Hello everyone,
After reading here for a while, I am now starting my first thread because I recently became a homeowner. I am currently thinking about basic energy-efficient renovations and have some questions about the fundamental purpose of heat pumps, which I have not really found answers to despite extensive searching and reading here. I have a basic understanding of thermodynamics but no technical knowledge about heating systems. However, I keep coming across statements about heat pumps that contradict my basic understanding of thermodynamics. Threads here or on Google are usually too detailed, and even a conversation with an energy consultant didn’t help much, as everything basically ended with "it depends."
My thoughts and what I know or understand:
What I don’t understand:
I want to exclude aspects like “independence from Russian gas,” “gas will eventually be more expensive than electricity,” and photovoltaics from this discussion for now. I understand those points, but as I said, I first want to clear up my knowledge gaps or logical errors that I apparently have.
After reading here for a while, I am now starting my first thread because I recently became a homeowner. I am currently thinking about basic energy-efficient renovations and have some questions about the fundamental purpose of heat pumps, which I have not really found answers to despite extensive searching and reading here. I have a basic understanding of thermodynamics but no technical knowledge about heating systems. However, I keep coming across statements about heat pumps that contradict my basic understanding of thermodynamics. Threads here or on Google are usually too detailed, and even a conversation with an energy consultant didn’t help much, as everything basically ended with "it depends."
My thoughts and what I know or understand:
- To keep a house at a constant temperature, a certain amount of energy is needed, which is lost through the walls. This energy must be supplied to the house by the heating system. The better the insulation, the lower the heat loss, and therefore, the less energy the heating system has to provide.
- Gas is generally (at least until 06/22) cheaper per kilowatt-hour than electricity.
- If a heat pump and a gas boiler have roughly the same efficiency in heating water, then each kilowatt of heat from one kilowatt-hour of energy source (electricity or gas) should generally be cheaper with the gas boiler at first.
- Heat pumps need underfloor or other surface heating systems for good efficiency, while gas boilers can work with any heating system. Installing underfloor heating does not necessarily mean switching to a heat pump, but it does open the potential for that later without much extra effort.
What I don’t understand:
- In discussions where someone plans to install a heat pump, there is always a warning that if the house has poor insulation, the heat pump will have a negative impact on heating costs. But shouldn’t it always do that? According to my reasoning above, even in a well-insulated house, the gas boiler should be cheaper because gas as an energy source is still less expensive for the small amount of energy needed.
- If my reasoning is wrong, and a heat pump does become more cost-effective than a gas boiler in a well-insulated house (due to some non-linearity in the efficiency of the heat pump or gas boiler) — is there any rule of thumb or benchmark to say roughly from which point a heat pump makes sense (for example, below “100 kWh/(m2*a)” or something similar)?
I want to exclude aspects like “independence from Russian gas,” “gas will eventually be more expensive than electricity,” and photovoltaics from this discussion for now. I understand those points, but as I said, I first want to clear up my knowledge gaps or logical errors that I apparently have.
D
Deliverer20 Jun 2022 18:23Yep – you always have to adjust a heating system yourself. This has also been true for fossil fuel systems.
4lpha0ne schrieb:is actually the most efficient way to operate a combustion engine vehicle. That’s how fuel economy contests are won. The reason lies in the power curve of combustion engines: the sweet spot is at nearly full load and about two-thirds of the rated engine speed. So it’s a bit different from heat pumps. ;-)
... accelerating constantly in low gear and coasting with an internal combustion engine ...
D
Deliverer20 Jun 2022 21:19Performance curves for many engines can be found online. These show torque, engine speed, and power. From this, one or more optimal operating points can be identified. It is important to consider that the power must actually be needed. It is pointless to run the engine at the optimal speed if the required power is not demanded. In such cases, reducing throttle makes more sense. Or, as mentioned above, cutting the throttle completely and coasting.
The issue of required power is why internal combustion engines typically achieve only about 10-15% efficiency in real-world driving. The theoretical 35% efficiency occurs for about 10 seconds, such as when merging onto a highway. For the remaining 99% of the time, most cars are overpowered and operate inefficiently, consuming more fuel.
This is exactly why Toyota developed the Prius: a non-plug-in hybrid that was and still is more fuel-efficient than any other internal combustion engine vehicle. It does this by shifting the operating point upward during inefficient low-speed driving, meaning it loads the engine more and uses that energy to charge the battery. Shortly after, the engine can shut off completely, which is the optimal state for saving fuel.
Okay, sorry for going off-topic. If I have explained anything poorly, please feel free to correct me—I’m not an expert and I’m not very fond of driving. ;-)
The issue of required power is why internal combustion engines typically achieve only about 10-15% efficiency in real-world driving. The theoretical 35% efficiency occurs for about 10 seconds, such as when merging onto a highway. For the remaining 99% of the time, most cars are overpowered and operate inefficiently, consuming more fuel.
This is exactly why Toyota developed the Prius: a non-plug-in hybrid that was and still is more fuel-efficient than any other internal combustion engine vehicle. It does this by shifting the operating point upward during inefficient low-speed driving, meaning it loads the engine more and uses that energy to charge the battery. Shortly after, the engine can shut off completely, which is the optimal state for saving fuel.
Okay, sorry for going off-topic. If I have explained anything poorly, please feel free to correct me—I’m not an expert and I’m not very fond of driving. ;-)
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