all articles 25. March 2020

Eco-friendly energy for homes and buildings (an overview)

Anyone building a house today who believes that sustainability and climate protection are important is faced with the question of how to generate green energy for electricity, heating and hot water.

As José Manuel Busquets Hidalgo, the energy expert advising us on our Casa Xaloc project, likes to say: "The most sustainable energy is the energy that you don't use". That's why the first step in your energy concept should always be to look at how energy can be saved - whether by changing your own behaviour, using technology or effective insulation. Nevertheless, we cannot do without the use of electricity and heat in everyday life.

This is why we are now looking at what possibilities there are for property owners to generate their own energy, what kinds of environmental energy can be harnessed for this purpose, and what technologies can support us in doing this.

We will take a closer look at these methods of generating energy:

  1. Photovoltaics: using solar energy to generate electricity
  2. Small wind turbines: harvesting the wind to generate energy 
    Related topic: battery storage
  3. Heat pumps: heat recovery from groundwater, soil or air
  4. Solar thermal - heat generation from solar energy

In doing so, we consider the different possibilities under these aspects:

  • How does it work?
  • How energy efficient is it?
  • How big is the investment and how profitable is it?

So, let's go for it!

1. Generating electricity from solar energy with photovoltaics

If you think about climate-friendly electricity, you immediately end up with a photovoltaic system. Photovoltaics has established itself worldwide as a technology for generating electricity from solar energy and pays off in both ecological and economic terms.

How does a photovoltaic system work?

A photovoltaic system collects solar energy via solar modules mounted on the roof or terrace. The solar cells contained in the collectors absorb the sunlight and convert it into electricity via an inverter. This can either be used directly for personal use, stored in a battery or fed into the public power grid.

In order to be able to compare the performance of various systems, the electrical output (nominal output) of a photovoltaic system is given in kilowatt peak (kWp), i.e. the maximum output of modules under standardised laboratory conditions.

If you install a system with a peak output of about one kilowatt, six to eight square metres of space are required. The electricity yield depends on the actual insolation, the seasons and other factors such as shade, reflecting light and how dirty the modules are.

When does a PV system work efficiently?

In order to capture as much sunlight as possible, the collectors should ideally be aligned to the south, with a roof inclination of 30 degrees. Inclinations below or above this can significantly reduce the electricity gain. Furthermore, the roof surface should not be shaded by surrounding trees or houses..

From an economic point of view, it often makes more sense to not optimise the size of the system to only cover individual consumption, i.e. not to make it too small. Small systems are often more expensive per kilowatt of output than larger ones. System sizes of up to 10 kilowatts have proven to be the best choice for detached or semi-detached houses..

For example, with a photovoltaic system with a 5 kWp output, a family of four can cover 20 to 30 percent of their own consumption without the use of a battery storage unit. With battery storage, this can be increased to 50 to 70 percent. In addition, providers of virtual storage, so-called electricity clouds, even promise a degree of self-sufficiency of 100 percent and more. More on this in the chapter on battery storage.

What does a photovoltaic system cost?

The acquisition costs for a PV system have fallen significantly in recent years. Per kilowatt of output, one currently has to invest 1,200 to 1,600 euros net. For detached and semi-detached houses with a system size of 5 to 10 kW, the net acquisition costs are therefore between 7,000 and 12,000 euros. In Germany there are various ways to apply for subsidies for this purpose.

The service life of a PV system is between 20 and 30 years.

Although a photovoltaic system often produces more electricity than can be consumed on warm, sunny days, it is necessary to resort to other energy sources in autumn and winter. Here, the combination with a small wind turbine power generator can be useful.

2. Electricity generation with small wind turbines

Another possibility to produce electricity yourself is to install a small wind turbine on your property. Combined with a photovoltaic system and a battery storage unit, you can generate your own electricity all year round, and ideally you will even be 100 percent self-sufficient.

Unlike photovoltaics, small wind turbines have not yet reached the mass market and are rather expensive in terms of investment.

How does a small wind turbine work?

Small wind turbines are installed directly next to the building, or wherever the energy is needed and consumed. Turbines with vertical or horizontal rotor axis are available. However, the horizontal type has become generally established.

Via the rotor blades, the energy from the wind is converted into electrical energy by a generator and either fed into the public power grid or fed into a battery storage.

Because of their compact size of up to four meters in diameter, so-called micro wind turbines with an output of up to five kilowatts are particularly interesting for private households. Incidentally, unlike large wind turbines, aspects such as noise or casting shadows are of less significance.

Costs of a small wind turbine

The acquisition costs for a small wind turbine are between 3,000 and 9,000 euros per kilowatt of rated output. The mast and foundation can sometimes be more expensive than the wind generator itself. Depending on the location and average wind speed, a taller mast can be profitable if the generation of electricity also increases.

Depending on quality and maintenance, the service life of a small wind turbine is between 10 and 20 years.

When does a small wind turbine make sense?

No wind, no power. If you are thinking about a small wind turbine to generate electricity, you should check whether the location offers enough wind at the height of the rotor blades. An key indicator here is the average annual wind speed. The stronger the wind, the more electricity is generated.

Incidentally, it is not the rated output of the generator that is decisive for the amount of electricity produced by a wind turbine, but the length of the rotor blades. These should be exposed to the wind as unhindered as possible.

When it comes to the economic viability of the turbine, particularly in Germany, Austria and Switzerland, it only makes sense to generate enough for your own consumption. Feeding into the public power grid receives very little remunerated. The turbine should therefore be optimised to only cover your .own consumption.

Related topic: Battery storage for more self-sufficiency

Particularly when energy production is for your own consumption, it is worth thinking about a battery storage. This would enable you to meet your own consumption significantly and thus gain self-sufficiency.

This is because energy is often not needed exactly when it is produced. With a battery, any generated energy that is excess to current consumption can be stored for later in the day.

Battery storage systems are mainly designed for use with photovoltaic systems. However, a small wind turbine can also be connected in order to increase the degree of self-sufficiency to up to 80 to 100 percent. However, the battery storage unit must fulfil the necessary technical requirements for this.

Lithium-ion batteries have established themselves as battery storage. They can be charged more often, last longer and are more powerful than other types. An independent study ("Compendium: Li-ion Batteries") sponsored by the Federal Ministry of Economics and Energy (BMWi) has also confirmed the safety of these devices.

Do battery packs pay off?

As with photovoltaic systems, the costs of home storage systems have fallen so much in recent years that they have now reached the threshold of economic viability.

However, it is less economically viable for those who are aiming primarily for self-sufficiency and therefore require a larger storage facility because they do not necessarily want to feed the electricity generated by photovoltaics into the public grid.

The higher the storage capacity, the less economically viable the electricity from the battery will be.

Does a power cloud make sense?

In addition to home storage, more and more providers are offering so-called electricity clouds, i.e. virtual storage facilities into which private electricity producers can feed excess electricity. The electricity fed into the cloud in summer can be used in autumn and winter, for example, when the PV system generally generate less electricity than required. In this way, it is possible to achieve 100 percent self-sufficiency in combination and exchange with other private PV electricity producers.

Depending on the provider, you can receive a payment if you use less electricity than you generate. Some providers now even allow you to access the electricity you have generated from other places, for example at selected filling stations to recharge your e-car.

When considering using an electricity cloud, you should take a close look at possible membership costs or even electricity costs that are incurred if one consumes more than was fed into the grid.

3. Heat pump: climate-friendly heating

Heat pumps are the heating system of choice in new buildings. However, a heating system with a heat pump only makes sense for well-insulated houses. Heat is extracted from three different sources: from the groundwater, the soil and the air. However, the three types of heat pump differ significantly in their efficiency and not all of them can be implemented on every property.

How does a heat pump work?

Actually everyone has a heat pump at home in the kitchen: The refrigerator works on the same principle, only in reverse: the interior is cooled and warm air is released to the outside.

The heat pump, on the other hand, transports heat into the house from the outside air, groundwater or the soil. As with a refrigerator, a refrigerant in a pipe system serves as a means of transport. With the aid of electricity, the refrigerant flowing from the soil, groundwater or air is compressed to raise its temperature to the required forward-flow temperature of the heating system. When the heat is released, the refrigerant cools down and the cycle starts again.

When does a heat pump make sense?

The heat pump is only efficient if it is designed to meet the heat requirements of the household. If more heat is required, an additional electrically operated heating element usually kicks in, which drives up electricity consumption.

Optimum conditions for the use of a heat pump heating system are therefore:
 - good building insulation
 - large radiator surfaces, for example underfloor or wall heating.

The three types of heat pump have different levels of efficiency. The most efficient is a groundwater pump, followed by the soil heat pump and finally the air heat pump.

Even in the winter, the groundwater is generally no colder than 10 degrees Celsius. This is what makes this type of heat pump so effective compared to the others. However, since two wells have to be drilled for the installation, a groundwater heat pump is not permitted everywhere.

Soil heat pumps use the heat from the soil, which like groundwater, hardly fluctuates in temperature. However, it depends on the composition of the soil whether enough heat can be extracted from it.

Air-source heat pumps are less complex to install and can be used anywhere, but are the least efficient, even though heat can still be extracted from cold air. Since fans are used to extract heat, sound insulation should also be taken into account in the planning stage.

What does a heat pump cost and when does it pay for itself?

How much the actual investment for the purchase and installation of a heat pump will vary greatly depending on location, environment and other factors. These figures can therefore only be taken as a guide.

  • Air heat pump including installation: 12,000 to 14,000 euros
  • Soil / ground heat pump: 9,000 to 11,000 euros for the unit; for the heat pipes an additional 50 to 75 euros per meter of borehole depth, or for horizontal soil / ground collectors approx. 25 euros per square meter.
  • Groundwater heat pump: 9,000 to 12,000 euros for the unit; 4,000 to 7,000 euros for installation costs.

4. Solar thermal: Hot water from solar energy

Solar thermal energy is used to convert solar energy into heat via solar collectors. This form of heat generation is mainly worthwhile for households with above-average hot water requirements, for example to operate a swimming pool.

How does solar thermal energy work?

In order to be able to use the sun's heat to produce hot water, solar collectors are used which are filled with a conductor fluid containing antifreeze. When the sun is shining, the fluid heats up and circulates between the collectors and the buffer tank with the help of a circulation pump. There, a heat exchanger heats the drinking water. Solar thermal energy can also be used to support the heating system, which can save up to 30 percent of the energy costs of the main heating system.

What does a solar system cost and under what conditions is its use reasonable?

The purchase cost of a solar thermal system starts at around 4,000 euros for a small detached house. With additional heating support it starts at 8,000 euro.

The heat yield of the system depends on the surface area, orientation and inclination of the solar collectors. Such a system only makes sense if the hot water it produces is actually utilised. Otherwise it is neither economically nor ecologically viable. The amount of hot water consumed or heating requirements should therefore be analysed carefully in advance.

Conclusion: There is not THE one solution

Even if some labels would like you to believe otherwise: there is no patent recipe for optimally generating energy in residential properties. Every new project, with its differing conditions in terms of location and the characteristics of the immediate environment, is a unique challenge that needs to be considered individually.

Irrespective of which option(s) you decide on, it is worth asking questions, not only about efficiency and cost-effectiveness, but above all about your own motivation. Because even if sometimes a certain investment is not immediately worthwhile in monetary terms, it is perhaps all the more worthwhile in ecological terms. With a view to the preservation of our wonderful planet, we believe that this can be a very worthwhile investment.

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