Your choices so far:
1 Process cooling (< 0 °C); 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) | Electricity | 1 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) |
From a thermodynamic point of view, cooling something to a temperature below ambient is a very
complicated process. Just imagine the home refrigerator:
When something warm is put into the refrigerator, then thermal energy becomes present in there. As this
thermal energy is given of inside the fridge, the temperature will rise and the thermostat will call for
cooling. What then happens is that a coolant at say 4 °C is circulated absorbing the extra
heat and attaining a temperature of, say, 6 °C. A secondary medium is then used to absorb
the extra heat from the primary coolant, restoring its temperature back to
4 °C. The secondary medium is now boosted to a temperature higher than ambient – say
30 °C – so that it may spontaneously cool down and leave off the extra energy when exposed
to the room air (which is assumed at 20 °C) in a heat exchanger coil on the back wall of the
fridge.
So what cooling requires is that heat is made to flow from a low-temperature confinement into a surrounding at a higher temperature. Such a complicated process – working against the spontaneous flow of heat from higher temperatures to lower – requires exergy.
Exergy can be supplied in two different ways: Either can a large amount of energy with only a small fraction of exergy be supplied or can a small amount of energy containing a large fraction of exergy be supplied.
- The first is what happens in absorption heat pumps where district heating water, solar energy or fuel firing (like in propane-fired refrigerators for boats or caravans) is used.
- In the second case, electricity is used to provide the exergy.
The lower the temperature desired, the bigger ne need for exergy and when the desired temperatures fall below 0 oC the use of absorption heat pumps becomes impractical.
There are two ways to use electricity for cooling:
- The first and most common is the compressor heat pump. This is what is used in home freezers, refrigerators and alike and also in larger applications such as freezing rooms in food manufacturing industries, butcheries and alike.
- The second is direct cooling by the use of Peltier elements where the electrical current directly – without any media – will provide a cold spot in the junction between two materials. Peltier elements are used in refrigerators for cars or other leisure-sized applications.
For large-scale cooling needs, the main alternative will be to use compressor cooling machines and to combine this with the use of renewable energy sources the demand will be that the electricity has been produced using renewable resources. There will also basically be only one energy carrier in the system – electricity.
The technologies to be considered then become, in priority order: 1-hydroelectricity, 2-wind-power, 3-solid or digestible biomass in large-scale CHP or tri-generation plants. Solar cell electricity production is still too insignificant to be treated.
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.