Energy Efficiency

What does 'energy efficiency' mean?

Energy efficiency is the term given to the ratio between the energy input and the useful energy output. The input usually takes the form of a primary energy source (fossil fuels, mechanical energy from the wind, solar radiation etc) and the output relates to the end use.  Thus a wind turbine has an efficiency of around 15-25 percent in converting the kinetic energy of the wind to electricity.

This figure is sometimes called the energy conversion ratio.Typically, any figures of energy efficiency given by manufacturers or suppliers in relation to their products refer to the maximum achievable efficiency in laboratory or factory conditions. Even where this relates to an ‘average’ output over time, this ‘average’ may be based on the device working at near maximum output and all other conditions being favourable. For example, wood or wood pellet burning stoves or boilers are often described as being 80-85 percent efficient in converting the theoretical energy value of the fuel into useful heat energy but this level of efficiency may only be achieved when the fuel has the lowest possible moisture content, during conditions of little or no wind, and when the occupants of the building require a large heat output. In real life conditions where the device is turned down low and the fuel quality is less than perfect, the real efficiency is often much less.

Where the heat source (conventional or biomass boiler, heat pump, solar panel) is located some distance away from its required destination, distribution losses can amount to as much as 30 percent of the total output of the device. These losses will be proportional to the amount of insulation and lagging around the pipes.Some typical thermal efficiencies of different types of domestic heating technologies (allowing for typical distribution losses) are as follows:

Oil Fired Boiler:                             50-70
Gas Boiler:                                   65-80
Biomass Boiler:                             55-75
Solar thermal Panels:                     15-40
Solid Fuel Stove in Dwelling:           40-70
Open Fire:                                    10-25

Claims of higher figures made by the manufacturers should be qualified.While efficiencies can never exceed 100%, heat pump based technologies are sometimes described as being three or four hundred percent efficient. What this means is that the heat pump can deliver more useful heat than it uses. This ratio is more accurately known as the coefficient of performance (COP). A COP may be as high as 4 but more typically is 2-3. This means that for every one unit of electrical energy used by the heat pump, two to three units of useful heat energy are delivered.  

Houses

Most houses are very inefficient at trapping heat energy and up to 90 percent of the available heat source may leave the building with little benefit to the occupants. Very significant improvements can be made by insulating the building properly and lagging all hot water pipes.Much has been said about the benefits of using heat recovery ventilation systems. These extract some of the heat from air expelled from the building and use it to warm incoming fresh air from outside.

Although theoretically the use of heat recovery ventilation systems could increase the energy efficiency of a building by up to about 30 percent, in the real world the figure is likely to be considerably less.  When installed in houses, heat recovery ventilation systems tend to over ventilate a lot of the time, as the air exchange rate is often set for the maximum number of occupants in the building and does not allow for the reduced requirements when the building contains less people or is empty. In some cases, heat recovery ventilation systems may use more energy than they can save.

Another point often overlooked is that only a tiny percent age of houses, probably under 0.1 percent, are so tightly sealed that  adequate natural ventilation will not occur through the fabric of the building without any further need for mechanical ventilation.Heat recovery ventilation systems are of most benefit in tightly sealed buildings with high rates of occupancy such as schools, colleges and offices and in which the number of occupants follows a clear daily cycle.  They can only work well in buildings in which the windows are kept closed. Once a window is opened, all the potential benefits of the heat recovery ventilation system are lost.

Electrical Energy

As stated above, typical efficiencies of wind turbines are usually in the 15-25 percent range. Efficiencies tend to be lower for very small turbines, or for turbines located near to the ground or on turbulent sites. In some cases the efficiency may be as low as 5 percent .Solar photovoltaic  panels (solar PV) have efficiencies of up to about 13 percent, though typically in Ireland 5-8 percent can be expected. The efficiency of solar PV is seriously compromised  if any part of the panel is shaded, even by water droplets.Diesel and petrol generators have a maximum efficiency of about 25-30 percent. The efficiency level plummets when the load is small, and is effectively zero when there is no electrical demand.

Transport

There is much hype from the car manufacturers about improved engine efficiencies as a possible means of reducing carbon dioxide emissions but for the most part this is a technological fantasy. Internal combustion engines, for example, are already close to their maximum achievable efficiency. While vehicles powered by electricity convert electrical energy to mechanical energy  at much higher levels of efficiency, this has to be balanced against the very inefficient and polluting methods commonly used to generate electricity, namely by the burning of fossil fuels.The only significant improvements which will be made in terms of fuel efficiencies will be made by making vehicles lighter, designing them for much lower maximum speeds, and having more passengers in each vehicle. Vehicles designed to have a maximum speed of 80 kmph (50 mph) would be able to travel up to twice as far  for the same input of energy as the current generation of road vehicles. Even  existing models will achieve much better fuel consumption at lower speeds.

In one recent study carried out by the Sustainability Institute, a 13 year old Peugot 405 with over 320,000 km ( 200,000 miles ) on the clock still managed  over 17 km per litre ( 49 miles per gallon)  averaged over a full tank of fuel. The maximum speed driven was 90 kmph (56mph).When fuel efficiency is measured in terms of passenger miles/kilometres travelled per given quantity of fuel, however, cars will never compete with buses or trains. Perhaps surprisingly, some studies have shown that buses have a greater fuel efficiency than trains. Typically, buses are more efficient than cars by a factor of 5 or more. At least with terrestrial transport there is plenty of scope for achieving greater fuel efficiency simply by designing  vehicles to go much slower.  This is not even a possibility with air travel as below a certain speed (known as stall speed) planes will simply fall out of the sky. Air travel will never be fuel efficient. The current trend towards larger and larger aircraft is unlikely ever to achieve its publicly stated aim of greater fuel efficiency and even if it did, it has to be viewed in the context of moving ever greater number of people to places to which they don’t really need to go.

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