Your choices:
1 Biomass (solid); 2 Electricity; 3 Process heat
| 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 |
| 1 Biomass (solid) | 2 Electricity | 3 Process cooling (< 0 °C) |
| Geothermal | Fuel: Gaseous | 3 Process heat/steam (50 - 150 °C) |
| Sunshine | Fuel: Liquid | 3 Process heat (150 - 1000 °C) |
| Water | Fuel: Solid | 3 Process heat (> 1000 °C) |
| Wind | Local cooling (ind. house) | Transport |
| Residual oils/fats etc | Local heating (ind. house) |
Electricity is the king of energy carriers. The unique thing with electricity is its high share of exergy which means that it can be converted into different types of energy: mechanical work, into illumination, into extremely high or low temperatures, into pressure, into radiation of different wavelengths; it can be used to run home electronics, for transportation and – basically – any number of applications.
Freezing temperatures at process industries, such as a food processing industry, must be produced locally, on-site. This is not necessarily the same as to say that the production of freezing temperatures must be seen as an isolated or stand-alone process, but a system perspective must be adopted.
Cooling and refrigerating temperatures can be produced basically in two different ways, by absorption cooling where the main energy supply is heat and only minor amounts of electricity are needed, or by compressor cooling machines where all the help energy is supplied as electricity.
From a thermodynamic point of view, the production of low temperatures is complicated and requires exergy. The exergy can be supplied either as relatively small amounts of electricity, which is a high-exergy energy carrier or via proportionally larger amounts of low-exergy energy carriers such as hot water. And the lower the desired temperature, the more exergy the process will demand. It must also be remembered that even absorption cooling system will always demand a base supply of electricity for the pumps. From a system point of view, the supply of relatively smaller amounts of electricity may always seem the better alternative but in such cases when thermal energy is locally available close to the cooling/freezing needs, other solutions should be considered.
Many industrial processes, like food manufacturing, washing and several biotechnological and/or chemical processes require only modest temperatures like 0-150 °C . In very many cases will the companies themselves have their internal energy supply system, often centred around one or two small hot-water or steam boilers producing a heat carrier (hot water or steam) that is distributed around the production site.
To obtain temperatures in this range, only minor amounts of exergy are needed and the energy carrier should be chosen accordingly. Steam, hot oil, superheated water as well as low-quality (i.e. not upgraded) fuels like raw biogas can all be considered. Obviously, environmental constraints must be put on the use of low-quality fuels so that the outdoor air quality does not suffer.
Temperatures ranging from 150 to some 1000 °C are common in many types of industries like asphalt works, metal manufacturing and heat treatment, metal casting, glazing of ceramics end numerous others. CHP-plants and hot water boilers for the production of district heating, district cooling and electricity also fall into this category.
With increasing temperature demands the share of exergy increases but in case fuel-firing is used for the production, then any fuel that is combustible will also be able to deliver any of the temperatures desired in this interval. To put it simple: In case something can burn it will already produce temperatures exceeding 1000-1100 °C. If such temperature is not reached, then the fuel will not burn. For simplicity, some industries prefer to use electricity also for these relatively low temperatures and it is obvious that the source of electricity can be changed freely. However, there are no thermodynamic demands or limitations that make electricity exclusive for applications in this temperature range.
For temperatures exceeding 1000 °C, such as glass melting, steel reheating for rolling, the burning of ceramics like building brick or cement, fossil fuel firing and electricity are the major sources of energy supply.
The demand for exergy increases successively with the demanded temperature and while solid biomass with moisture contents up to some 50% or raw biogas may well be sufficient for the lower temperature range will purified bio-methane, wood pellets, dried wood powder or torrefied biomass have to be used in cases when the demanded temperature exceeds 1500 °C. The refined and stabilised bio-oils (FAME) can also be used for these temperatures but the market for these fossil-oil substitutes is mainly the transport sector.
With electricity, king of the energy carriers, temperatures up to about 3000 °C can be reached in plasma torches.
Hence again the energy carrier should be selected with respect to the energy service desired but electricity is often the preferred choice because it is easy to control and it is clean at the end user.