Living a low carbon life
The science of going green
About 100,000 homes in the UK have some form of solar hot water heater and this number is growing by several thousand a year. Most solar water heaters take the form of a flat metal plate or a set of vacuum tubes on the roof of the house. These devices can provide hot water for most of the year, but will only supplement conventional water heating in the winter months. They don't replace central heating in any way. Adverts for these products claim that they can replace 50 to 70 per cent of the energy used for heating water. The truth of this statement depends upon the relationship between the size of the installation on the roof and the hot water demand of the household. The underlying difficulty is that only six months of the year give us enough solar energy to get water anywhere close to a high enough temperature to avoid using the boiler.
How does solar water heating work? Solar energy falling on a plate or a glass tube heats up the fluid passing through it. This hot liquid, usually water with anti-freeze in it, is pumped through a coil inside the hot water tank, heating the water. Having transferred its energy to the water in the tank, the now cooler liquid is pumped back to the roof. The technology is simple and reliable, and primitive versions have been used for centuries, particularly in countries where sunshine is plentiful. Efficient solar hot water panels - sometimes just called collectors - will work well in strong sunshine, even when air temperatures are low. However, collectors in cold climates will tend to lose a greater part of the energy that they collect. More heat is conducted or radiated to the air or to the surroundings of the solar thermal panels. Solar collectors will heat water from mains temperature of about 10°C up to 60°C or more. Depending upon the time of year and the number of hours of sunshine, the energy transferred from the collectors will either provide all of the water heating needed or will preheat the water so that it requires less fossil fuel energy to get it to the correct temperature. How much water will the Sun heat?
The savings in gas or electricity both depend upon the amount of the Sun's energy falling on the plates and the efficiency with which it is collected. In southern England, one square metre of surface receives about 1,200kWh/year in solar energy. The number is lower in the north. Of course, it is also unequally distributed between the months. The total energy received in December is about one-tenth of the June level. On a typical day in June, a four square metre solar collector in England and Wales south of the Mersey will receive about 20kWh to 24kWh, compared to no more than 2kWh in December. People often ask me whether one can use solar systems for central heating. The answer, unfortunately, is that when we need heating, there is so little solar energy reaching the UK that it would require a huge collector plate to capture more than a fraction of the heat energy we would need. The typical house requires 65kWh a day of heating, and 2kWh is only a small percentage of this.
The most efficient solar collectors are evacuated tubes, which turn about 70 per cent of this energy into useful heat, provided that the installation is facing reasonably close to south at an angle of tilt of about 30 degrees. Evacuated tubes not facing approximately south, or placed horizontally or vertically, will receive less energy. Provided that it is correctly oriented, in an average year one square metre of evacuated tubes will carry about 840kWh to a hot water tank in the south of England. Typical domestic installations are between two and four square metres, giving 1,680-3,360kWh/year, or an average of up to 9kWh per day for a larger collector (for reference, this compares to a figure of about 19,000kWh for the annual gas consumption of a typical house on the mains gas network). The average daily figure will vary by a factor of more than 30 between a long sunny day in a summer month and a short overcast period of daylight in the depth of winter. On a sunny day in late June, the figure from a four square metre installation might be as much as 30kWh.
Is 545 litres per day enough for a typical house? The rule of thumb is that hot water demand in the UK averages about 50 litres a day per person. So, a household with two people will be hugely overprovided for in June. But this also depends upon how the household does its bathing. Eighty litres will provide a full bath, so a sunny day should deliver almost seven baths. On the other hand, a quick gravity-fed shower might take only 30 litres. The real question is whether the house has pumped showers, usually called power showers. These water guzzlers use up to 16 litres or more per minute, so the June solar energy will give a maximum of about 34 minutes of hot water. Most sources suggest that the average length of time spent in a power shower is about five minutes, meaning that the water should last for six or seven people even if they are all using a pumped shower. In a household of two adults and two children, a sunny day in June should give enough hot water for all bathing needs, whether the family uses baths, power showers or the gravity-fed variety.
Actually, it is not quite as simple as this. When solar collectors are installed, the household usually gets a new hot water tank of about 300 litres capacity. So unless family members spread their showers throughout the day and give the hot water tank a chance to reheat, there won't be enough water. Or the 300 litre tank needs to get to a hotter temperature so that it can be diluted with cold water. The summer temperature of hot water from solar collectors can rise well above 80°C, meaning that the hot water has to be diluted with at least one part cold for two parts hot. The 300 litre tank delivers hot showers of at least 450 litres (28 minutes in a powerful shower), enough for five or six people.
Most users of solar thermal systems are very happy with their systems. The apparently free supply of copious hot water during summer gives people particular satisfaction. Owners I know are all enthusiastic advocates of solar collectors. But it is worth mentioning a few quibbles. The water can get very hot in some systems. Of course, it can be mixed with cold water to achieve the right temperature; but the risk of scalding is ever present in high summer unless a temperature controller is installed. Also, the effectiveness of solar thermal hot water in the summer months means that the boiler is very rarely on. Many households use the boiler cupboard or room as a place in which to dry clothes. From April to September, clothes won't get dry because the room isn't being heated by the operation of the boiler.
Solar photovoltaic (PV) panels generate electricity through the action of the sun's rays falling upon a silicon layer and causing the passage of electrons. Now 30 or 40 years old, the technology has always slightly disappointed. Expectations of rapid improvement have come to little, and the most efficient panels still convert no more than about 20 per cent of the energy falling upon them. Typical figures are lower than this, and the average installation in the UK today will probably turn no more than 15 per cent of solar energy into electricity. Solar PV installations on the roofs of UK homes are few in number - probably less than 3,000. The cost is daunting, with an array that generates 2kW in bright sunshine possibly costing more than £10,000. No one seems to be certain about whether solar PV grants will continue into the foreseeable future, not least because of the considerable scepticism that seems to exist in official quarters about the underlying rationality of pushing PV technology in the cloudy UK. Indeed, it does make far more sense to try to capture wind energy, rather than Sun. As with solar hot water, the orientation of the roof and its degree of tilt towards the Sun are important for solar electricity, although perhaps not as vital as one might think. A roof facing due east will probably capture about 80 per cent of the energy of panels facing due south. Similar degrees of attenuation accompany roofs that are either flat or very steep. The amount of solar radiation reaching England and Scotland varies greatly by latitude. The tip of Cornwall gets as much as 30 per cent more than northern England. As a result, a PV installation in the south-west, where electricity costs are generally higher as well, is more economically rational than in the north of England or Scotland (in September 2006, the price of electricity is about 12 per cent higher in Cornwall than it is in London, based on the British Gas price list, so the saving from PV panels might be as much as 50 per cent greater in the far south-west of England). As with wind turbines, solar PV installations are generally connected to the grid. The house uses the electricity first, if necessary, and any surplus spills over to the grid. The panels generate direct current that needs to be converted into alternating current, synchronised to the 50 cycles per second pattern of the electricity network. Output from the roof has to go through expensive electronics, a meter that registers how much is generated and, possibly, a meter that says how much is exported to the grid, rather than being used in the home. These all add to the cost.
Cost of domestic PV
PV installations are rated like wind turbines in terms of the amount of electricity they can generate under optimum conditions of sunshine or wind speed. In the case of PV panels, they will generally capture most at about midday during the summer months. The maximum solar radiation reaching the panels will be about 1kW per square metre at this time. At about 15 per cent efficiency, it therefore takes about 12 square metres to provide two 'kilowatt peak' (2kWp). Under typical southern England conditions, such an installation will gather about 1,800kWh/year (the equivalent of about 900 hours operating at peak potential). The figure might be 1,500kWh/year for an installation facing due west, rather than directly to the south. In financial terms, solar PV attached to the grid offers two benefits to the owner: first, a reduction in bills and, second, a payment for the renewable energy generated. The major electricity companies are prepared to pay for the energy from the sun because it helps, however minimally, to meet their obligations to generate renewable energy.
In the case of solar PV, house-owners get three benefits: a reduction in electricity bills from the units they consume themselves; a payment for units exported to the grid; and a payment for all the electricity they generate in the form of money for the Renewables Obligation Certificate (usually known as the ROC). Assume that one-third of the electricity generated is exported to the grid. This seems to be the assumption used by most installers. Generating 1,800kWh/year will therefore reduce a household's electricity bill by 1,200kWh/year. This will save approximately £120 at today's prices. In addition, generators may pay four pence for every unit exported. This means another £24. Finally, the ROC payment might be £72. So the net income from investing £5,000, after grants, is about £216 a year, or just over 4 per cent. The life of such systems is probably about 25 years or perhaps more, although the efficiency will slowly fall over the last years of life. To put it bluntly, the system will therefore probably just about pay back its cost. Given the poor economics, it is not surprising that the UK has only a tiny number of domestic solar PV systems. In some other countries, solar PV has had a much more rapid take-off. In Germany, for example, the number of domestic solar electricity homes is now well over 100,000. An extremely favourable public policy regime means guaranteed high prices for the electricity produced and low-interest loans to reduce the barriers to install the equipment.
Extracted from Chris Goodall's book 'How to live a low-carbon life' (£14.99), published by Earthscan. Available from all good bookstores or direct from www.earthscan.co.uk [new window].
We installed a solar PV and solar hot water system in December 2004. The search for an installer was not simple; prices varied widely and most companies were reluctant to come to Oxford. Our installer provided a quote for both systems separately and offered a substantial discount if we bought both at once. The firm would only have to pay for one set of scaffolding, and a lot of the work on the roof would not need to be duplicated. After grants of 50 per cent for the PV and £400 for the solar hot water, the cost was just over £8,000. The installation itself was trouble free, but took the best part of a week. Both systems have worked perfectly since being inaugurated, except for a short period when one of the sets of PV electronics mysteriously failed. It was replaced under guarantee. The meters tell us every second of the day how much electricity is coming into the house. The track record of the system has been slightly better than we thought. 2005 was a sunny year and the installers are forced to be conservative in their sales promises by the conditions of the grant-making bodies. The total amount of electricity generated was about 1,430kWh. The solar thermal collectors worked very well indeed in their first year. For the six months when the Sun was highest in the sky, the collectors gave us a reliably large tank of hot water, except on the cloudiest days. Even in early February, when the Sun is strong the tank will be heated to over 45˚C, and possibly reach 50˚C, after being only 10˚C after the morning showers. The water is scalding in summer and care needs to be taken. The profligate nature of our water use means that we still use a small amount of gas in the summer to heat up part of a tank of hot water for washing before the Sun comes out. The total savings from our investment in solar energy of £8,300 are about £300 a year at current prices.
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