Most alternative energy sources are inherently variable and unpredictable. Solar and wind power are the worst offenders in this respect, while being in many ways the most convenient and cost-effective. Technologies based on wind and solar power, therefore, have to cope with a widely fluctuating energy input that rarely matches demand for more than short periods. A practical alternative energy system must be able to smooth out any mismatches that occur without an overdependence on back-up supplies from national energy grids. The only practical way of doing this is to store surplus alternative energy for use when input falls short of demand.
Surplus electricity from wind turbines, solar PV panels and mini hydro is normally best 'stored' on the national grid, via net metering, if it is available (see Chapter 1). Buildings not connected to the grid normally have either large conventional batteries or standby fossil-fuelled generators for when supplies from the alternative energy source are inadequate. Purpose-designed deep cycle or deep drawdown batteries are essential, of the type often used for forklift trucks. Automotive starter batteries are not suitable. Current battery installations mostly use lead/acid technology, and have a limited lifespan.
Alternatives under development include the sodium-sulphur battery, which has a high energy density and efficiency of charge/discharge and is manufactured from inexpensive, non-toxic materials. Unfortunately, it operates at temperatures of up to 350°C, and sodium is highly corrosive and reacts violently with water. Another high-performance option is the molten salt battery . This needs a temperature of up to 700°C to function properly, with obvious safety issues. Several companies have announced progress with lower temperature high-density batteries utilising non-toxic materials, but none are (at the time of writing) on the market.
A promising development is the flow battery, which uses liquid electrolyte. This is stored in two separate tanks outside a flow cell, which contains two chambers separated by a thin membrane. One chamber contains a positive electrode, the other a negative electrode. When surplus electricity is available the two electrolytes are pumped separately through the flow cell, where each acquires a different electric charge, and returned to their storage tanks; if the energy supply drops below demand, the electrolytes are recycled through
Flow batteries are a promising development (Reproduced with permission from New Scientist Magazine)
the flow cell, where they give up their stored energy as hydrogen ions move through the membrane to balance electrons released at the negative electrode. Two variants have been tested, one based on vanadium sulphate, one using zinc bromide.
Developed in Australia, vanadium flow batteries are said to have a much longer potential operating life than lead/acid or other conventional battery types, and to be much easier to scale up for larger installations. Two installations of more than 200 kW have already been completed on wind farms in Japan and Tasmania and one that is designed to produce 1.5MW for up to eight hours planned to go into operation on the Sorne Hill wind farm in Ireland at the end of 2007.
On the larger scale, and where site topology allows, pumped storage might be feasible. Surplus electricity is used to power a pump that moves water from a lower to a higher reservoir. When a shortfall occurs, the system goes into reverse, with water from the upper reservoir flowing back through the pump, which then acts as a generator. Other options becoming available include superconducting magnetic energy storage, which has the quickest response time of any electrical energy store. Units capable of producing 10MW for two hours are now available, and are said to be very reliable as there are almost no moving parts. Also available are flywheel energy storage units. These typically will utilise advance carbon composite rotors mounted on magnetic bearings that can operate at more than 50,000 rpm inside a vacuum chamber. Energy efficiency, defined as the ratio between energy input and output, can top 90%, and storage capacity available ranges from 3kWH to 130 kWH. The Australian Powercorp has installed a number of its 1MW Powerstore units to back up both wind and hydropower installations. Early experiments with flywheel energy storage were dogged by problems with flywheel explosions, which could only be mitigated with rugged containment vessels. Even with modern composite rotors many users prefer to embed the systems in the ground for safety reasons.
Compressed air energy storage may be the answer for some projects (Reproduced with permission from New Scientist Magazine)
On the larger scale, and where site geology is favourable, compressed air energy storage (CAES) may be a realistic option. This uses surplus energy to compress air up to 35 bar or more and pump it into underground voids such as worked out salt mines and aquifers capped with impervious rock. Energy can be recovered via a gas turbine with a bypassed compressor stage - the high pressure air from the store passes directly to the combustion chambers. Between a half and two thirds of the power generated by a gas turbine is needed to power the compressor stage, so bypassing it allows the unit to generate up to three times as much power from the same quantity of fuel.
Regenerative fuel cells are another possibility. Basically an electrochemical device that directly converts the chemical energy of oxygen and hydrogen-rich gases or liquid hydrocarbons into electrical power and water, a fuel cell can also be considered as something that reverses the well-known electrolysis process, which uses electrical energy to break down water into hydrogen and oxygen. Combining both functions produces the regenerative fuel cell (RFC), also sometimes known as the reversible fuel cell . Some types of fuel cells operate at very high temperatures (see Chapter 14). However, the RFCs now becoming commercially available are based on the proton exchange membrane fuel cell (PEMFC), which has an operating temperature of around 200°C or less.
The first generation of RFCs were developed in the US and aimed at aerospace and defence applications, although there is still considerable interest in their use as prime movers in road transport. Energy density is approximately ten times higher than that of conventional lead-acid batteries. Some early versions separated the functions of electrolysis and power generation. These days, it is more usual for both functions to be combined in one cell, consequently known as a unitised regenerative fuel cell (URFC). Later developments include the substitution of metal hydrides for the traditional noble metal catalysts in the hydrogen electrodes.
This would appear to have the potential to slash cell prices and eliminate the need for separate storage of the hydrogen gas produced by electrolysis. Technological benefits should include almost instantaneous start up and excellent performance in low ambient temperatures. The leading developer is the Ovonic Fuel Cell Company of Michigan, which claims that when they come on the market, metal hydride URFCs will be slightly cheaper than conventional batteries in terms of initial cost and much cheaper over the long-term.
An alternative and much simpler strategy on the smaller scale is to use surplus electrical energy to generate heat, which is then stored. A straightforward immersion heater is one alternative, or a ground, water or air source heat pump might be preferred. Some of the surplus electricity could be used to power anti-legionella systems (see Chapter 2), drive irrigation pumps and perform other tasks that do not require regular operation.
Storing surplus heat is somewhat more straightforward. Thermal store technology is developing rapidly, and there are now four basic subdivisions. The main distinction is between diurnal storage, which absorbs heat during the daylight hours and releases it at night, and seasonal storage, in which summertime heat is saved to maintain supplies during the winter. In practice, on larger projects where the main input comes from solar energy, it may be preferable to go for an intermediate thermal store, which can provide enough heat for several days. A further distinction is between low temperature and high temperature storage. Low temperature storage normally involves heating a large mass - often the ground near to or below the building - to a relatively low temperature, usually below 40°C.
Early prototypes of zero energy or autonomous buildings used the earth below, around (and sometimes above) the building as a low temperature seasonal thermal store, relying on passive heat transfer to transmit passive solar gain through the walls and floor (and sometimes the roof) into the earth during the summer. A waterproof membrane curtain extends out from the building to stop rain penetration into the storage area. Keeping the moisture content of the earth used for storage relatively low minimises energy losses to the surroundings.
More sophisticated designs relied on solar collectors to heat either air or water, which was then ducted or piped into the ground. The earth reached a higher temperature than with a completely passive system, so the area that needed to be protected by a waterproof curtain was smaller. Heat movement back into the building in winter was still often down to uncontrolled conduction through the ground floor slab, but temperatures within the building were generally better controlled in both summer and winter than with the totally passive system. More control could be achieved in air ducted designs, which used either a fan or a solar chimney to draw air back through the underground ducts in winter.
Larger, more sophisticated forms of low temperature seasonal underground thermal energy storage (UTES) have been used successfully to supply housing developments, offices, schools and factories with sizeable fractions of their annual demand for both DHW and space heating. There are three basic types: aquifer thermal energy storage (ATES), borehole thermal energy storage (BTES), and the least common, cavern thermal energy storage (CTES). ATES systems can only be used when ground conditions allow - specifically, there must be at least one aquifer at a convenient depth that will yield adequate groundwater supplies. There must be very little lateral water movement as well. Most systems installed to date use cold groundwater pumped up from a water bearing strata a convenient distance below the surface, heated in solar collectors and pumped back down another borehole some distance away. The cold water may also be used for space cooling purposes before it passes through the solar collectors (see Chapter 13).
In winter the flow is reversed although in some installations, a heat pump is used to abstract heat from the fairly low temperature store and concentrate it in the building's DHW and space heating systems (see Chapter 12). An aquifer with a high rate of ground-water flow is only suitable for summertime cooling, although there is at least one installation, the Reichstag building in Berlin, that uses two aquifers at different levels, one for heating, one for cooling.
If no aquifer is present below the site, BTES might be the answer, especially in impervious soils and rocks with high specific heat. Boreholes up to 200 mm diameter and 200 m deep are usually positioned 5 to 8 m apart in a hexagonal pattern to minimise heat loss. U-pipes or more sophisticated forms of heat exchanger are installed in the boreholes and are often bentonite grouted after installation to maximise heat transfer to the surrounding substrate. A heat transfer fluid, typically water containing glycol antifreeze, circulates through the boreholes and solar collectors in the summer, and usually transports heat collected from the cooling system as well. In winter the stored heat in the ground is drawn off for DHW and space heating, either directly or via a heat pump.
A variation of BTES is ducts in soil, also known as earth tubes or ground coupled heat exchangers . These are usually a horizontal installation of pipes and/or ducts at relatively shallow depth. This has the advantage of tapping into the natural solar gain of the ground, and overlaps in operation with ground source energy systems (see Chapter 12). Using a relatively high mass of soil heated to relatively low temperature means that a heat pump is normally needed to obtain DHW and space heating at acceptable temperatures. Against this can be set the opportunity to use less sophisticated solar collectors, which may be more durable and vandal-proof, and hence cheaper overall. Constructing a CTES can be expensive. The best strata are stable, impervious igneous or hard sedimentary rock with low thermal conductivity and potential for leaching. A configuration with low surface to volume ratio is essential; even then at least 10% of the energy stored in the water inside the cavern will be lost, and it will take at least 12 months for energy flows to stabilise.
Low temperature seasonal pit storage is more popular, and its performance is easier to predict. Many forms have been tried, with varying degrees of sophistication, and development work continues. At its simplest, a thermal storage pit consists of an excavation lined with a suitably rugged insulation material and waterproofed by a heavy duty waterproof membrane. In some examples this is then filled with plain water, but such a tank would need either a loadbearing cover slab or perhaps a floating cover, which would involve long-term maintenance issues. An alternative is to use either gravel/water or gravel/sand/ water storage. The latter design positions the heat transfer pipes in layers of sand between layers of gravel to protect them from damage. A loadbearing cover slab is not needed, as the cover is supported by the mineral fill. Unit storage capacity is somewhat less, but it costs little extra to enlarge the pit to compensate. Some installations have a history of leakage problems. There is also the potential problem of legionella disease, and local regulations might demand the periodic sterilisation of the store to kill any legionella bacteria that might be breeding. Against that, legionella disease is unlikely to spread from a covered store with no significant water movement.
Double the physical dimensions of a storage tank and its surface area will increase by a factor of four. Its volume, however, will increase eightfold. Thus the larger the tank the more efficient it will be at storing heat, as the area through which heat can escape by conduction through the insulation will be proportionally lower. Large water thermal stores have been successfully used for seasonal storage on a number of projects, operating at temperatures of 40°C or less to minimise heat loss even further. These usually take the form of heavily insulated underground concrete tanks, either in the basement in the case of a single large building or located centrally amongst a number of smaller buildings or homes. Heat input can be either through an indirect solar water heating system or a solar-heated air supply passing through heat exchangers. Energy for DHW and space heating is most conveniently extracted by passing water through another series of heat exchangers, or by the use of a heat pump.
Capital cost is higher than most other forms of low temperature seasonal storage, but the greater efficiency of thermal water stores can make concrete tank stores attractive, especially if they are constructed at the same time as the rest of the building. There are problems associated with the construction of insulated watertight concrete that can push up both capital and running costs. Waterproof lightweight concrete, which has a significantly lower thermal conductivity, is now a practical proposition, and might be preferred to more traditional forms of construction. Again, attention must be paid to the legionella potential.
Almost all UTES installations require a buffer tank between the thermal store proper and the DHW and space heating/cooling circuits. This damps down the inevitable short-term variations between demand and supply temperatures. No buffer tank is needed in one hybrid low temperature thermal storage system which combines both pit and borehole storage. A central underground concrete water tank insulated only on its top surface is surrounded by a ring of boreholes fitted with heat exchangers. Heat transfer fluid from solar collectors and/or space cooling passes first through a heat exchanger in the central tank until the maximum operating temperature is reached, then into the boreholes. Heat lost through the walls and floor of the tank is largely retrieved by the boreholes when the system goes into reverse during the winter. The central tank acts as a buffer, and the elimination of insulation to walls and floor significantly reduces capital cost. In practice it can take up to three years for the system to stabilise and achieve its maximum efficiency.
Prototypes of this design have seen temperatures in the central store reach 90°C. Other high temperature UTES systems have also been trialled. These raise ground or water temperatures to 70°C or more, but have a mixed record in practice. There have been problems with clogging, scaling, corrosion and leaching, and most installations have struggled to reach their design performance. On the other hand, such high temperatures effectively eliminate the risk of legionella disease.
Another possible source of input into UTES of all types is waste heat from industrial processes, or from cogeneration or trigeneration installations (see Chapter 14). Industrial waste heat can be stored for DHW and space heating needs, or as a back-up supply should the industrial process suffer any interruption of primary heat supply. It could also be stored for use in spells of severe weather, to de-ice car-parks and access roads.
Rock bins and rock beds, also known as gravel beds, are popular low temperature diurnal thermal stores when used in conjunction with solar air heating (see Chapter 3). Both involve the use of single size clean dry high density gravel or crushed rock, usually of a 20 m or 40 mm nominal size, stored in insulated waterproof airtight containers. Recent research indicates that, where available, rocks of a 150 mm nominal size offer the best combination of heat storage and airflow. Although relatively cheap, simple and requiring little maintenance, rock bins and gravel beds can suffer problems with mould, mildew and insect infestations. Their biggest drawback is size - a rock bin can store only 30-40% as much heat as a water-based store of the same volume. Rock bins, which have a predominantly cubical design, are more effective than under-floor gravel beds due to their more advantageous surface/volume ratio and associated lower rate of heat loss. Normal practice, when space allows, is to construct a rock bin large enough to store enough heat to meet up to five days space heating demand.
High temperature diurnal thermal stores are usually just larger, more sophisticated versions of the ubiquitous domestic hot water cylinder. Water is stored in large prefabricated insulated tanks constructed from steel or glass fibre reinforced plastic. Sometimes insitu concrete is preferred. The main distinction is between vented and unvented stores: vented stores, operating at atmospheric pressure, can deviate from the obligatory cylindrical form of the pressurised unvented store and can be manufactured in more convenient shapes, such as cuboids. This can take up less floor space than the cylindrical form, at the cost of slightly greater conductive heat loss due to their greater surface to volume ratio.
Whatever the geometry, stratification is encouraged, heat is extracted for DHW and space heating via one or more heat exchangers: more effective stratification is said to be possible when external plate heat exchangers are fitted. Prefabricated tanks are easier
to plumb into the pipe networks than the in situ concrete alternative. Against that must be set the practical limitations on the size of prefabricated tanks set by site logistics.
A more advanced and less space hungry alternative currently coming onto the market is thermal storage utilising phase change materials (PCM) . The most convenient change of phase is that from liquid to solid and back again, although solid to solid PCMs exist and liquid to gas PCMs have been investigated. Below the critical phase change temperature liquid to solid PCMs behave like any other solid: temperature rises in proportion to the heat input.
At the phase change temperature, however, the solid PCM melts, absorbing large amounts of energy as latent heat without any significant temperature rise. As heat supply increases, the PCM behaves like any other liquid store. As heat is extracted and the temperature falls, however, the liquid will solidify again at the critical temperature, releasing its stored latent heat. There are a number of materials such as salt hydrides, fatty acids and esters, and paraffins such as octadecane, which change phase in the convenient 20°C to 30°C range. In practice, most of these only work efficiently when the operating temperature range is relatively small, less than 20°C, which makes them suitable for many alternative energy applications.
Some PCMs are available in microcapsulated form. These are basically minute spheres of PCM coated with an inert polymer, which have been incorporated into traditional building materials such as fibreboard, plasterboard and floor tiles. As a typical PCM can store up to more than 10 times as much thermal energy as masonry or reinforced concrete, this has the effect of significantly increasing the building's thermal mass without a corresponding increase in structural weight. Thermal and Trombe walls (see Chapter 3) could be made much slimmer and lighter.
A PCM high temperature diurnal thermal storage tank could be only one-fifth of the volume of the equivalent water-based storage, or even less. Or five times or more energy can be stored in the same volume. Most commercial PCM products on the market are aimed at the cooling sector (see Chapter 13). However, a number of suppliers offer custom-made PCM-based thermal stores, usually involving PCMs packed in corrosion-proof metal shallow metal cassettes or stainless steel spheres, typically 100 mm in diameter. These are housed in insulated tanks through which water is circulated as the heat transfer medium. As these tanks can operate at relatively high pressures and temperatures they are particularly suitable for storing waste process heat. PCM-based thermal stores could also utilise air as the heat transfer fluid.
Larger-scale installations utilising several different technologies could well use two distinct types of thermal store, e.g. a UTES which is topped up with a mixture of heat recovered from cooling systems and DHW, and a high temperature diurnal store fed by solar water heaters, which is backed up by a heat pump in the UTES.
Without an effective energy storage system most microgeneration technologies will be neither practical nor economic. Luckily, there is a wide range of practical options available. Whatever the nature of the project and the peculiarities of the site, it should not be difficult to come up with an effective solution that will make the numbers look attractive.
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Global warming is a huge problem which will significantly affect every country in the world. Many people all over the world are trying to do whatever they can to help combat the effects of global warming. One of the ways that people can fight global warming is to reduce their dependence on non-renewable energy sources like oil and petroleum based products.