Embedded Generation Demand Side Management Economic Aspects Enviromental Aspects
   
 
Currently there are several types of technology which can be used to generate electricity from renewable sources. In our study we have considered the most common and advantageous technology types which include wind turbines, photovoltaics and combined heat and power. However if there is a good resource for a more specialised renewable technology it should be considered.

Energy from the sun

 

Energy from the sun has been harnessed for thousands of years. We utilise this energy in three main ways [15]:

Passive heat – This is the heat that we receive from the sun naturally. This can be taken into account in the design of buildings so that less additional heating is required.
Solar thermal – This is when the sun's heat is used to provide hot water for buildings or swimming pools.
Photovoltaics (PV) – Use energy from the sun to create electricity to run appliances and lighting.

The solar resource
   
Solar thermal

Solar water heating systems gather energy radiated by the sun and convert it into useful heat in the form of hot water. This technology is well developed with a large choice of equipment to suit many applications. Solar water heating systems work alongside your conventional water heater to provide hot water. If one has been sized and installed correctly it can provide almost all of a buildings hot water during the summer months and about half of it for the remainder. It will reduce carbon dioxide emissions by 0.25-0.5 tonne per year, depending on the type of fuel that the system replaces [15].
   
 

A range of factors will affect the size and type of system that would be suitable including; the amount of south facing roof space, the existing water heating system and the capital available for the project. Approximately 2-4 m2 of southeast to southwest facing roof space that receives minimal shading during the main part of day will be needed and space to place an additional water cylinder if required [15].

Solar water heating can be used for domestic water heating and also for larger scale applications such as swimming pools. A solar water heating system for domestic hot water comprises three main components: solar panels; hot water cylinder and a plumbing system.

The principle and operation of these systems are quite simple. Solar panels are fitted to the roof and retain heat from the sun's rays. This heat is then transferred to a fluid and pumped through the plumbing to a hot water cylinder where it is stored for later use.

Solar collector (Water heating)
 
Photovoltaics

Photovoltaics (PV), use energy from the sun to create electricity to run appliances and lighting. The PV cell consists of one or two layers of a semi-conducting material to convert solar radiation into electricity. When sunlight shines on the cell it creates an electric field across the layers, which causes electricity to flow. When the light intensity increases, so will the flow of electricity that has been generated.

 

PV systems generate no greenhouse gases and save approximately 450 kg of carbon dioxide per year for each kWp (Kilowatt peak ) [15]. The three main types of solar cells are:

Monocrystalline – These cells are thin slices of silicon, cut from a single crystal and have a typical efficiency of 15%.

Polycrystalline – These cells are thin slices of silicon, cut from a block of silicon single crystals and have a typical efficiency of over 13%.

Thin Film – These cells made from a very thin layer of semiconductor atoms deposited on a glass or metal base and have a typical efficiency of 7%.

Polycrystalline PV cells
   
 

Individual PV cells are connected together to form a module. Modules are then linked into an array and are sized to meet a particular load. The output from the modules has to be changed for dc to ac to be used in standard electrical networks with the use of an inverter. The conversion from DC power to AC power results in an energy decrease from approximately 5%-10%, and varies for each inverter with the wasted energy being lost as heat. Higher losses may occur when the inverter is only operating at a part load [21].

 
 

PV arrays

PV arrays may also be isolated or connected to the local electricity supply network. If they are connected to the local network any excess electricity that has been produced can be exported. However, when demand is high, backup electricity can be supplied from the grid through the utility companies. Where there is no mains supply to export the excess, the PV arrays will be put to use charging batteries, or the load can be dumped as waste heat.

PV arrays now come in a variety of shapes and colours, ranging from grey 'solar tiles' to be built into the roof structure to more conventional looking panels. PV systems can be used for a building with a roof or wall that faces within 90 degrees of south provided that there is no overshadowing of the area by vegetation. For some buildings another consideration may be whether the roof will be strong enough to hold the weight of the panels. The number of panels and hence weight applied to the roof will be dictated by the amount of electricity required to be generated.
   
  Energy from wind

In the UK we have a large potential wind resource. Although we have a high proportion of Europe 's total wind energy resource, it remains largely untapped, currently meeting only a very small proportion of our electricity requirements.

Power from the wind is proportional to the cube of the wind speed therefore relatively minor variations in wind speed can result in large changes in output from the turbine. Individual turbines vary in size and power output from a few hundred watts to 2-3 MW. Turbine uses range from very small turbines, supplying energy for battery charging systems, to turbines grouped in wind farms supplying electricity to the grid [22].

Wind speed increases with height so the best location will be high on a mast or tower. An ideal siting for the tower would be a smooth topped hill with a flat, clear exposure, free from excessive turbulence and obstructions such as large trees, houses or other buildings. Often the ideal locations are far from the urban centres and other connection issues arise. However, small-scale building integrated wind turbines suitable for urban locations are currently being developed and will be available to install in homes and other buildings within the next few years.

   
 

Knowledge of the local wind resource is critical to designing a wind energy system and predicting output. Planning issues such as visual impact, noise and conservation issues also have to be considered and the installation will require permission from the local authority.

Most small wind turbines generate direct current (DC) electricity. Off-grid systems require battery storage and an inverter to convert DC electricity to AC (alternating current). A controller will also be required to ensure the batteries are not over or under-charged and can divert electricity to another useful source when the battery is fully charged. These types of systems are usually combined with diesel generators as back up generators for periods of low wind speeds. With the combination of the two systems, if there is little wind, the generator can be used to charge batteries at optimum load for short periods of time. If the generator is constantly used with a varying load, it will considerably reduce its efficiency.

   
 
Wind turbine

Large wind turbines are generally used in groups and will be connected directly into the national grid. These vary in size from approximately 600 kW, with a hub height of approximately 40 metres, upwards with current developments pushing generating capacity towards 2 MW per turbine [22].

Wind systems can also be installed where there is a grid connection. An inverter and controller will convert DC electricity to AC at a quality and standard that will be acceptable for export to the grid. No battery storage will be required as all excess electricity can be exported to the grid and sold to the local utilities.

   
 
Provided the wind resource is there, community wind projects can be a viable proposition. Potentially, there are great benefits for the community to join together and increase the capital available to invest in the project. Community projects may also be eligible for substantial grants from the relevant government agencies.
   
  Combined Heat and Power

Combined heat and power (CHP) is the production of electricity and useful heat in a single process. In CHP plants the heat produced during the generation of electricity can be put to good use, rather than being wasted.

The basic elements of a CHP plant comprise one or more prime movers usually driving electrical generators. The heat from the engine block, oil cooler and exhaust, which would normally be wasted, is absorbed by coolant water through heat exchangers. The heat generated in the process can then be used for a variety of purposes including; industrial processes, community heating and space heating. In general, the production of 1 kilowatt of power will create 2 kW of usable heat energy. CHP plants can generate useful energy, at the point of use, in the form of both electricity and heat, with overall efficiencies that can exceed 80% [13].

   
 
CHP unit (Internal combustion engine)
The most efficient CHP systems, exceeding 80 percent overall efficiency, are those that satisfy a large thermal demand while producing relatively less power. As the required temperature of the recovered energy increases, the ratio of power to heat output will decrease. The decreased output of electricity is important to the economics of CHP because moving excess electricity to market is technically easier than is the case with excess thermal energy [12].
   
 
CHP provides an excellent solution to the energy needs of many different building types which have a simultaneous requirement for heat and power in excess of 4,000hrs per year. There are approximately 1000 individual buildings with CHP in the UK, representing an installed capacity of approximately 320 MWe. The types of buildings that have proved particularly suited to CHP include: hospitals, hotels, leisure centres, universities, police stations and residential homes [14].

CHP in district heating

In addition to the individual sectors, there is considerable advantage in energy linking buildings so that aggregate loads are met by centralised plant. This can be built up into community (or district) heating networks, with significant extra CHP potential. District heating is where a group of buildings, whether domestic, non-domestic or a combination, are supplied with heat from a single source. CHP provides an excellent heat source for district heating schemes with the energy being distributed using hot water or steam as a medium.
   
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