Heat pump systems + geothermal energy: The ideal combination for the energy and heat transition
Heat pumps (HP) are considered climate-friendly and environmentally friendly.
In order to achieve the goal formulated in the German coalition agreement of climate-neutral heat generation by 2045, fossil-fuelled gas and oil heating systems must be replaced by more environmentally friendly alternatives. A large part of this will be accounted for by electric heat pumps, which will replace boilers as the new heating standard, according to the assessment of the German Renewable Energy Federation in its current heating scenario.
The classic anergy sources for heat pumps include the soil, groundwater and ambient air. Waste water and exhaust air from ventilation systems are also suitable heat sources.
Heat pump function
Heat pumps use the energy contained in the surrounding temperature for heating and cooling. In the cooler season, the ambient temperature heats up the liquid refrigerant (with the aid of electricity) and turns it into a state of gas. The contained thermal energy is transferred to the heating water in the heating system and thus tempers the room air. The most efficient way to do this is to use underfloor heating, which requires less high temperatures ( 35°C) than radiators ( 55°C). One advantage of environmental energy is that it is available in almost unlimited supply.
In summer, many HP systems are able to reverse the process and switch from heating to cooling mode. With active cooling, heat is extracted from the building via the heating circuit, as cooler water circulates in the heating system compared to the room temperature. With borehole heat exchanger systems, the extracted heat from the building can be transferred to the ground, so that it regenerates better. Surface heating systems are also particularly suitable for cooling.
Anergy-sources for heat pumps
One can basically distinguish between four different heat pump systems:
- Air-source heat pumps, or more precisely air-water heat pumps, use the ambient temperature of the air as a heat source. Naturally, the temperatures here are subject to seasonal fluctuations from -10 to +35 degrees.
- Geothermal heat pumps and near-surface geothermal energy (2-3 metres deep): These extract thermal energy from the ground - usually by means of a brine-water mixture. Geothermal energy from near-surface layers of earth can be tapped using surface collectors, ring trench collectors or short GeoCollect collectors. In these near-surface layers of earth, the seasons and air temperatures still play a role - the ground is heated by solar radiation and air and, conversely, also cooled seasonally. However, this happens more weakly than with air HP and also with a slight time delay.
- Geothermal heat pumps and geothermal probes (30-200 m): As the probes reach into deeper layers of the earth, they encounter more stable ground temperatures. At a depth of 50 metres, there was a constant temperature of about 10 degrees Celsius. Below that, the temperature even rises by about 3 °C per 100 metres.
- Groundwater HP (3-25 m, depending on level) The most efficient of the heat pumps is the water-to-water heat pump. Groundwater serves as the heat source. However, it is also possible to use lake or river water. The temperature of groundwater is relatively stable and is around 10 degrees Celsius in Munich, for example. Especially in winter, this source is much warmer than the outside temperature.
- (We do not consider the topic of deep geothermal energy in this blog post.
A special form among the heat pump sources is the ice storage, which we have described here: http://bit.ly/3qW4VpB)
Heat pump = heat pump? A fallacy!
It is true that modern heat pumps do not differ fundamentally from each other technically. But depending on the type of heat pump, different sources are used and these have different temperatures. For the efficiency of a HP system, the source temperature makes a big difference. (Since this article is about the choice of sources, the sink temperature is not dealt with separately here).
"If we look at the source over a longer period of time, it is crucial for efficiency whether my source has a temperature of -5 or +5 degrees Celsius," says Markus Pröll, authorised signatory at goodmen energy. That's why he advocates heat pump solutions with geothermal energy. Because in terms of technology and efficiency, near-surface geothermal energy is a tried and tested solution and has the potential to cover large parts of the heating and cooling demand in buildings. Analogue to the increasing use of heat pumps in buildings, the use of geothermal energy as a heat source will also increase.
Energy and heat efficiency and new economic efficiency
The efficiency of a heat pump is determined by the ratio of the usable heating energy to the electricity required to operate the heat pump. In order to also take into account any energy losses during provision, the primary energy demand must be defined.
- The coefficient of performance COP
The Coefficient of Performance, COP, reflects the ratio of heating power to electrical drive power under standard conditions and can therefore vary depending on actual application conditions. - The annual performance factor (APR)
It describes the ratio of the heating energy provided annually to the amount of electricity consumed annually. For example, an annual performance factor of 4 means that 1 kWh of electrical energy is required to provide 4 kWh of heating. The remaining 3 kWh are taken as environmental heat, e.g. from the ground. 25 % must therefore be provided electrically. In contrast to the COP, the AWP reflects the efficiency of a heat pump system in use and is intended to represent the overall system of the heat pump. This means that in order to determine the figure, the electricity demand for the heat pump including hot water preparation, the electricity demand for pumps or fans to tap the heat source and the electricity demand for any heating rod must also be recorded. And as already mentioned at the beginning: a lower temperature difference between the heat source and the heating circuit leads to a higher annual performance factor. Ground-source heat pumps usually achieve higher annual performance factors than air-source heat pumps. - Primary energy demand
The energy losses along the supply chain of electricity generation are shown by the primary energy factor. Since 01.01.2016, the primary energy factor for electricity purchased from the German grid is (by definition) 1.8. This means that for every kWh of electricity purchased by consumers, 1.8 kWh of primary energy must be expended until the electricity reaches the consumer. Combining the HP system with photovoltaics to operate the heat pump therefore makes sense and reduces the primary energy demand.
In order to assess the efficiency of a heat pump, let us now take a closer look at the performance of air and ground heat pumps. First of all: It is not a question of playing heat pump A off against heat pump B. Every heat pump is better than burning fossil fuels, which are harmful to the climate.
In terms of the energy performance ratio, even poor heat pumps are still economical and climate-friendly compared to natural gas1! As field measurements have shown (source: Miara, Forum WP Bestand), air-source heat pumps also achieve an AAC of about 3.1 on average, and even up to 3.8 at the peak (depending on climate and building efficiency). Here, relatively low investment costs are offset by comparatively higher operating costs.
Marek Miara, ISE blog.innovation4e.de/2022/04/08/warmepumpen-oekonomische-betrachtung-der-betriebskosten-neue-sichtweise/
Focus heating element
We will now discuss the design of (air) heat pumps with a bivalence point. This is the outdoor temperature at which the (air) heat pump is no longer sufficient to provide the necessary heat output. In this case, a purely electric heating element is switched on in order to be able to generate the necessary heat.
Although the heating rod rarely kicks in2 with air heat pumps (this is partly due to the hopelessly exaggerated heating load calculation according to DIN12831 - there is also a need for action here - and partly due to the fact that the bivalence temperatures are rarely undershot), the days on which it does have a great effect.
We would like to illustrate this with a model calculation that shows the significance of the maximum electrical output:
We consider the operation of two heat pumps at an outdoor temperature of -7°C.
- Below the bivalence point of -4.15°C, the air heat pump output of 11.18 kW is not sufficient for the heating load (according to DIN12831) of 13.63 kW. The electric heating element switches on with 2.46 kW. This reduces the heat pump COP from 3.05 to an effective 2.28.
- A brine heat pump with a well-dimensioned geothermal probe should provide a source temperature (assumption here: 0 °C) regardless of the outside temperature, resulting in a COP at the operating point of 4.4 and an electricity demand of 3.1 kW.
With the use of heating rods, the air HP requires 78 % more electricity than the geothermal brine HP (admittedly, this operating point is not (or hardly) decisive for the overall annual economic efficiency)."But, and here is the key message of this blog post: The worst operating point very much determines the design of the infrastructure at the higher level. This may mean that medium-voltage grids and supra-regional transmission grids have to be dimensioned larger, which is likely to trigger enormous economic costs. Imagine 20 million heat pumps that require 78% more electricity than absolutely necessary for one week in winter," Dr Markus Pröll explains.
Future outlook - a look into the crystal ball
What could be the consequences of too high a share of heat pumps with supporting heating rod operation on the energy transition? Three possible scenarios:
- The transmission grids are being expanded. This triggers high investment costs. Short-term peak electricity demand in Germany is balanced out via the European electricity market.
- Electric storage buffers the increased electricity demand of electric auxiliary heating3. Also very high investment costs for households.
- Probable: To avoid high investment costs at grid level, variable electricity tariffs (via digital electricity meters) are introduced to control electricity purchases during these periods via high electricity costs.
Bottom line:
Air-source heat pumps are preferable to heating with fossil fuels, especially in new buildings. The annual performance factors are economically and ecologically satisfactory and have been increased in recent years. Furthermore, climate change and the associated warmer winters are reducing the operating hours below the bivalence point (heating rod operation).
In addition to evaluating the technologies in terms of annual electricity demand and CO2 emissions, the short-term maximum electrical output must also be considered. This has a direct economic impact on medium-voltage and transmission grids, where it can trigger very high costs that have to be passed on to consumers. We are committed to anticipating these effects in the interests of the customer and the German energy transition and to taking them into account in planning.
"However, in order to cover our electricity demand from renewable energies in the future, we will not need 100 % of today's consumption, but 180 %," believes Dr Markus Pröll. And further: "In my opinion, we will soon be discussing peak load efficiency as heat pumps successfully enter the energy transition. Additional electricity consumption, such as electromobility, is a challenge, but it can basically be controlled over time. Heating demand could only be shifted in time by large thermal storage units."
Especially on the coldest days in winter, when the heating rods of air heat pump-fed heaters switch on and the demand then almost doubles, power bottlenecks are to be expected. The financial consequences could be further electricity price increases or flexible electricity tariffs.
"For me, it is not only the annual performance factor that is decisive, but also the COP on the coldest days and weeks of the year with maximum energy load. Because this has an impact on society as a whole and can prevent the energy transition from succeeding," Markus Pröll judges. "Therefore, I plead for a move away from an economic efficiency consideration, which is optimised purely according to annuity."
On the eligibility of HP systems, read our blog on the innovations of the BEG.
[3] Example calculation: A common 5kWh battery storage in a single-family home would only be sufficient for about 2 hours of electric heating.