Direct electric resistance heating

Berthold Kaufmann

12.4.1 Electrical space and water heating

Electricity is a high-level energy form (100 per cent exergy) that can provide almost all energy services, from supplying electric light, the mechanical drive of electric tools and modern information technology. For all of these activities, there is no substitute for electricity.

On the other hand, there are many possible substitutes for the task of space and water heating -for example, the needed heat can be produced directly from burning fuels. From a technical point of view, combustion technology is highly advanced and cost effective.

When combustion has the task of producing electricity, the heat must first be converted by a thermodynamic process - for instance, by a steam engine or turbine. This conversion results in substantial losses of energy. The thermodynamic efficiency of such machines is only about 0.3, or 30 per cent. Only the best combined gas and steam turbines that combine the two functions in subsequent steps reach a thermodynamic conversion efficiency, in total, of about 56 per cent. These highly efficient systems are, up to now, very rare so that the average end-to-primary energy conversion factor in Europe is, at present, about 38 per cent (CEPHEUS-GEMIS, 2004).

The conversion factor will hopefully improve in the near future as conventional sources of primary energy are increasingly replaced by renewable sources. The growth rate of all renewable energy sources together has been anticipated to increase to about 25 per cent of the total energy production by 2020. Yet, even if this ambitious goal is reached, the amount of renewable energy available will still be limited and prices for electricity can be expected to rise. So there is a strong argument that electricity is and will continue to be too valuable to be 'burned' merely to produce low temperature heat!

12.4.2 Alternative technologies for heat production

Small heat pumps using exhaust heat from a ventilation system with heat recovery can provide seasonal performance factors (SPFs) higher than 3, making heat production by this method more efficient than direct electric heating. The investment costs for these combined systems are higher than for an ordinary resistance heater, but the energy savings during the system lifetime can offset this.

12.4.3 Peak power supply and freeze protection by direct electric heating

There are some applications where direct electric heating to cover peak heating demand may be sensible and cost effective.

When domestic water is heated by a low power heat pump coupled to a 150 litre to 200 litre boiler, there may be hot water shortage for short periods during the year. To economically solve this problem, a flow-through water heater is a reasonable alternative. This heater should be arranged after the heat pump so that direct electric heating is only called for when the heat pump is unable to deliver the needed heating power. If the system is properly dimensioned, such occurrences should be rare. Accordingly, the absolute amount of electricity consumed for this purpose should be minimal over the year.

By the same reasoning, a reserve radiator in a living room can be sensible. It can be regulated to only switch on when the system control recognizes that the heat pump cannot deliver enough heat. If this occurs too frequently, an error message can be generated to inform the occupant or building manager.

A third plausible application is heating air. A pre-heater in the outside air duct can be activated to ensure that the supply air is always above -5°C. This prevents humid room exhaust air from freezing up and shutting down the ventilation heat exchanger. The duct thermostat should be set so that incoming air is not heated above +2°C in order not to waste heat (Kaufmann et al, 2004). The air heater can be a simple unit comprised of an inexpensive electric resistance grid positioned in the air flow. When the regulator is properly adjusted, heating time will be kept minimal in Central European climates.

References

CEPHEUS-GEMIS (2004) Global Emission Model for Integrated Systems, available at

www.oeko.de/service/gemis/de/material.htm#infos Feist, W. (2005) Zur Wirtschaftlichkeit der Wärmedämmung bei Dächern, Protokollband no 29,

Arbeitskreis Kostengünstige Passivhäuser (AKKP), 1, Auflage, Darmstadt, Germany IEA (International Energy Agency) (2001) World Energy Outlook 2000 Highlights, OECD, Paris Kaufmann, B., Feist, W., Pfluger, R., John, M. and Nagel, M. (2004) Passivhäuser erfolgreich planen und bauen: Ein Leitfaden zur Qualitätssicherung im Passivhaus, Erstellt im Auftrag des Instituts für Stadtentwicklungsforschung und Bauwesen (ILS), Aachen, Germany Reiß, J. (2003) Messtechnische Validierung des Energiekonzeptes einer großtechnisch umgesetzten Passivhausentwicklung in Stuttgart-Feuerbach, Fraunhofer IBP Stuttgart, Ergebnisse des Forschungsvorhabens Passivhaustagung, Hamburg, Germany

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