Exploiting the opportunities for solar hot water

There are two types of solar thermal collector — evacuated tubes (main picture) and flat plate collectors. Evacuated tube collectors are about 20% more efficient than flat plate.
The UK has an ideal climate for solar heating, and consulting engineers are more involved than ever in harnessing this valuable natural resource, says Paul Jakeway.It might seem hard to believe during a wet British summer, but meteorologists insist that the UK climate is ideal for solar heating. The Met Office says we enjoy about 60% of the sunlight experienced at the Equator — more than adequate for solar collectors to produce at least half of a typical British family’s annual hot-water needs. The figures According to solar installation guidance issued by the Chartered Institution of Building Services Engineers (CIBSE), a high-radiation summer day in the south of England provides 7 to 9 kWh of solar energy per square metre of collector area. Assuming 50% efficiency with a flat-plate collector, you could expect 3.5 to 4.5 kWh to be collected as useful heat. The cold-water supply temperature will be about 12°C and will normally require heating to 60°C for use in a hot water system. The available 3.5 to 4.5 kWh should, therefore, heat about 70 litres of water. The Solar Trade Association calculates that a 4 m2 array will provide half of a family's hot water needs for the year and reduce the heating fuel used by 40 to 60% — about 1500 kWh per year. More consulting engineers are involved with residential design these days, thanks in part to the explosion in mixed-use developments. Heating systems that use renewable energy have an important role to play in these projects because of the need to provide sustainable solutions that deliver improved lifecycle benefits and inflation-proof energy costs. The Code for Sustainable Homes, the measures of which will be compulsory for all publicly funded projects from next year, will also form the cornerstone of the next revision to the Building Regulations. This, along with the Government’s target for all new homes to be carbon neutral by 2016, promises to open up a huge potential market for renewable-energy systems. Consultants More clients and developers are turning to consultants for advice on these requirements, as they need expert guidance early in the design process. Solar water heating is being identified more widely as a quick win because it is a tried-and-tested solution that is relatively easy to install. It is also less expensive and disruptive than other emerging technologies such as geothermal heat pumps and combined heat and power. However, the skills issue is a potential stumbling block. While there is nothing daunting about the basics of solar installation, any new renewable system must be properly integrated with existing building services to deliver its full potential. This means the system designer and installer must have an in-depth knowledge of how conventional services work and how the renewable system can be designed to work in partnership with condensing boilers, for example. If you understand British domestic central heating and all its vagaries, you have a much better chance of delivering a solar thermal solution that integrates correctly and provides a meaningful amount of hot water at minimal carbon penalty. System types There are two main types of solar thermal system for engineers to consider: flat-plate collector panels and evacuated tubes. Flat-plate collectors consist of a metal sheet embedded in an insulated box covered with glass or clear plastic. Solar energy absorbed by the sheet is trapped by the glazing above and insulation behind the panel and transmitted into the water which passes through copper pipes. The heat energy is then transferred via circulating pipes containing a mixture of water and glycol and through a heat exchanger in the hot-water cylinder. On average, this indirect approach will convert about half the energy received into useful heat. System designers need to take into account issues like the danger of freezing in winter. Where a direct drain-back system is used, anti-freeze must be circulated through the water pipes, and the system may have to be drained down for the winter. However, this will reduce the effectiveness of the installation by removing any possibility of solar heat from winter sunlight. Evacuated-tube collectors are generally considered to be about 20% more efficient than flat plate because of their ability to capture energy from low levels of sunlight. Being cylindrical, they have a 180° absorbing surface facing the Sun for most of the day, whereas flat plates are in an ideal absorbing position for only a short period. The tubes are actually rows of glass vacuum flasks, which have a high-efficiency absorbing surface inside the vacuum. The vacuum insulates the tubes from low temperatures and traps the heat energy. The heat is transmitted to the hot-water system in a similar way to plates. The cost of installing a solar hot water system ranges from £2500 to £4000, depending on the size of the system and whether flat plates or tubes are used. A typical domestic installation in the UK would have a panel area of 3 to 4 m2 when using flat panels and about 2 m2 with evacuated tubes. The storage cylinder will typically have capacity for 200 to 300 litres. Accurate sizing is critical in solar systems, with a key aim of preventing stagnation of water in the collector — the point at which the collector does not pass any useful heat into the hot-water system. This raises the risk of Legionnaires’ Disease and is avoided by making full use of the captured heat and not oversizing the collectors. An electronic controller should constantly compare the temperature of the solar collectors with the temperature of the water in the cylinder. Whenever the collectors are hotter than the cylinder, the controller switches on the system’s circulating pump. A mixture of antifreeze and water travels through the collectors and the heat exchanger, so the hot-water cylinder is heated in much the same way as a central-heating boiler. Design engineers should calculate the most economic system size, noting that the cold water-supply temperature can vary by 8 K between summer and winter. Other seasonal variations, such as the temperature used in baths and showers, must be taken into account, as these all affect the base installed solar-heating load. Bright outlook While long-term weather forecasts are more uncertain than ever, the outlook for solar technology is bright so long as design engineers are involved from the outset and the industry ensures it has the trained installers to match the right system to the application. Paul Jakeway is marketing communications manager for Vaillant
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