Energy
Differences Between Geothermal and Aerothermal Energy
*Update: Please contact our team for current pricing.
As is well known, at Slow Studio, we specialize in nearly zero-energy buildings, which reduce energy demand through passive systems and supply the remaining energy through renewables.
Therefore, it is crucial to analyze which energy system will be most efficient based on the climate, available budget, and lifestyle.
Sometimes, one energy system or another is marketed as the solution to all problems: aerothermal energy is the future, geothermal energy is highly profitable, biomass is very ecological… For the end consumer, it is already difficult enough to understand what each system entails, let alone having to evaluate the advantages and disadvantages of each based on their climatic location, type of housing, and its use.
Thus, in today’s article, we aim to break down two of the most current energy systems, understanding their operation and the advantages and disadvantages of installing one or the other.
What is Geothermal Energy?
Let’s take it step by step, starting with geothermal energy, whose etymological origin already indicates that it is based on harnessing the temperature of the ‘geo,’ meaning the earth. It is a practice whose origin dates back to volcanic regions, which are points of internal-external energy exchange of the earth.
More than 10,000 years ago, our ancestors already used hot springs directly for cooking, but today the system works differently. Basically, there are different enthalpy levels, and in Spain, we have low levels, classified as ‘low enthalpy.’
Therefore, we do not extract temperature directly from the earth, but rather rely on the principle that the earth’s internal temperature is constant throughout the year from about 20 m deep. As we will detail later, what is harnessed is the temperature difference between the interior of the home and the interior of the earth.
What is Aerothermal Energy?
Aerothermal energy, on the other hand, extracts energy contained in the air, which is actually a process very similar to photovoltaic energy, as this air is heated by solar radiation. In this case, through its cooling, we will capture the heat it holds and convert it into thermal energy for heating and domestic hot water (DHW).
To capture this air from outside, a free and renewable source, we must place an outdoor unit containing the necessary machinery, which we will detail later.
It is important here to pause and understand that, on one hand, we have the temperature exchange source, and on the other, a heat pump that will generate hot or cold water. Both aerothermal and geothermal systems operate connected to a heat pump that requires electrical power.
In the case of aerothermal energy, all providers claim it produces 70% of the energy relative to the 30% electrical input it requires, while for geothermal energy, it would be 85% vs. 15%.
Installation Components
To correctly analyze the differences between geothermal and aerothermal energy, we must first understand the theoretical operation of the installation and its various components throughout the circuit.
For any energy installation, whether geothermal or aerothermal, there are three stages in the operational scheme: (1) collection system, (2) energy generation system, and (3) distribution system.
(1) The collection system consists of the mechanism for extracting energy from the ground or air, either through an outdoor unit, vertical or horizontal wells, or directly from groundwater aquifers.
(2) Secondly, the energy generation system transforms the temperature difference between the subsurface/air and the interior of our home into useful energy for heating, cooling, and DHW.
(3) Finally, the distribution system is the means we use to distribute this energy throughout the house, whether it’s a device or a building element. This can involve air distribution, radiant floors or walls, or low-emissivity radiators.
This installation can also be classified into two types of systems: closed system or open system. A closed system is one in which there is no direct fluid exchange; instead, it consists of a refrigerant fluid that travels through pipes and transfers thermal energy. Generally, residential installations involve closed systems, unless a directly obtained resource can be used, such as rainwater harvesting in a tank, which would be an open system.
Differences Between Geothermal and Aerothermal Energy: Collection Systems
In the case of aerothermal energy, it is more difficult to distinguish between the collection and energy generation systems, as they are more closely related and located in the same space. Collection is carried out through an outdoor unit that houses a heat pump and an inertia tank. However, the collection system as such is usually found within the heat pump itself, which draws in ambient air to harness its temperature and convert it into thermal energy.
This is similar to a solar panel that captures solar radiation and releases thermal energy, as the panel itself acts as both a collection and generation system. In fact, the mechanism is even more alike if we consider that the heat we harness from the air we are capturing has been directly warmed by solar radiation.
When talking about geothermal energy, things get more interesting, as there are up to three different collection methods: vertical wells, horizontal wells, and groundwater collection. The first two systems are considered closed, while groundwater collection represents an open system, as we will illustrate below.
The most widespread system, due to its efficiency, is vertical wells, a circuit composed of pipes about 10 cm in diameter that reach depths of more than 20 m, specifically 25 to 150 m. This is based on the knowledge that beyond this depth in the Spanish climate, a constant temperature is maintained throughout the year, and for every 100 m deeper, the temperature increases by 3 °C.
This system also has the advantage of being able to utilize the building’s foundation in the case of new construction. Recently, ‘energy piles’ have even been invented, which integrate the piles used in construction with the conduits for transporting the fluid.
Horizontal wells are less efficient as they do not reach the 20 m depth, ranging from 0.6 to 1.5 m, thus utilizing the superficial layer of the ground heated by solar radiation. Therefore, the climate has a greater influence, similar to the aerothermal system. Like the vertical well system, it consists of a circuit of polyethylene pipes that transport water with antifreeze. However, another disadvantage in this case will be the need for a large plot of land where trees cannot be planted or developed, making it more sensible to centralize it for several houses.
Finally, groundwater collection is the only open system, as it directly harnesses constantly replenished underground aquifers: extracting water directly and circulating it for our benefit. The main drawback, apart from the fact that it applies to very specific locations, is that we will need the help of a hydraulic pump for its impulsion, which in turn consumes electrical energy and makes it less efficient.
Differences Between Geothermal and Aerothermal Energy: Generation Systems
Heat pump
At this point, we realize that the energy processes are not so different despite coming from distinct sources, as they are all based on the basic principles of thermodynamics. In both cases, geothermal and aerothermal, a heat pump is necessary to transform heat into energy, as its name suggests. Direct use would be possible in some geothermal cases if the ground temperature exceeded 100 °C, but this is not the case in Spain, as that temperature does not fall within the low enthalpy range.
The transformation process carried out by the heat pump involves cooling the ‘hot’ air or liquid obtained using a refrigerant gas, and the resulting enthalpy change converts it into thermal energy.
However, the drawback we mentioned for aerothermal energy in this regard is its low efficiency in winter, when we want to extract heat from the air but there is very little to extract, resulting in very little energy generation.
In contrast, geothermal energy utilizes the constant heat of the earth and the temperature difference between the home’s interior and the earth’s interior. Therefore, in summer, the heat pump’s operation is reversed, transferring excess heat from the home to the ground. The process is exactly the same as just detailed, simply with the liquid heating up inside the home by utilizing its distribution systems.
Inertia tank
The inertia tank basically consists of a container where domestic hot water (DHW) is stored, ensuring immediate availability of this service when needed. Systems like gas, which operate at much higher temperatures, can heat water more quickly and perhaps omit this device. This is not the case for geothermal or aerothermal systems, where the tank is indispensable. Additionally, it usually includes an expansion vessel that allows it to regulate the volume increase of cold water.
Differences Between Geothermal and Aerothermal Energy: Distribution Systems
Both aerothermal and geothermal systems allow for the same distribution system installation possibilities, encompassing virtually all types found on the market. Therefore, they are highly versatile systems, a value increasingly appreciated in modern society.
Water-based Heating/Cooling Systems
First, let’s discuss radiant floors or walls, which allow for both heating and cooling of the room using water. The truth is that cooling is less common as it carries the risk of condensation, which can be more problematic than useful.
Basically, the system involves creating a network of water pipes that extend across a surface, making heat emission much more extensive and avoiding the localized heating we are accustomed to with conventional radiators.
A disadvantage of this system is the loss of inertia when perforating an entire building element for the installation, which can be climatically inefficient by neglecting this essential passive strategy in our passive house design. One strategy would be to alternate high-inertia accumulating elements with installation elements to ensure both requirements.
The other option is low-emissivity radiators, a system that, like radiant floors, operates at maximum temperatures of 50 °C, so it does not function in the same way as conventional gas radiators. It is a much more expensive heating system, but also much more efficient, as it completely eliminates cold zones by operating through convection. Similar to radiant elements, it takes longer to heat a room, but the heat is better distributed.
Air-based Heating/Cooling Systems
Air heating is carried out identically to air conditioners we are all familiar with, using a device that provides hot or cold air depending on the desired temperature. The fact that both aerothermal and geothermal systems allow for the installation of this system offers great versatility, as other energy generation systems do not permit its installation.
However, in our studio, we extensively analyze the health implications for the occupants of such environments, and in this regard, air-based heating diffusion is not highly recommended.
The health issue stems from the fact that the air expelled into the environment has very low relative humidity and reduces ambient humidity, which ultimately dries out the mucous membranes of users, leading to a decrease in defenses and a greater predisposition to illness.
Differences Between Geothermal and Aerothermal Energy: Profitability
When choosing one system or another for the construction of our future home, economics plays a very important role. Let’s remember that every passive house will inherently involve an extra budget for various bioclimatic strategies, meaning a higher initial investment that will later save us energy costs.
The same applies to active systems like geothermal and aerothermal energy; we must evaluate the cost of each and the performance it offers to calculate how many years it will take to amortize the investment and which one we prioritize.
Within these systems, the cheapest option would be to distribute thermal energy via splits, i.e., entirely by air, but as we mentioned earlier, this is not optimal for health. The most expensive option would be low-emissivity radiators or radiant flooring; the latter is also not optimal because the installation of radiant flooring, being insulated, reduces the inertia of the slab, which is its natural ability to accumulate heat.
In our studio, for the same project, we requested quotes for both geothermal and aerothermal energy to make a comparison, as detailed in the article A House with Geothermal Energy. The result was a quote of €24,000 for aerothermal energy compared to €30,000 requested for the geothermal installation for the 220 m2 project.
This difference of €6,000 more for geothermal energy, approximately 20%, primarily corresponds to the installation of the probes. The amortization of the installation is thus calculated at 7 years, with almost zero maintenance costs for the probes.
When to Install Geothermal or Aerothermal Energy?
Having gathered and analyzed all this information, we will now present the conclusions reached in our studio. We start from the understanding that every location, family, and design is unique, and it is difficult to make definitive statements. However, there are various trends that suppliers themselves share with us.
One of them is that aerothermal energy is cheaper but less efficient, while geothermal energy requires a greater investment that will be more profitable in the long term. This is relative, as it will always depend on the climate of each area and its level of continentality.
Thus, a more continental climate, meaning a greater temperature difference between summer and winter, will highlight the value of geothermal energy and its consistency throughout the year. However, locations closer to the coast, and therefore with milder winters, will allow the lower investment in aerothermal energy to be profitable despite its common inefficiency in winter.
What aerothermal energy does best is cool in summer, when it can capture more thermal energy from the air and transform it into cold air or water – precisely the installation that can most easily be replaced by passive strategies. Its poor performance on the demand graph is found in winter, which is when we need more energy but are capturing less.
In any case, our recommendation is to ensure that we will be able to make the most of the system we install in our future home. Any investment, no matter how small, if not utilized effectively, becomes more expensive than we realize, both for us and for the planet.
Our advice is that, after understanding the differences between geothermal and aerothermal energy, we should make informed decisions, always with the help of energy demand calculations and with the goal of efficiency, but also health.