Passive solar water heating

Ample supplies of hot, clean water ' on tap' are a basic signifier of an advanced society. Ever since ancient times the heating and distribution of domestic hot water (DHW) has been central to a civilised lifestyle - even if only the rich could enjoy the convenience and comfort that DHW can provide. Over the last century or so, fossil-fuelled DHW systems have become the norm, and the last 50 years in particular has seen DHW usage rocket. Daily power showers have replaced the weekly bath in water heated by coal or crude town gas 'geysers'. Hot tubs are spreading. Washing machines are bigger. At least 30% of the energy demand for a typical modern home is accounted for by DHW needs -this will be much higher in well-insulated homes located where prolonged cold spells are rare. At the same time, the emergence of leisure centres and the like are boosting the need for DHW outside the home. Even offices now have to provide showers for the increasing proportion of their staff who chose to cycle or run to work every morning. Fortunately, replacing some or all of the fossil fuel-derived energy currently utilised for DHW is relatively straightforward. Several well-developed technologies exist, of which the most convenient is usually the most obvious.

It may be more than 150,000,000 km away, but the thermonuclear furnace that is our sun radiates enough raw energy across space to satisfy the needs of human civilisation many times over. Some is absorbed by the atmosphere, but even so, more than a kilowatt can arrive on every square metre of the Earth at the equator. Vast quantities of solar energy reach the Earth's surface even at high latitudes, well north or south of the equator -but there are inherent drawbacks.

Solar power delivery to any given point on the Earth's surface is highly variable. Simple calculations can allow for latitude and the diurnal and seasonal variations in solar delivery -solar energy varies from dawn to noon to dusk, there is obviously much less energy available at high latitudes during winter months - but these give no more than average figures. Unpredictable variations in cloud cover can have major impacts on available energy. Even on a clear day, no more than 75% of the energy reaching the Earth's surface will be direct solar radiation. The balance will be diffuse radiation that has been scattered by the atmosphere, and on days with unbroken cloud cover this proportion will rise to 100%. On a day of broken cloud, available energy levels can vary from minute to minute. As a result, only systems with some form of energy store to smooth out the peaks and troughs of energy capture are practical and economic.

Technologically, the simplest way of utilising the sun's energy is to absorb it into air, taking advantage of the well-known 'greenhouse effect' and using glass to trap infrared radiation from a surface heated by sunlight. Heat is transferred to air between the surface and the glass; the hot air is drawn off for use inside a building. Solar hot air space heating systems have been used for many years (see Chapter 3) and in some installations heat is extracted from the solar-heated air via a heat exchanger to provide DHW. Solar air systems are often the most practical solution in locations where there are very big differences in day and night temperatures - such as deserts - as the solar collectors are immune to the risks of both freezing or 100°C+ temperatures that dog water-based systems. In practice, however, solar hot air water heating systems usually depend on fans and/or pumps, so belong properly in Chapter 9.

Passive Heating Strategies

In more temperate climates a water-based solar water heating system is usually the best compromise. Water is a very effective absorber of solar heat. Sometimes this is done directly, sometimes by first heating fluid in a primary circuit that then passes through a heat exchanger. Heated water can then be stored in a well-insulated tank, or in a larger-scale thermal store of some sort (see Chapter 15), and drawn off as needed for a wide range of end uses.

Small-scale solar water heating systems aimed at the domestic market have been available for decades. The technology, originally based on standard plumbing components, has become substantially more sophisticated over recent years. But small-scale solar water heating on its own is far from being the ideal solution for the typical family home, largely because a large thermal store is usually impractical (see Chapter 15). Peak demand for washing and heating occurs on winter mornings, when the water in even the best insulated storage tank is likely to be below optimum temperature. And after a sequence of cloudy days a severe shortfall is inevitable.

Homeowners, therefore, are generally advised that they can expect no more than 50 to 70% of their DHW, i.e. non-space heating needs to be supplied by individual installations, and that they should fit a more reliable standby water heater. This could, of course, use zero or low carbon energy from sources such as mini hydro, or ground source energy (see Chapters 6 and 12), but most individual installations use standby heaters connected to an energy grid.

Larger-scale housing developments are more likely to have effective communal thermal stores (see Chapters 15 and 18) and alternative zero or low carbon supplementary energy supplies. Against that must be set the energy losses inherent in a large-scale hot water distribution system, however well insulated. In practice, therefore, it would seem that the most effective applications for solar water heating would be where the demand for hot water more closely matches the availability of solar energy.

Swimming pools are the first obvious case. Public pools need the water in the pool to be maintained at a certain minimum daytime temperature, and there will be a high associated demand for hot water in the showers. Much the same applies to private pools, but they will normally cool down faster and heat up quicker than an Olympic-sized facility, which tends to act as a massive heat sink. Sports and leisure centres, with or without pools, have high daytime DHW needs.

Solar water heating can work very well with swimming pools (Reproduced with permission from Energie Solaire)

Many other public buildings have a demand profile that is more in keeping with supply. Schools and offices need space heating and hot water mainly during the day; care homes and hospitals have high daytime hot water needs but require high levels of space heating both day and night. For larger installations like these there are benefits of scale, which can yield significantly greater efficiencies, but there will also be a need to consider aesthetics, acoustics and safe access for maintenance.

One of the oldest technologies for extracting energy from sunlight and using it to heat DHW is the integrated collector/storage (ICS) solar water heating, or batch system. In its simplest form, an ICS system consists of a black painted tank exposed to sunlight and with one face angled upwards. Usually the tank sits within an insulated box with a glazed sunward face. Mains water flows into and out of the tank as hot water is drawn off from the system, and there is usually an internal storage tank with a conventional water heater as back-up. Traditional ICS tanks are heavier than other types of passive solar collectors and are often located at ground level.

Passive Solar Air Connective Loop

Insulated box Cold water

Ground mounted batch system

Insulated box Cold water

Ground mounted batch system

In practice, batch systems are largely confined to mild climates at lower latitudes, due to the risk of freezing during long spells of cold weather. In suitable areas, however, the most practical option for smaller-scale installations would be a modern ICS system, where a storage tank is mounted directly above a more efficient passive solar collector. Several manufacturers supply well-developed systems, but storage capacity is usually limited, making them more suitable for daytime DHW supplies in offices, etc.

Thermosyphon solar water heating is the next simplest option in technological terms. It uses the differential between the densities of hot and cold fluids to circulate a heat absorbing fluid through some form of solar collector. As the fluid is heated in the collector it expands, its density is reduced and it is displaced by denser, cooler fluid entering the collector below it. The heated fluid rises into a thermal store, which may contain a heat exchanger and which must be positioned above the solar collector. Circulation will occur automatically as long as solar energy is available without the need for any form of pump, control system or powered valves, or for any secondary energy supply. If a collector with integral header tank is used, this can be positioned at roof level and used to feed a thermal store in a more convenient location.

Batch Collector Passive System
Integrated solar collector and header tank (Reproduced with permission from Powertech Solar)

Direct thermosyphon systems use potable water as the heat absorbing fluid, and hence no heat exchanger is required. This eliminates the potential loss in thermal efficiency associated with all heat exchangers. Water will circulate and heat will be absorbed as long as the sun continues to shine, although the amount of heat absorbed will reduce as the mean temperature of the water increases. Hot water drawn off for use is replaced by cold, usually from the mains. Deliberate stratification - where the hot water from the collectors is delivered into the top of the storage tank and 'floats' on the top of the cooler water below without mixing - can maximise the availability of domestic hot water early in the morning.

Such simplicity has its drawbacks, not least the risk of the water in the collectors and connecting pipework freezing in prolonged sub-zero temperatures (see below). Potential problems also exist during periods of prolonged sunny warm weather, when the demand for heated water is usually low. In such conditions the temperature of water in smaller scale thermal stores and storage tanks can soon rise to undesirable levels - above 60°C for domestic supplies. The risk of scalding is obvious - hot water at 70°C takes less than half a second to cause third degree burns - but, if the temperature begins to approach 100°C, there may be serious problems with high pressures within the system. Safety valves are not always 100% reliable. A number of methods have been adopted to deal with the problem, which inevitably adds to the complexity and maintenance requirements of the installation.

Air Vent

Pressure relief valve

Air Vent

Pressure relief valve

Pressure Relief Valve Solar Water Heater

Drain

JTTJ Drain

Drain

Collector

^ Check

- valve

- Bell valve

Drain

JTTJ Drain

Drain

Bell

Collector

^ Check

- valve

- Bell valve

Hot water out

Drain

Drain valve

Stup-off valve

PT, relief valve By-pass j-f valve

Cold water in

Thermosyphon system using integrated collector and header tank

The simplest is to design a system with variable thickness insulation to the storage tank. As hot water from the collectors is added to the tank, the temperature increases from the top down, where the insulation is thickest and heat losses are lowest. As water lower down the tank begins to reach temperatures close to a safety level - usually around 75°C - heat losses through the thinner insulation begin to balance the heat gain from the collectors. Eventually, if the system is well-designed, all the water in the system will stabilise at what is known as the stagnation temperature, although some circulation through the collectors will continue. Where flat plate solar collectors are used (see below), their heat collection efficiency almost disappears as the temperature of the water flowing through approaches 100°C, so the additional energy input is easily balanced by increased heat loss in the system.

More efficient evacuated tube collectors (see below) still absorb heat at temperatures close to 100°C, so additional precautions would be needed. These can take the form of

'heat dissipation pipes', which are basically sealed devices fitted about halfway down a storage tank. They transfer surplus heat to an external radiator - or 'heat dissipators' -simple water to air heat exchangers on a bypass loop through which the return flow from the solar collector is diverted when the preset temperature limit is exceeded. These should be mounted outside, preferably on the northern side of the building. On larger-scale installations, the solar collectors can have adjustable shading actuated by the same type of temperature-sensitive self-powered units used to control horticultural glasshouse ventilation.

Schools, hospitals, nursing homes and leisure centres have been using thermostatic blender valves to protect hot water users for many years, especially children and the elderly. These limit the temperature of water leaving hot taps in wash hand basins and baths to around 48°C by mixing in cold water as the heated water leaves the storage tank (although 38°C is the recommended maximum temperature for bathing children and the elderly). Blender valves may well become mandatory in the future, even for private homes, but will add complication, restrict flow and increase maintenance.

A further complication is that in hard water areas softened mains water will usually be needed, although on many projects water softening will be desirable for other reasons. Overall, however, the lower cost of installation and minimal maintenance requirements can make direct thermosyphon solar heating systems an attractive option for both domestic scale and larger developments. The most effective installations can be where the solar collectors can be positioned a short distance away from, and below the level of, the main building, feeding into a thermal store at ground or basement level.

Indirect thermosyphon systems capture solar energy in a closed loop primary heating circuit filled with a heat absorbing fluid. The oldest and simplest systems use water with high concentrations of automotive antifreeze, although standard refrigerant fluid is

Expansion

City water supply in

Hot water

Expansion

Warmer antifreeze liquid

Cooler water

Hot water

City water supply in

Warmer antifreeze liquid

Indirect Cooling Water Circuit
Less dense gravity flow

Cooler water

Heat exchanger

Cooler antifreeze liquid

Heat exchanger

Cooler antifreeze liquid

More dense gravity flow

Basic thermosyphon system with wraparound heat exchanger sometimes used. This heated fluid then passes through a heat exchanger, which could be a simple coil within the main thermal store or a more elaborate and more efficient 'wraparound' design in which the thermal store is double skinned and the heated fluid flows between the outer and inner skins. Softened mains water is not essential, but the potential for deliberate stratification within the heated water is limited, reducing the availability of hot water in the mornings. Saturation temperatures and pressures within the primary loop can be higher than with direct thermosyphon systems, and similar precautions have to be taken to control storage tank temperatures and scalding risks. Maintenance requirements are higher than with direct systems if antifreeze is used in the primary loop.

A more modern way of coping with freezing temperatures is to use burst proof polymer for all external pipework, including that within the solar collectors themselves, or to insulate the pipe network effectively (see Chapter 9). Direct thermosyphon systems using this technology appear to offer a practical and realistic zero carbon alternative for smaller-scale installations, provided precautions are taken against legionella disease (see below).

Passive solar collectors are fixed in position and have no moving parts. Properly aligned and located (see below), passive solar collectors can capture more than 80% of the solar energy falling on them. Two types are readily available.

1. Flat plate collectors are generally the simplest, cheapest and most rugged, consisting of little more than shallow weatherproof insulated boxes with transparent lids and internal dark absorber plates backed by sealed piping through which the heat transfer fluid flows. Many variations on this theme are available: the glazing can be one or two layers of high performance glass or polycarbonate, absorbers and piping can be metal or

Solar Energy From Aluminium Plate
Flat plate solar collectors are now well developed (Reproduced courtesy of Solarnor)

plastic, coated with a range of special 'selective' materials that reduce the radiation of infrared from the collector at night. Some designs are particularly sophisticated, with serious attention paid to aesthetics.

2. Evacuated tube collectors usually circulate the heat transfer fluid through copper-filled U tubes or heat pipes in direct contact with aluminium heat transfer fins all located inside the glass tubes from which air has been evacuated between the two glass tubes to minimise heat loss, although more complex and theoretically more efficient variants are available. This type of collector offers higher potential efficiency (up to 25% greater), due to its lower heat loss to the atmosphere and its ability to operate at lower solar energy levels and to produce water at higher temperatures. Against this, evacuated tube collectors are up to three times more expensive than flat plate collectors, due to their more complex technology. There have also been reports of problems with some evacuated tube collectors, with both explosions and implosions occurring, and at least one range of all glass evacuated tube collectors is said to have been taken off the market as a result.

Example Passive Solar Energy

Evacuated tube solar collectors are more complex (Reproduced with permission from Powertech Solar)

...but give higher performance. This example is from Viesmann (Reproduced with permission from Solarcentury.com)

Evacuated tube solar collectors are more complex (Reproduced with permission from Powertech Solar)

...but give higher performance. This example is from Viesmann (Reproduced with permission from Solarcentury.com)

The higher water temperatures possible, however, make evacuated tube collectors the obvious choice if solar water space heating is contemplated (see Chapters 3 and 10). Again, most modern versions are designed with aesthetics in mind, and ' vandal-proof' designs are also available, as are units glazed with ultrasmooth 'self-cleansing' glass. Soaring copper prices could well see solar collectors switch to alternative materials such as stainless steel, aluminium and polymers in the near future.

Actual performance in terms of solar energy collected over the course of a year depends on many factors, most of them site-specific. Latitude, orientation and any shading from nearby buildings, trees or geographical features can influence performance more than collector type. Shading is particularly undesirable between 9 am and 3 pm solar time, when nearly 85% of the daily solar energy is available. Ideally, passive collectors will be aligned with true south, although a deviation of up to 30° east or west would have no significant effect. Angle of inclination - the angle the collectors are tilted up from the horizontal - is always a compromise (see Chapter 9). Other factors being equal, however, and allowing for heat losses within the system before useful hot water is produced, a conventional flat plate collector on average will collect about 400 kWh/m2 annually at moderate latitudes, a typical evacuated tube collector around 500 kWh/m2 or more. How much of this energy is available in practice is also a function of thermal store size (see Chapter 15).

Most solar collector suppliers have standardised on an approximately 1.2 m by 2.4 m unit, and larger installations are made up of numbers of these panels plumbed together. Some manufacturers will produce significantly larger individual units if requested. A standard panel, in operation with its tubes filled, weighs in at around 30-50 kg.

Legionnaire's disease, or legionellosis, is an unpleasant bacterial infection that kills up to 15% of those unfortunate enough to catch it, usually the old and those who smoke. Infection follows the inhalation of fine water mists or aerosols infected with the Legionella bacillus, a particularly tough and persistent organism that can flourish in warm water environments. Luckily, 95% of those infected by legionella develop only the much less serious Pontiac Fever, which is usually mistaken for influenza. Legionella flourishes in water at temperatures between 30°C and 45°C, but can survive for years at temperatures not far above freezing and even up to 70°C. All forms of warm water facilities from domestic hot water systems to air-conditioning chillers have been associated with outbreaks of the disease. Only installations where there is no production of aerosols and the water temperature remains permanently below 25°C or above 60°C can be said to be low risk.

Most developed countries have strict regulations on what precautions must be taken to minimise the risk of legionellosis. A common practice is to ensure that water temperatures in all parts of the mains water system reach 60°C at frequent intervals to kill off the bacilli. This is particularly important in systems that use deliberate stratification within the thermal store to maximise higher temperature water availability. The water in the lower part of the tank may never reach temperatures at which the bacilli are killed for weeks at a time unless precautionary measures are taken. These can include regular mixing of the tank's contents using 'destratification' pumps - if the water entering the tank is frequently well above 60°C - or, more commonly, using a low mounted back-up heater to raise the whole thermal store temperature to a level at which the bacillus will be killed on a regular basis - pasteurisation. If this heating and/or mixing are powered by a renewable source, such as wind or mini hydro, only a simple control system will be needed. On larger installations the use of ultraviolet sterilisation could be justified, perhaps in conjunction with ultrasound to help remove any biofilms within the pipe network. Regular flushing and sterilisation with chemicals are other options. Precautions should be taken in all cases to minimise the production of the dangerous aerosols.

Passive systems are always worthy of consideration for every project, and passive solar water heating can be a realistic option on many projects, especially when site topology is favourable. Where one of the objectives is to minimise the use of high technology systems -and hence maintenance needs - or where the preference is for equipment with low embedded energy, passive solar water heating can be a worthwhile component in the final microgeneration mix. Its sensitivity to the relative locations of the collectors and the thermal store can make it difficult to combine with some other technologies and may impose architectural compromises, but its simplicity and reliability are very attractive.

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Getting Started With Solar

Getting Started With Solar

Do we really want the one thing that gives us its resources unconditionally to suffer even more than it is suffering now? Nature, is a part of our being from the earliest human days. We respect Nature and it gives us its bounty, but in the recent past greedy money hungry corporations have made us all so destructive, so wasteful.

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