Your choices so far:
1 Electricity
| What is your resource? | What do you want to deliver? | What is the service the customer wants? |
| Biomass (digestible sludge) | District cooling | Comfortable indoor climate |
| Biomass (fermentable sludge) | District heating | Electricity |
| Biomass (solid) | 1 Electricity | Process cooling (< 0 °C) |
| Geothermal | Fuel: Gaseous | Process heat/steam (50 - 150 °C) |
| Sunshine | Fuel: Liquid | Process heat (150 - 1000 °C) |
| Water | Fuel: Solid | Process heat (> 1000 °C) |
| Wind | Local cooling (ind. house) | Transport |
| Residual oils/fats etc | Local heating (ind. house) |
Electricity – un-threatened king of the energy carriers – is often supposed to be the main demand by the customer. The reason for this is that electricity is the most flexible of all energy carriers, it can be converted into heat, it can be used for cooling and it can be used for mechanical work.
In thermodynamic terminology, it is said that electricity has high exergy content, independent of the surrounding. Exergy is the kind of energy that can be converted, and since electricity can be converted it is obvious that the main part of the electrical energy is exergy. The remaining part of the energy is called anergy. The use, or the "consumption", of energy is really the successive loss of exergy through its transformation to anergy.
Using electrical energy is hence a simple thing: It can be used more or less for anything and in western and industrialised countries are the distribution grids expanded all the way out to individual houses with only few exceptions. Likewise, the distribution is simple: The electricity grid is there and what you need is to establish a connection point and connect your production unit.
In Europe electricity is delivered as so-called alternating current (AC) with a frequency of 50 Hertz. That means that your production, to be accepted by the grid, has to be AC/50 Hz. It is also three-phase. But that is not all: Electricity is finally characterized by two values, namely its potential (measured in Volt, V) and its current (measured in Ampere, A).
The electricity grid can be seen as a tree structure where the stem is built up from a high-voltage main grid 400 000 – 800 000 V which is then successively branched all the way down to the most local grids at 400 V. Individual customers would typically be connected to the 400 V three-phase grid and in the individual dwellings the sockets will be 230 V AC, single-phase.
As the first and roughest estimate one may say that electrical power (Watt, W) equals voltage multiplied by current so that if a motor takes 10 A at 400 V its power will be ten times four hundred = 4 000 W or 4 kW. Strictly, this is valid only for direct current (DC) and only under specific conditions, but for the simplest estimates it is a useful approximation.
As said above, the electricity grid operates at a frequency of 50 Hz. For many reasons, this frequency must be kept within very narrow limits. At the same time, electricity cannot be stored in the grid. So when electricity is fed into the grid at one point, there has to be a corresponding and simultaneous extraction of electricity at some other point. In principle, the grid works so that when you switch on the light in a room, then a power station somewhere in the European electricity system will increase its production to supply the power you use.
In reality it does not work exactly that way but when you switch on the light in a room, what happens is that the frequency drops a tiny bit from the prescribed 50 Hz and becomes lower. The central power control will have sensors to detect the frequency and once it drops below a threshold the production at one or more power stations is increased so that the frequency is restored. This works also the other way around: if the feed to the electricity grid at some moment exceeds the outtake of electricity, then the frequency increases and once the frequency exceeds a threshold production is decreased so that the frequency is restored.
Now imagine an electricity production system with random fluctuations. You will then realize that if the production cannot be thoroughly controlled, then the frequency will fluctuate. This is a serious problem since many modern electronic and electrical apparatuses are strongly depending on a stable frequency for their function and for their lifetime. You will realize that the European electricity grid is much more powerful than any single wind farm, so there is a certain "buffering" capacity but in case you want to connect a wind generator at a low voltage (i.e. to a local grid), then the sensitivity is much more pronounced.
The conclusion becomes that local considerations concerning the capacity of the grid to accept random feed may be limiting factors for – just as an example – the amount of wind power or solar power to be installed in a region or in a locale. In general, this sensitivity is more pronounced in the low-voltage (local) grids at 400 V than at higher voltage. But then, on the other hand, a reasonably large wind farm will typically "want" to deliver at maybe 10 000 – 45 000 V (regional grid) and then will also the total power delivered be significant.
Hence the electricity grid demand for stable frequency (i.e. the fear of "flicker") may well be a major constraint for the installation of unpredictable electricity production units, i.e. wind power and solar power.
Such restrictions do not apply to steam- or gas-turbine cycle power generation in CHP-plants or condensing power plants, nor does it apply to hydroelectricity, all of which can be easily controlled.
So the electricity production must be based on a number of stable production techniques to maintain the frequency base-line. Then there has to be some techniques that allow for rapid control to keep the frequency within narrow limits. Finally, there must also be room for new, intermittent and un-controllable input from solar cells and wind-power stations.
Co-generation plants (district heat and electricity) as well as tri-generation plants (district heat, district cooling and electricity) fired with solid biomass or with biogas qualifies for the first group. The production in these plants will certainly depend on weather, since the demand for district heating and -cooling will be weather dependent, but the variations will be slow.
Hydropower stations qualify for the second group. The control of the production in a hydroelectric power station is done by opening or closing for the water running through the turbines and this can be done almost momentarily.
Solar cells and wind-power fall within the third group: electricity production that is welcome to the grid but which has to be closely monitored and compensated for so as to not upset the quality (i.e. the frequency) of the grid electricity.
In case of micro-scale production, sufficient only for the individual house or so, the electricity production unit can be connected to the electricity system in the building, i.e. at the consumer side of the electricity meter. Doing that, what happens is that when electricity is produced the demand for electricity from the external grid drops and the meter slows down. In case the local production overrides the use, electricity will be exported from the house to the grid and the meter will move backwards. Thus the local production will be automatically discounted from the electricity bill.
Today, one may contract the delivery of different qualities of electricity. The contracting works just like the bank system with its cash machines: If you have a job and a salary, then your money is paid to a bank account by your employer and you can withdraw the money from a cash machine almost anywhere. The bill you get from the cash machine has – most likely – never even been touched by your employer, but it is still considered being paid by the employer to you. This is achieved by regular balancing of all bank accounts between banks, companies and individuals. These balances are done on a 24-h basis.
The same system applies to the electricity market: If you have a contract for wind power, then your withdrawal (i.e. use) of electricity from the grid must balance the input of windpower from your contractor. The balancing can be hourly, daily or any period of time depending on federal and national law and on contract.
Because of this system it becomes meaningful to talk about the use of e.g. Danish wind power for e.g. the production of bacon in Spain or the use of solar-cell electricity from Cyprus for climate control in a hotel in northern Sweden.
Due to the unique features of electricity and the following difficulties to produce it, it should be produced only in processes best aimed for it and it should be used only in processes where the unique features are fully appreciated.
But there will also be a total volume constraint: Since – just as an example – solar cells so far have a low efficiency and the total production of electricity from solar cells is limited, then it becomes un-realistic to consider solar-cell electricity as the base supply for – again just as an example – the European virgin aluminium production. However, based on a 24-h balancing, it may well be reasonable to consider solar-cell electricity generated during the day as the main source for illumination used during the dark hours in a single building. Doing the balancing on a 24-h basis will mean that the fact that electricity cannot be stored falls out of the argument.