Solar Trolley FAQs
How long does it take the Solar Trolley to heat up the water?
It depends on how much sunlight is available. On a bright, sunny day the unit will heat 6 litres of water from cold to 50 degrees C (hand hot) in 2 hours.
Does it need direct sunshine?
No, it will still heat the water when it is cloudy but it will heat up faster in direct sunshine.
What angle should the solar collector be set at?
Ideally it should be perpendicular to the sun. In the summer the sun is high in the sky so the unit should be set to a fairly low angle. In the winter, the sun is very low in the sky so you need to have the panel more upright. If it is cloudy, it doesn't really matter what angle you set the panel to.
Does the Trolley have to face south?
Not necessarily - it will work best if it is facing the sun. In cloudy weather it doesn't really matter which way it is facing.
How much energy is produced by the Trolley?
About 1 megajoule in 2 hours on a bright sunny day. Here is the explanataion:
- The unit holds 6 litres of water
- Water weighs 1 gram per millilitre so the weight of the water that has to be heated is 6000 grams.
- We know that the specific heat capacity of water is 4.2 Joules per degree C per gram. That means for each gram of water we need to supply 4.2 Joules of energy to raise the temperature by 1 degree C.
- Cold water from the tap is around 10 degrees C and we want to heat it to 50 degrees C. Therefore we want to heat 6000 grams of water by 50-10 = 40 degrees C. To do this we need to supply 4.2 x 6000 x 40 = 1,008,000 Joules (or about 1 Megajoule) of energy.
How long will the Trolley's battery last before it needs recharging?
The battery will last a full day before it needs recharging. It takes a few hours to recharge it fully.
Isn't this some kind of a trick? The Trolley has a battery in it, couldn't the battery be used to heat the water by itself?
Yes - but not very much. Here is the explanation:
- The battery is a 12 volt type with a capacity rating of 3 amp-hours which means it can supply 3 amps for one hour before it is exhausted.
- Power (in watts) is the product of current (amps) and voltage (volts) so another way of expressing the battery capacity is 3 x 12 = 36 watt-hours.
- Power is the rate of transferring energy. One watt is defined as one joule per second. Therefore the battery can supply 36 joules per second for one hour.
- One hour is 3600 seconds so the battery can supply 36 x 3,600 = 129,600 Joules before it is exhausted (0.1296 megajoules). Therefore if we tried to heat the water using the battery alone, it would only be able to heat water by 129,600 / 4.2 / 6000 = 5 degrees C.
Now assuming it stayed sunny for 10 hours, you could heat the water from cold using the Solar Trolley up to 5 times before the battery would need recharging. We could therefore say that the Solar Trolley is able to produce 5 x (1,008,000 / 129,600) = 39 times more energy than the battery can by itself.
Of course the battery charging process is not 100% efficient so it is a bit lower in practice, but you can soon appreciate that you are really harvesting the free energy of the sun.
How does the Solar Trolley compare with a domestic solar thermal system?
It is basically the same but bigger. Also the pump is usually mains-powered rather than battery-powered.
A typical house would need about 3m x 3m of solar collector and a hot water cylinder in the region of 150 to 200 litres. Domestic solar thermal installations always have some kind of auxiliary heat source that is automatically controlled (e.g. a gas boiler) to make sure the water is properly heated even during cloudy winter days.
Solar thermal collectors can be fitted to most houses as long as there is enough roof space which is not shaded. They do not need to face due south and in some cases east or west facing roofs can be used.
Domestic solar thermal systems are normally designed so that they produce about half of the hot water over the course of a year and generate in the region of 1000 kWh of energy per year. Installation costs vary but a typical installation is in the region of £3500 including a new hot water cylinder.
Does our school have to give back our Trolley in March 2010 when the project ends?
No - when the Explore Renewables project ends in March 2010, schools will be able to keep their Trolleys and associated learning materials. However, support via the technical helpline and website enquiry form will no longer be available after this time.
Should we insure our school's Trolley?
Yes - we recommend that you insure your Trolley in order to cover against damage that falls outside the warranty. If your unit suffers damage that you believe is outside the warranty, please contact the Technical Helpline for guidance. For insurance purposes the unit cost per Trolley is approximately £1200.
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