world! |
| Energy Consultancy is a very expensive business. Often the cost of consultancy can outweigh the cost of making a wrong decision....buying the wrong size turbine.....siting it in the wrong location etc. However, by applying some knowledge it is possible to get the best results out of your system. |
| Customers often ask us to advice them on what size turbine they require or how many solar tubes they need to provide their hot water....but it is important to realise that as well as the differing environmental factors that may apply to a particular site, there are also personal considerations that need to be taken into account. In fact, YOU are ideally placed to make the decisions - as you know your situation better than any consultant can. |
| We have summarised below some of the basic points about each technology, to help you to make a decision on the technology/technologies that you are considering using. Always bear in mind that renewable energy is generally unreliable by its nature. Therefore, although it may be relatively easy to produce a system that will provide 50% of your power, it will be much more difficult to produce a system that will provide 75% of your power. If you want to produce 95% of your power by renewable means, it often requires a very large investment. |
| If the following text does not answer your question, please also read our other FAQ pages. |
| Solar Photovoltaic (electric-generating panels) |
| Solar photovoltaic panels are available in three types - monocrystalline, polycrystalline and amorphous. The most efficient generally available is only 14-15% efficient. Even NASA-grade panels do not go much over 20%. First and foremost you must consider the cost of photovoltaic's against the potential rewards. For example, a 1kW solar photovoltaic panel will produce around 700kWh of electricity per year. This is worth £53 per year at the best buy-back rate available. You would get the same financial return from investing £1000........so how much does 1kW of solar PV's cost?! As you can see, there is no good economical reason for investing in solar PV's - and that is before you have considered the cost of ancillary equipment - isolation switches, cable, connectors, mounting systems or the grid-tie inverter. Solar PV's do have their place - remote power systems, off-grid application and as replacements for battery-based power systems, they can provide an economic alternative. For most customers, they are a luxury only afforded by people dependant on off-grid power generation means, or enthusiasts. Uptake of solar PV's has been largely hampered by the uneconomical price tag. Manufacturers have long promised to reduce the price of PV's....but greed and high demand particularly from Germany (where government grants are extremely generous) has maintained a high price - which has actually risen by over 50% over the last few years. |
| Solar water heating (Evacuated tube solar hot water panel/flat plate) |
| There are various solar water heating technologies. Flat plates are very effective in the summer, not not so good during the other seasons. Vacuum tubes will perform with similar results in the summer, but will vastly exceed the performance of flat plates during the rest of the year. Ideally a vacuum tube system should be orientated facing South, although SW or SE is almost as good, losing only around 15% of the available heat. If you are forced to use an East or West roof slope, then you will need to double the number of tubes to get the same output as a south facing system. The panels should be angled at your angle of latitude - so for example in the UK, it would be around 53°. In fact, up to 15% deviation from this will only result in a small reduction in efficiency (~5%), so it usually makes more sense to mount the panel at the same angle as the roof. |
| Off-grid solar water heating |
| The constant power requirement of the controller, and the 40 watts required by the pump is often viewed as excessive by off-grid system designers. Many have seen a system where the pump is powered by a solar PV panel. This is a bad idea for two reasons: 1. Solar-powered pumps do not work in low-sunshine conditions... whereas the vacuum tubes can still get very hot even in overcast weather. 2. The low voltage DC pumps are too low a wattage for solar vacuum tubes. A 20 tube panel requires a minimum of 20-30 watts of pump power in hot weather, and a 30 tube system really requires 40 watts. An alternative is to power the system using a UPS (uninterruptable power supply) which can in turn be charged by solar PV's. This way, it is possible to run a full 40 watt pumped system, without relying on mains power. Be careful - some UPS systems use a lot of power in standby mode - 70-80 watts minimum power consumption is quite common. We have selected a French system which requires only 3-4 watts whilst powering the solar controller - which we can supply if required. |
| Number of solar hot water panels |
| Even a small number of solar vacuum tubes will make a difference to your hot water heating requirements, regardless of cylinder size or demand. For small households, a single 20 tube 58mm south facing panel should be sufficient. For larger households, a 30 tube 58mm panel is a better option. For swimming pools, you should use 25-30% of the pools surface area (eg. 3 panels for a 10x4m pool). |
| Cylinder size |
| if you do not use vast quantities of water, then a smaller cylinder will yield hotter water (albeit lower quantities). a 1200x400 cylinder (135 litres) is usually sufficient. Many people chose larger cylinders - up to 175 litres for a standard 20 tube south-facing panel. Larger cylinders up to 260 litres can be accommodated by a 30 tube standard panel. If you want to fit a mains pressure hot water system, now is a good time to do it. Solar water heating will work with mains pressure hot water. Use 'Thermal store' option to provide mains pressure hot water without the expense of a stainless steel pressurised cylinder. |
| Underfloor heating |
| Large panel arrays (4-8) can provide a useful addition to heating a house in the winter. However, they will generally produce heat at the wrong time - excessive heat in the summer, and little or no heat during overcast winter weather or at night. Conclusion: Very cost-effective, massive energy return for your capital, relatively short pay-back period, virtually every house can benefit from solar water heating. |
| Water Turbines |
| If you have a potential water
turbine site, then there is no question
that it will be a worthwhile investment.
We provide very competitive prices on
small scale hydro generation - so
payback times can range from 2-6months -
much faster than any other form of
renewable energy. You will need a good flow and/or a good head of water. You can easily calculate the potential power of a water resource using the formula given on our water power webpage. Our water turbines have a built in dump load, which will soak up any excess power to prevent the water turbine over-spinning (important for voltage/frequency regulation). Conclusion: Exceptionally fast payback time. Very cost-effective, dependable power source. If you have a suitable water course, make use of it! |
| Wind Turbines |
|
Two things determine wind turbine choice
Both are very difficult to measure. If
you have no idea, then chances are that
no-one else does either! You can check
the average wind speed at your location
at the BWEA website. This will give you
the wind speed high-up, where there are
no obstacles to cause turbulence. This
will NOT tell you how well a wind
turbine at low height will perform. |
| What size turbine should I buy? |
| If this question was accompanied by a cheque for £10, we would no longer need to sell turbines! It is not a question that can be answered. However here are a few pointers: |
| A 200W
turbine is capable of running all of the
household lighting circuits, if in a
suitably windy location. Sometimes the decision can be based on battery voltage, if it is to be integrated into an existing battery system. Wind turbines can be used as direct
heating with suitable controllers. This
is an excellent way to heat a house, as
wind tends to accompany bad weather, and
is especially prevalent in the winter. Wind turbines make excellent grid-connect power systems. However the cost of the grid-connect equipment is high, making it a longer term investment. No wind turbine can be connected directly to electrical loads (except for direct heating systems). This is because the wind power varies constantly, and if connected directly, the voltage/frequency would be constantly changing. You must either use a grid-connected system, which basically uses the national grid as a battery feeding excess power into it, through an export meter. To work out what size wind turbine you require, you firstly have to identify your demands. If you average out household demand over 24 hours, most houses use around 500-750 watts. However, peak demand can exceed 10kW. If you want to be dependant on wind power, you will need to reduce your peak demand to a level that can be sustained by your inverter. Wind turbines are generally assumed to produce around 30% of their rated power, if you average things out, so to provide for a constant load of 500W, you would need a 1.5kW wind turbine. Of course, this assumes 100% efficiency and that storage issues are not a problem. |
| Grid-connection systems |
| These are the best way to run your
entire house on wind power, but the
grid-tie equipment cost exceeds the cost
of the wind turbine by some margin.
Conclusion: Great energy return on the
investment. Very effective for off-grid applications. Simplest solution is to use a battery bank/inverter, but this cannot be connected directly to household wiring. Alternative is grid-connect system, which can be directly interfaced with domestic wiring (i.e. no batteries). This is an excellent option, but grid-connection equipment is costly (will exceed the cost of the wind turbine kit by some margin). |
| Heat Pumps |
| Heat pumps produce more heat energy than the electric energy used to drive them. In fact, heat pumps will often provide 4 units of heat for each unit of electricity that is fed into the system. This is called the COP (coefficient of performance). For very small temperature differences (possible with a good heat source and underfloor heating), it is possible to achieve COP's greater than six. Ground source heat pumps can utilise heat from: 1. pipes buried in the ground 2. well water 3. bore holes, 4. streams 5. lakes. For the 9kW heat pump using buried pipes, you will need to run 3 loops (each 150m). You should make up a manifold, and use restrictors to balance the three loops, making sure that each loop has the same temperature differential. You will need quite an area of land to do this, so this is really only an option for small-holders and farmers. It is possible to install a heat pump in a conventional home, but this requires a water heat-source - such as a well, bore holes or a running stream. Conclusion: Very effective and dependable source of heat. Requires major earthworks unless a water source available as a ground heat source. |
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Posted
on
Wednesday
29
August
2007
Swithenbanks Alternative Energy Ltd for all your renewable energy needs: solar panels for your home - wind and water turbines - solar hot water evacuated tube panels systems - solar photovoltaic panels (pv) etc. |
