Heat pumps are a means of converting work energy into heat energy in a process analogous to a reverse heat engine. Thermodynamically, an ideal heat pump cycle is the most efficient means of generating heat energy at a temperature above ambient - i.e. heating a room.What is more, heat pumps extract most of this heat energy from the environment - a renewable energy source. A smaller part of the energy is supplied in the form of work energy, renewable or otherwise ....The heat transferred between a cool environment and a heated room for a given amount of work is theoretically given by the system's COP. For an ideal cycle operating between a environment at Tc and a heated room at Th this is given by 1/(1-Tc/Th). For typical exterior and interior temperatures of 10 degrees C (283K) and 20 degrees C (293K) the COP is an impressive value of more than 29.
This means that an ideal heat pump consuming 1kW of electric power for work delivers 29 times more energy than an electric element heater, with at least 28 kW of this heat energy being extracted, renewably, from the environment.
In real systems, with their non-ideal cycles and realistic system limitations, the COP rarely exceeds 4. Nevertheless, this still looks distinctly impressive compared to simple resistive electric and fossil fuel combustion heating systems. Who wouldn't want a heating system with an "efficiency" of 400% ? (more on the topic of heating efficiencies later maybe).
A so called "Primary Energy" analysis reveals a slightly different picture, perhaps. If the electricity is generated in conventional fossil-fuel combustion power plants, where work is generated from heat and then converted into electricity, a plant "energy efficiency" of 30% is quite typical. In the UK, for instance, the remaining 70% of the energy is mostly discarded as waste heat energy to the environment (in those distinctive wide parabolic-sided chimneys). If electric energy is used to drive a heat pump with a COP of 3.3, we have just recovered the heat energy initially lost to the environment - and we haven't even accounted for distribution losses. Now our "energy efficiency" appears to be well under 100%. Not as impressive! (conventional gas condensing boilers routinely exceed energy efficiencies of 90%)
So why not use the fossil fuel, on-site, in an engine to supply work directly to the heat pump and then capture the "waste" heat energy of the engine in our heating system - a classical co-generation approach. A quick calculation shows that a gas engine with a work efficiency of 25% driving a heat pump with a COP of 3.3 yields a primary "energy efficiency" of 158%. Now that's a bit better! ..... (unless these efficiencies of >100% are beginning to irritate you - in which case you'll have to wait for the next instalment on this topic)
A so called "Primary Energy" analysis reveals a slightly different picture, perhaps. If the electricity is generated in conventional fossil-fuel combustion power plants, where work is generated from heat and then converted into electricity, a plant "energy efficiency" of 30% is quite typical. In the UK, for instance, the remaining 70% of the energy is mostly discarded as waste heat energy to the environment (in those distinctive wide parabolic-sided chimneys). If electric energy is used to drive a heat pump with a COP of 3.3, we have just recovered the heat energy initially lost to the environment - and we haven't even accounted for distribution losses. Now our "energy efficiency" appears to be well under 100%. Not as impressive! (conventional gas condensing boilers routinely exceed energy efficiencies of 90%)
So why not use the fossil fuel, on-site, in an engine to supply work directly to the heat pump and then capture the "waste" heat energy of the engine in our heating system - a classical co-generation approach. A quick calculation shows that a gas engine with a work efficiency of 25% driving a heat pump with a COP of 3.3 yields a primary "energy efficiency" of 158%. Now that's a bit better! ..... (unless these efficiencies of >100% are beginning to irritate you - in which case you'll have to wait for the next instalment on this topic)
No comments:
Post a Comment