Aerothermal energy. Next step ahead to nearly zero-energy buildings

Aerothermal energy

It is a fact that we are in a period of energy transition in Spain with a progressive change of trends in the design of thermal installations for heating and domestic hot water. we can find in the market a greater number of thermal comfort solutions, increasingly efficient and with a more significant importance of the origin of the sources of renewable energy. Aerothermal energy. Next step ahead to nearly zero-energy buildings


Aerothermal energy. Next step ahead to nearly zero-energy buildings


First thing first

We architect find a changing regulatory scenario and with some ambiguity until the imminent publication of the update of the Technical Building Code which is the Spanish Building Regulation known as the “CTE” for short, which is now the guide to the design of new buildings towards a lower consumption of energy and with a more relevant importance for renewable energies.

In a very near future (there are talking for 2020) that the CTE will require new buildings designed in 2020 must comply with Almost Null Energy Consumption Buildings or nZEB. Compliance with these indicators will force us to increasingly design better and more efficient buildings, which will have a dramatic reduction in heating and cooling demands.

In this scenario of nZEB buildings with increasingly reduced air conditioning demands, the demand for hot water. it will be increasingly important in the building in terms of consumption and required performance (since, in addition, our standard of comfort in sanitary hot water is increasingly high and demanding in terms of flow, with the use of shower and hydro massage columns, etc … ), must be very efficient in this provision since it will have greater weight in the energy bill of the building.

Heating comfort is essential, but in the case of hot water is more important if possible. The displeasure on the part of the user when he cannot shower at the desired temperature is notable, since it is a much more sensitive type of demand and that is also carried out throughout the year (unlike the heating that is only used for a few months at year).


Sufficient power but with little energy.

Therefore, the installation as a whole and the heat generating equipment that is selected must be sufficient and adequate to deliver the power demanded at all times depending on the needs of the users.

The aforementioned, puts us in a much more complex and diverse context when we undertake the design of an installation for hot water. At this point it is also important to comment on the existing regulation on new build regarding the renewable contribution to meet the demand for hot water. So far, the energy contemplated is solar thermal (allowing the use of other renewable sources as long as it is justified that the proposed system has a primary energy consumption and CO2 emissions equal to or lower than the reference system formed by boiler and solar thermal).

The renewable percentage to meet the demand of hot water. In this case it is variable depending on the size of the installation and the climatic zone considered (the references for north Spain is very different compared with south-east).

In the draft of the future CTE, will require that 70% of the annual energy demand of hot water and pool heating (value to be confirmed until the final publication of the document), be covered with renewable energy (as it is contemplated in the EU Directive 31/2010), being a fixed renewable percentage regardless of consumption and area Climate considered. This will facilitate the use of other renewable energies to meet the demand for hot water. In addition to the solar thermal.


Aerothermal energy. Next step ahead to nearly zero-energy buildings


New market solutions and trends for hot water production.

There are currently many technical solutions to produce domestic hot water (gas condensing boilers, diesel boilers, electric boilers, biomass boilers, electric water heaters, gas heaters, storage tanks, coil coils, aerothermal systems, systems geothermal, solar thermal systems, photovoltaic, etc. …), and probably more systems and technologies will appear in the near future.

The fact of having a greater number of solutions in the market is positive, since it allows us to choose the best and optimal solution depending on the characteristics, type and size of the installation considered.

Of course, rational criteria should also be applied in terms of proposing the most efficient and economically viable solutions within the various technologies existing in the market. Optimal and more efficient solutions will reduce both installation and operating costs.

In relation to the hot water production, the fundamental variable when considering the design of such an installation is to meet the hot water needs of the users during the peak periods of the installation (understanding that the calculation poses the difficulty of not knowing with total precision the number of users who are going to use the installation, or when they are going to do it but we use the bedrooms as a guide).

This service, essential for any type and size of installation, is especially sensitive for the facilities of type services (hotels, gyms, etc …), since if the client does not have hot water when requested, it can mean the loss of it (specially today in which hot water systems are raised with high comfort of use such as showers with rain effect, hot tubs, whirlpools, etc …).

Final Part

In the previous article we discussed how the update of the Technical Building Code known as the “CTE” for short which is the Spanish Building Regulation, is pushing us the architects to design new buildings which must comply with Almost Null Energy Consumption Buildings or nZEB. This means buildings that will use nearly 0 none renewable energy.


Aerothermal energy. Next step ahead to nearly zero-energy buildings


Architect gravitate towards aerothermal systems.

Having to look for very efficient installation systems the aerothermal system seems to be the one that we architect are betting for.

These systems allow to meet both the demands for air conditioning (cold-heat), and the demand for domestic hot water using the same equipment.

The pros of this type of solution are clear: high energy efficiency, relative ease for compliance with the indicators of non-renewable primary energy consumption of the future Spanish Building regulation for most climatic zones within Spain, consideration as renewable energy, simplification of the installation because it does not require a gas connection in the building and the possibility of hybridizing with other renewable systems such as thermal or photovoltaic solar energy.


And against…

The main point against these systems is their direct dependence on the ambient temperature from which they take advantage of the energy, a fact that can substantially influence the temperature of hot water reached, as well as the overall efficiency of the machine. In cold areas with low cooling loads, the traditional system with condensing gas boiler combined with thermal solar can be a solution to take into account to meet the demand for heating and domestic hot water.

Another issue, at least today and with the CTE in force today, is its consideration as renewable for domestic hot water, which can be complex to justify in certain areas of Spain and in some climatic zones with a lot of solar radiation (basically zones IV and V). Sometimes, for domestic hot water of the machine, it may be necessary and mandatory to install these systems together with solar thermal energy to be able to comply with the aforementioned CTE.

Continuing with the area of housing, but with e optics of the reform, aerothermal systems may have their place, but in direct competition with the condensing gas boiler. The replacement of obsolete boilers with current condensation technologies is the most appropriate solution in terms of comfort and performance, energy efficiency (think that a condensing boiler can provide fuel savings of the order of 30% compared to one that is not), and return on investment in a reasonable time.

Another issue is when it will be profitable and appropriate to install a natural gas boiler as we approach 2050 and want to achieve decarbonization objectives. In a new building the rules are clear, but in reform it is not very clear today as we will have to undertake in the future in terms of technical and economic feasibility on the building stock of a certain age that exists in Spain.


Aerothermal energy. Next step ahead to nearly zero-energy buildings


What about the service sector?

Speaking of centralized facilities oriented to the service sector, the scenario is somewhat more complex compared to the housing market. It is true that the aerothermal system spreads rapidly in the building of collective type (being prescribed on a recurring basis for gyms and hotel sector, for example), but in this type of facilities there are more variables to consider when selecting the optimal product.

A first point to consider would be the type of consumption curves that can be found in centralized facilities. In this type of installations, the consumption points can be very high (up to 60-70% of the total daily consumption). If we want to satisfy and maintain the comfort of use, a fundamental premise in any installation, to solve the installation with aerothermal systems we will have to go to very specific and efficient technologies that allow working at high temperature with a minimum variation of its performance depending on the area Climate considered (systems with CO2 cycle, for example), relying on large accumulation systems to store the energy needed to meet the consumption tip.

Regardless of the issue of comfort, in centralized facilities (except in residential buildings), there is a regulatory requirement to comply with in relation to RD 865/2003 regulation and UNE 100030: 2017, regarding the prevention and control of legionella on site. In installations within the scope of this sanitary regulation, it is necessary to carry out mandatory thermal shocks at 70 ºC, which require a high temperature primary circuit water production.


Reaching higher temperatures.

Other solutions that naturally cannot reach these temperatures, need support systems with low energy efficiency (usually by means of electrical resistors) to be able to reach these working values. In the case of aerothermal technology with R744 (CO2) refrigerant, this temperature need can be met without problem.

It is very likely that, as the various technologies try to find a place in both type and size of installation and depending on the climatic zone where the installation has to be undertaken, the trend in new building will be hybridizing production systems both to the part of air conditioning as production of domestic hot water.

Everything seems to indicate that the alliance between the aerothermal systems and the photovoltaic production systems will be a recurring solution and aligned with the carbon free objectives that are intended to be achieved in 2050. In that sense, The Spanish Royal Decree 244/2019 regulating self-consumption simplifies the installation of photovoltaic solutions.

In any case, there are still 30 years still to go by and on that road, the combination of condensing gas boiler and solar thermal energy to produce A.C.S. should not be forgotten. (especially for installations of centralized type with high demands of hot water).

Regardless of the market’s perception of this technology, solar thermal energy is the most efficient way to produce A.C.S. in a country with a level of solar radiation as high as Spain. In relation to carbon free, it is interesting to note that the carbon footprint for each kWh generated is 14.9 times higher in the case of photovoltaic panels in relation to thermal collectors. We should not, therefore, forget this technology for the production of A.C.S.


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