Issue 10: Analysis of photovoltaic and heat pump data for 2022.
A deeper look into what my private roof-top photovoltaic system's and heat pump's performance in 2022.
This issue is intended to give you some insights in real life energy data of a modern house with an up-to-date energy system. It might help you to plan your own project, optimize an existing system or just compare yours.
The year 2022 is over and from the perspective of a private owner of photovoltaics in Germany, it was a great year. Let’s have a deeper look into the data I have collected along the way.
For details about my photovoltaic system and heat pump, check out Issue #1 of this blog.
Please note: In my texts I avoid the word “energy production”, because energy is neither produced nor consumed, just converted. Therefore I use the word “harvest” or “yield”, but haven’t found a better word for “consumption”, even if it’s actually not accurate.
Photovoltaic & Battery System
Sunshine hours 2021: 1.960 hrs (source: Munich Airport weather data)
Electricity harvested in 2021: 16.235 kWh
Specific annual yield: 1.137 kWh/kWp/a
Sunshine hours 2022: 2.224 hrs
Electricity harvested in 2022: 17.280 kWh
Specific annual yield: 1.210 kWh/kWp/a
The specific annual yield of 1.210 kWh/kWp/a is particularly noteworthy because the installer of my system once predicted an annual yield of slightly below 1.000 kWh/kWp/a. Obviously this prediction was too pessimistic.
The energy harvested per month follows the typical pattern of every year: very little yield in the months around winter solstice, very high yield during the months around summer solstice. Yet 2022 was already very sunny early in the year.
Overall a very successful year from the perspective of the energy harvested. But that’s just one part of the story. The other part is the consumption and the energy balance between harvested and consumed energy.
The graph shows losses of 1.101 kWh, while there’s no way to measure them directly. The loss is determined by balancing all energy flows. The amount that remains unaccounted for are losses. While the amount sounds a lot, it’s just ca. 6% of the annual energy harvest.
Where did this these losses go? It’s pretty easy to find out: when the photovoltaic system delivers power, the DC/AC and battery converter heats up, which can be felt easily at the heat sink on the back of the DC/AC converter.
Also noteworthy that despite a significant surplus of harvested energy and 8.251 kWh fed into the grid, my house still had to consume 2.737 kWh from the grid and overall autarky was at 74%. Why is that? Let’s look at the monthly energy balance.
My houses is being heated by a heat pump. During winter the heat pump consumes significantly more energy than in summer for obvious reasons. Therefore the monthly energy balance turns negative in winter and as there’s no way to store the surplus during summer, the grid had to kick in. The battery system in use is far too small for long term storage of energy. With “too small”, I mean two orders of magnitude too small.
Let’s face it: batteries will not solve this in the foreseeable future.
In that regard, I’d like to pick the two extreme months to show the difference between summer and winter. Let’s look at June 2022 - what a month!
In June the energy harvested was by far higher than the consumption, resulting in ca. 62% of the harvested energy being fed into the grid. Noteworthy that 50 kWh have been consumed from the grid anyway. Mostly due to high charging power of my electric car or other temporary peak consumption.
Let’s look at the other end of the spectrum now.
December 2022 was a bad month regarding photovoltaics: snow, cloudy, cold. The harvest from the roof was low, the consumption by the heat pump was high. Photovoltaics helped a bit with meager 219 kWh to cover the energy consumption, but the balance was nowhere close to autarky.
Let me also emphasize that charging an electric car with a private photovoltaic system during winter does not work because the heat pump’s consumption is higher than the energy harvest by the photovoltaic system. Switching off the heating in winter to charge your car? Not really an option.
Keep that in mind, if someone repeats this myth that charging an electric car in winter is possible with a private photovoltaic system.
Another interesting aspect in the battery utilization, which is relevant to choose the right size of battery. See here for details.
In 19 out of 24 months, the utilization was around or below 70% of the maximum capacity. From a purely financial perspective, a slightly smaller battery might have been the better choice. But there’s also the aspect of using the battery as a backup source, which requires to reserve some capacity.
Heat Pump
The consumption data of the heat pump in 2022 were as follows.
Electricity consumption heating: 2.107 kWh
Electricity consumption warm water: 1.280 kWh
Electricity consumption cooling: 129 kWh
Heat consumption heating: 7.670 kWh(th)
Heat consumption warm water: 3.754 kWh(th)
Specific heat consumption per square meter for heating: 31 kWh(th)/qm/a
COP heating: 3,64
COP warm water: 2,93
COP overall: 3,37
The specific heat consumption per square meter is surprisingly low. This source says that modern houses are in the range of 25-90 kWh(th)/qm/a. Despite being built according “Effizienzhaus 55” standard, which is not the highest building standard in Germany, it’s pretty good. This begs the question if higher isolation standards are actually worth it, or if the same money would have a higher effect being invested in active measures like a photovoltaic system. Unfortunately I lack the data to further investigate this.
The COP for heating 3,64 was within the range of a air source heat pump in a region with cold and humid winters. The influence of humidity and subsequently icing, is an aspect often neglected. COP for warm water was significantly lower, which is caused by the higher process temperatures.
The consumption of electricity is not equally distributed over the year.
Please note: To make the graphs comparable as much as possible, I scaled them to a maximum of 2.500kwh, which is the highest monthly energy harvest I have recorded so far.
The chart shows the average temperature of the months in blue. The data source measures at Munich Airport, which is close enough to be a reliable value for comparison.
For obvious reasons, the electricity consumption curve for heating is pretty much invers to the outside temperature. In summer heating was off.
In that regard, I’d like to emphasize that the major rollout of heat pumps will lead to a higher demand both for assured capacity and electric energy during winter. While wind can compensate the lack of energy by photovoltaic during winter to some degree, just compensation will not be enough - we will need both more guaranteed power and more energy during winter. I fear this fact is not being considered appropriately in the public debate about heat pumps.
As a reader of my blog, you might have read the series about optimizing heat pumps. In the 4th issue of this series, I discussed the actual effects of the proposed optimization efforts. December 2022 has brought some new evidence in that regard: average temperature in December 2022 was lower than in December 2021, yet the energy consumption was also lower. Subsequently the Coefficient of Performance was better, as shown in the next chart. A clear indication, that these optimization measures had an effect.
A heat pump does not only have to heat the house, it also needs to provide warm water for the water tap, shower, etc. In a well isolated house, the temperature of the heating circuit should be significantly lower than the temperature of the warm water. Subsequently the COP for warm water is worse.
The graph above shows, that in all months in 2022 the COP for warm water was higher than in 2021, even in December 2022, which was colder than December 2021. The optimization measure for warm water described here, obviously had an effect.
Also noteworthy that the COP both for heating and much more obvious for warm water correlate with the outside temperature. A air source heat pump gets more efficient the smaller the temperature difference between outside and water temperature is.
For the sake of completeness, it needs to to be mentioned that my heat pump has a cooling functionality. It basically goes in reverse mode and cool water is pumped through the heating circle to cool the house. The cooling performance is not on the same level as a real air condition, but it cools down the house to moderate temperatures. Sufficient for the climate in Germany.
The electric energy consumed for cooling doesn’t matter much from a financial perspective. The 129 kWh consumed came completely out of the photovoltaic system’s surplus, as cooling is necessary when the photovoltaic system harvests a lot. Therefore it’s a cheap way to cool your house.
Conclusion
2022 was a great year: the sun delivered more energy than the year before and the measures to optimize my heat pump had an effect. While we can’t influence the sun, you can influence your heat pump and if you have one, I’d like to encourage you to read this series of blog issues.
The challenge of supplying our houses and electric cars during winter remains. Direct usage of solar energy will not solve this, as it’s not available in a sufficient quantity, at least not in Germany. Seasonal storage combined with energy imports might be an option.
However, these challenges ahead are by no means an excuse not to do what’s possible today and this whole blog is intended to show that a lot is possible. Therefore I’d like to motivate you to follow the ideas described in my blog to do the steps possible to fight climate change. Even if they won’t assure success alone, these are steps we can take today and they are necessary to get there.
Comments are welcome, don’t hesitate to ask me for an advise and I’m looking forward to interact with my readers.
With sunny regards,
Andreas