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Revision as of 01:42, 24 September 2010
Basics
Energy sources
Energy storage media are matter that stores some form of energy that can be drawn upon at a later time to perform some useful operation. Energy storage is essential with autonomous devices as well as with vehicles. Also, to provide household electricity in remote areas (specifically, areas that are not connected to the main electricity grid), energy storage is required for use with renewable energy. Energy generation and consumption systems used in the latter case are usually stand-alone power systems.
Some practical examples from the list below are:
- energy carriers such as hydrogen, liquid nitrogen, compressed air, oxyhydrogen, batteries, to power vehicles.
- flywheel energy storage, pumped-storage hydroelectricity is more usable in stationary applications (e.g., to power homes and offices). In household power systems, conversion of energy can also be done to reduce odors. For example organic matter such as cow dung and spoilable organic matter can be converted to biochar. To eliminate emissions, carbon capture and storage is then used.
A device that stores energy is sometimes called an accumulator. All forms of energy are either potential energy (e.g., chemical, gravitational or electrical energy) or kinetic energy (e.g., thermal energy). A wind-up clock stores potential energy (in this case mechanical, in the spring tension), a battery stores readily convertible chemical energy to keep a clock chip in a computer running (electrically) even when the computer is turned off, and a hydroelectric dam stores power in a reservoir as gravitational potential energy. Ice storage tanks store ice (thermal energy) at night to meet peak demand for cooling. Even food is a form of energy storage, chemical in this case.
Grid energy
Grid energy storage lets energy producers send excess electricity over the electricity transmission grid to temporary electricity storage sites that become energy producers when electricity demand is greater. This means that energy storage is mostly not used, as the main electricity grid is organized to produce the exact amount of energy being consumed at that particular moment. Grid energy storage is particularly important in matching supply and demand over a 24-hour period. Grid energy, like stored energy, can be completely composed of renewable and/or green energy. Set-up of the main electricity grid in this fashion is, however, tricky on some levels. Energy production on the main electricity grid is always set up as a combination of (large-scale) renewable energy plants, as well as other power plants such as fossil-fuel power plants and nuclear power. This combination, however, which is essential for this type of energy supply (e.g., wind turbines, solar power plants, etc.) can only produce when the wind blows and the sun shines. This is also one of the main drawbacks of the system because fossil fuel power plants are polluting and a primary cause of global warming (nuclear power being an exception). Although fossil fuel power plants can be made emissionless (through carbon capture and storage), as well as renewable (e.g., if the plants are converted to biomass), the best solution is still to phase out the latter power plants over time. Nuclear power plants, too, can be more or less eliminated from their problem of nuclear waste through the use of nuclear reprocessing and newer plants such as fast breeder and nuclear fusion plants.
Renewable energy power plants do provide a steady flow of energy. For example, hydro power plants, ocean thermal plants, and osmotic power plants all provide power at a regulated pace, and are thus available power sources at any given moment (even at night, windstill moments, etc.). At present, however, the number of steady-flow renewable energy plants alone is still too small to meet energy demands at the times of the day when the irregular producing renewable energy plants cannot produce power.
Besides the greening of fossil fuel and nuclear power plants, another option is the distribution and immediate use of power solely from renewable sources. In this set-up energy storage is again not necessary. For example, TREC has proposed to distribute solar power from the Sahara to Europe. Europe can distribute wind and ocean power to the Sahara and other countries. In this way, power is produced at any given time as at any point of the planet as the sun or the wind is up or ocean waves and currents are stirring. This option, however, is probably not possible in the short term, as fossil fuel and nuclear power are still the main sources of energy on the main electricity net and replacing them will not be possible overnight.
Several large-scale energy storage suggestions for the grid have been done. This improves efficiency and decreases energy loss but a conversion to an energy storing main electricity grid is a very costly solution. Some costs could potentially be reduced by making use of energy storage equipment the consumer buys and not the state. An example is car batteries in personal vehicles that would double as an energy buffer for the electricity grid. However, besides the cost, setting up such a system would still be a very complicated and difficult procedure. Also, energy storage apparatus, such as car batteries, are also built with materials that pose a threat to the environment (e.g., sulphuric acid). The combined production of batteries for such a large part of the population would not be good for the environment. Besides car batteries, however, other large-scale energy storage suggestions for the grid include less polluting energy carriers such as compressed air tanks and flywheel energy storage.
Other storage methods
- Chemical
- Biological
- Electrochemical
- Electrical
- Mechanical
- Thermal
- Fuel Conservation storage
Some energy sources have a higher energy value, meaning that more energy could potentially be extracted. Fuel types, however, need a specific engine type to extract their energy, and some of these are highly inefficient, meaning that selecting a more energetic fuel does not always allow you to use more power. This information was obtained from http://en.wikipedia.org/wiki/Green_energy#Using_green_energy and http://en.wikipedia.org/wiki/Energy_storage
Energy conversion losses
Human efficiency
A human is also capable of providing work. When we compare a human to a machine, we see that he has an efficiency of about 1 to 5 percent (depending on whether he uses arms, or a combination of arms and legs). Internal combustion engines have an efficiency of about 20 percent, meaning that a machine is more efficient than a person in order to supply work. However, the economic cost of purchasing a machine versus hiring a labourer make the use of people still competitive in certain situations, at least until a certain duration of time. Also, certain tasks simply can't be done using a machine and another reason is that a person can provide several types of work, where a machine is often able to only provide one type (i.e., the task its specifically intended to perform).
The combined work capability of a human is also much lower than that of a machine. An average human can work for around 250 Wh per day, while a machine (depending on the type and size) can provide far greater amounts of work. For example, it takes four days of hard labour to deliver only 1kWh, which a small engine could deliver in less than one hour while burning less than 3 liters of hydrogen (or less).
Combining both the inefficiency as well as the low cumulative work capability, we can see that a boss will pick a machine over a human anytime. This, as in practice, means that a gang of 20 to 40 men will require a financial compensation for their work at least equal to the required expended food calories (which is at least 4 to 20 times higher). In most situations, the worker will also want compensation for the lost time, which is easily 96 times greater per day.
Even if we assume the real wage cost for the human labour to be at US $1.00 per day, an energy cost is generated of about $4.00/kWh. Despite this being a low wage for hard labour, even in some of the countries with the lowest wages, it represents an energy cost that is significantly more expensive than even exotic power sources such as solar photovoltaic panels (and thus even more expensive when compared to wind energy harvesters or luminescent solar concentrators).
Engine efficiency
Engine efficiencies differ depending on the engine. Gasoline IC engines have an efficiency of 20 percent. Electrical engines have an efficiency of 90 percent. [2] Hydrogen IC engines have an efficiency of 30 percent.[3] Hydrogen fuel cell engines have an efficiency of 40 to 60 percent.[4]
Inverter losses
The use of an inverter causes an energy loss of 10 percent. Excessive use of energy inversion (AC to DC or DC to AC) must thus be avoided as much as possible. To power homes, the use of either DC (direct current) or AC (alternating current) is advocated for the entire home. Some appliances may specificly require either AC or DC power (e.g., LEDs). In front of these devices, an inverter can then be placed, yet no separate AC or DC circuits are best made for homes, as this complicates the wiring and as the use of many separate electrical devices are best avoided as much as possible as well. Refer to Chapter 4 on how to decrease the use of electrical devices by using single (large) communal appliances.
Wire losses
Energy may also be lost via the wires. Wires create an electrical resistance to energy which increases the length of the longer. Thus, wire losses can be decreased by shortening them or by using better wires (with less resistance).