What are the heat technologies that could be used in the UK to reduce the requirement for oil or cut electricity usage?
3.6.1 Biomass
Burning coal or oil to provide heat and electricity depletes a nonrenewable resource and produces carbon dioxide. But other fuels can be substituted for these fossil fuels that are carbon-neutral over their life cycle. They are a renewable source of energy that the UK’s Department for Environment, Food and Rural Affairs (DEFRA) describes as ‘offering a new opportunity for rural areas’.
Five dedicated types of biomass are at various stages of development.
Willow and miscanthus are at the stage of commercial availability and they are being grown now, and there are now several thousand hectares of these crops under cultivation – mostly willow, but some miscanthus.
Willow (and sometimes poplar) is planted as short-rotation coppice (SRC) – densely planted, high-yielding varieties where the rootstock or stools remain in the ground to produce new shoots after harvesting. Willow is cut back in the first year and then harvested every three to four years. A plantation could be viable for up to 30 years before replanting becomes necessary, although this depends on the productivity of the stools.
Miscanthus is a woody, perennial, rhizomatous grass. On most sites it will take around two years to produce a stable crop. After that time it can be harvested annually for at least 15 years. Miscanthus is not native to the UK – it comes from South East Asia – and the current lines being planted in the UK are sterile hybrids, which cannot seed.
Work on using willow and miscanthus has been under way for some time and the two crops are being promoted by DEFRA, with grants available both for planting and for developing producer groups.
Reed canary grass may be next in line for development. It is native to the UK and it is already planted as game cover. In Scandinavia, reed canary grass is being used both as an energy crop and to produce fibre (for paper making, for example). So far, canary grass is a rather less attractive crop than willow or miscanthus. It has a lower yield and a shorter productive life, and there are potential problems in removing it because it is spread both by rhizome and through seed dispersal.
Switchgrass – also known as prairie grass – is a native of the USA, where it is the most interesting energy grass. In the UK a R&D programme has been started.
The heat connection and cogeneration 35 Described as ‘having potential, but still furthest from the market’, the final energy crop under investigation is Arundo donax – known variously as bamboo reed, Danubian reed, donax cane, giant reed, Italian reed, Spanish reed and Provence cane. Until now, this plant’s main claim to fame has been that it is the source of reeds for woodwind instruments. It offers high yield, but there are mechanical and technical problems to be overcome.
All the new energy crops could be grown on set-aside land.
Biomass may also refer to various types of wood waste, such as bark chippings, or recycled wood from urban areas. This type of recycled wood, however, has to be very carefully selected, as if it is contaminated it will come under EU directives on waste incineration and must be burned in a dedicated and qualified incineration plant.
3.6.2 Solar water heating
Energy from the sun warms water left in a bucket on a sunny day. In fact, most of the extra warmth in the water does not come directly from the sun but via the bucket itself: the sun heats up the bucket, which in turn heats the water. A black bucket will heat the water up faster because it is better at trapping the heat from the sun and passing it on. This is a ‘passive’ system – it has no moving parts and does not require electricity or other external power.
The simplest solar hot-water systems, also known as solar thermal systems (and not to be confused with solar photovoltaic systems, which produce electric- ity directly – see Chapter 4) are pretty close to being black buckets. These ‘batch’ collectors are black-coated containers or tanks that are housed in an insulated metal box and covered with a solar glass or glazing material, and are larger than buckets. Usually batch collectors are filled with pressurized water.
Batch collectors operate without the need of ‘active’ pumps or controls, so they don’t need much maintenance. Also, because they don’t have many parts, they can be the cheapest system to purchase or build. But their effectiveness is limited, and they are at risk of freezing, so during cold weather they may have to be drained.
The efficiency of the collectors was increased by using flat plates, usually made out of a set of parallel copper pipes on a thin copper ‘fin’ that runs the length of the tubes. The ‘fins’ increase the heat absorption. Water, or one of various other kinds of fluid that may have better heat-transfer characteristics or are not prone to freezing, is circulated through the tubes. The solar absorber plate is then installed in an aluminium-framed box surrounded on the bottom and sides with insulation and covered with tempered glass. Flat-plate solar panels require a constant flow of fluid through the panels.
There are two types of panel setup. Open-loop systems directly heat the water. Circulation of the fluid through the solar collector is accomplished via a small pump mounted on a solar storage tank. The solar pump is activated by a differential ther- mostat controller that senses when heat is available in the solar collectors. The solar storage tank connects to the existing hot-water heater and feeds the preheated solar water into the gas or electric hot-water heater as hot water is used. The solar collectors and feed lines are protected from freezing by automatic drain-down controls, which
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allow the water in the pipes and panels to fall safely back out of the solar collectors and feed pipes. These types of system get the description ‘open-loop’ because the energy-collection loop is not separate from the rest of the hot-water system – i.e. it is ‘open’ to using the same water.
Active solar hot-water heating systems can also employ the use of heat exchangers that circulate heat-exchange fluids through the panels and feed pipes. This type of system is called a closed-loop system, because the solar exchange fluid is closed off from the external atmosphere or isolated from the potable water through utilization of a heat exchanger. In a closed-loop system the heated solar fluid is pumped through the solar collectors. The heated solar fluid flows through a copper or stainless-steel heat exchanger located near the solar storage tank. The heat from the solar fluid transfers to the potable water within the solar storage tank. Another small circulator pump may be used to circulate the water through the potable side of the heat exchanger.
There are several advantages to these systems. One is that the anti-freeze heat- exchange fluids can withstand freezing temperatures, allowing the system to operate during periods when there is the greatest temperature difference between cold incom- ing water, and temperatures reached in the solar collectors. The system can have the greatest performance benefits at this time. Also, if maintained properly, these systems will not corrode or scale the passageways in the solar collectors and pipes. Closed-loop systems tend to have the lowest overall operating costs, other than passive systems, since they do not have to be drained and maintained, but they tend to have the high- est installation cost. They heat water slightly less efficiently than direct open-loop systems, but can work more and longer when it is risky to operate open-loop systems. Thermosiphon systems are a kind of ‘passive’ solar hot-water heating that employs flat-plate solar collectors. The solar panels are usually mounted at a lower elevation than the storage water to be heated. Thermosiphon systems can circulate potable water or utilize a heat exchanger and heat-exchange fluid.
For potable water systems, the cooler water at the bottom of the storage tank is thermally siphoned to the hotter water near the solar collector by the rising temperature and volume of the warmer water, initiating a circulation of the storage water through the collector’s fluid passageways back into the top of the storage tank. The circulation continues until the temperature at the bottom of the storage tank is about the same as the temperature of the outlet pipe at the top of the solar collector.
Since the early 1970s, the efficiency and reliability of solar heating systems and collectors have increased greatly and costs have dropped. Low-iron, tempered glass is now used instead of conventional glass for glazing.
Improved insulation and durable selective coatings for absorbers have improved efficiency and helped to reduce life-cycle costs.
3.6.3 Ground-source heat
In the winter, scraping ice off the car and seeing frost on the grass, it is hard to think of the ground as a source of heat. But in fact the earth is being bombarded with energy from the sun all day – even in winter – and it absorbs much of it. That energy is stored in the earth’s huge mass, so, while the surface may be frosty in winter or cracked and
The heat connection and cogeneration 37 dry in summer, even at depths of just a few feet the temperature is fairly constant all year round. It varies, depending on where you are on the earth’s surface, between 5◦C and 28◦C.
Ground-source heat takes advantage of this constant temperature – and very often it can be used all year round, so that it helps keep a building cool in summer and warm in winter.
Ground-source heating has three main components. Within the building there is a heat-distribution system, which can be very similar to the radiators that distribute hot water around the house in a conventional heating system. Air ducts that can be used for heating or cooling flows are another possibility.
Outside the building is the heat-exchange system. If this is a so-called ‘closed’ system, it consists of loops of pipe in which water is circulated. Sometimes another fluid with better heat-transfer properties is used. Depending on the characteristics of the site and the requirements of the building, the pipework is buried horizontally or vertically, in wells bored for the purpose. In some cases horizontal tubes need be only 2 m or so under the surface. Cold water in the tubes is warmed by the surroundings and pumped back to the house. Horizontal tubes are cheaper to install, but vertical tubes are likely to have better performance because, at greater depth, the temperature is more stable.
In some areas there is free water deep below the ground – known as an aquifer. In this case an ‘open’ system can be installed. Warm water from the aquifer is pumped up through one tube, and cooled water is pumped back to the aquifer through a second pipe.
The internal and external systems are joined by the third part of the system, the heat pump. This transfers the energy between the water pumped through the earth and the internal distribution system. The heat pump can ‘step up’ the heat that comes from the ground, concentrating the energy to increase the temperature. To do this, it uses a property of gases as they are compressed and vaporized. The principle is similar to the systems used to extract heat from inside a refrigerator, turning it from cold to icy inside and ‘dumping’ the energy as heat at the back of the fridge.
In the summer the system can work in reverse (and exactly like the fridge). The heat inside the building is reduced and is ‘dumped’ through the underground pipes.
The system does require an energy input for pumping and the heat exchanger. But generally the energy required to run the system is only a quarter of the energy that can be produced – and that may be supplied by PV cells or a turbine. Typically 1 kW of electricity used to drive the equipment will produce between 3 and 4 kW of heat output – very energy-efficient.
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