Building a house with biomass

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Among all the energy sources available to us for heating a space, biomass is perhaps one of the most controversial. It is a renewable fuel, as it comes from the pruning and felling of trees or shrubs, but it is not free of CO₂ emissions during its combustion process.

While emissions during the combustion of wood, pellets, or wood chips are considered neutral because the tree absorbs CO₂ during its growth process, this claim is not without controversy. This is because a net-zero balance in a climate emergency scenario—where we need to drastically reduce emissions—suggests that accepting we emit CO₂ simply because it has been offset does not seem to be the solution or the path toward a zero-emissions future.

Furthermore, biomass also takes other forms of combustion when transformed into biofuels, the cultivation of which is displacing other strategic food crops and whose transport—sometimes crossing oceans from the point of production—is being widely criticized.

In this article, we analyze all biomass production options, their origin, transport, and use as an energy source in construction, specifically for its application when building a house with biomass.

Types of active energy for generating heat

Today, more than 80% of our energy supply comes from fossil fuels—oil, gas, and coal—13% from nuclear energy, and only around 6% is generated from renewable sources.

In the case of Spain, the percentage of non-renewable sources in 2020 represented around 94% of the total energy consumed. In addition to the environmental implications this entails, depending on fossil fuels in a country with scarce fossil raw materials implies a strong reliance on external supply.

Consumption in buildings, homes, offices, and facilities accounts for around 60% of total consumption, so the construction sector will play a key role in the energy transition.

Of course, the first step is to take advantage of natural and renewable sources, primarily the sun through direct irradiation, accumulation via thermal inertia in materials, or natural ventilation through the air currents it generates. This allows us to reduce our energy demand; this is what is known as passive construction.

But what happens when we need to generate domestic hot water for bathrooms or kitchens? Or what happens when we are in a location with low temperatures that do not allow us to achieve a comfortable indoor temperature without heating throughout the year?

This is where we must provide active energy to our buildings. This active energy can be the production of electricity that powers machinery generating heat energy, or it can be the direct production of hot water using a fossil or renewable fuel.

Let’s look at the options we have before going into detail in the analysis of energy through biomass combustion.

Petroleum derivatives

Oil is the most exploited fossil fuel worldwide, the so-called ‘black gold’. It has also been one of the main sources of economic and political conflict in the last century and seems to be entering a slow decline following the new awareness taking shape worldwide, especially among younger generations, to address the climate emergency.

It is considered a non-renewable fossil fuel, as it takes thousands of years to form at great depths and under high pressure.

Its extraction and transport require large investments; it is a highly polluting fuel that generates high CO₂ emissions.

Oil as a fuel in construction is very residual, as it is not typically used as a direct source to generate energy beyond portable gasoline-powered generators in some isolated rural constructions. However, it is the raw material used for manufacturing many synthetic building materials, from insulation to waterproof membranes.

Fossil gas

Commonly known as natural gas, a gentler way of referring to the most polluting fossil gas today. Although its combustion process emits less CO₂ and nitrogen compounds harmful to health than alternatives like coal, it can have methane leaks—a greenhouse gas 28 times more potent.

Today it is one of the most expensive fuels, and its producers try to promote it as a clean alternative for an energy transition, something that does not seem very logical as it is a non-renewable fuel.

Electricity

We can buy electricity from the grid, whose production may have been generated through renewable sources or not (there are many companies already on the market that guarantee production through renewable sources), or we can generate it using photovoltaic panels.

While electricity itself is not a source of heat energy, it must be transformed either directly by powering radiators—we have high-efficiency options on the market today—or through systems such as heat pumps, heat recovery units, or electric boilers.

Heat pumps can exchange energy with the ground—geothermal—or with the air—aerothermal—but they always operate with an electric power source to ensure the movement of the fluid.

These systems, if used in passive houses, can be difficult to amortize, as the demand in a zero-consumption passive house is minimal and the investment in a geothermal or aerothermal system is very high.

Solar thermal energy

Thermodynamic panels generate hot water through heat exchange with the fluid inside the panels. These are high-performance panels, around 70-80% compared to the 15% of photovoltaic panels, but they involve high maintenance of the installation.

Biomass

The last of the options we analyze is biomass fuel, which we will examine in detail to assess the pros and cons of building a house with biomass.

Biomass is a very broad term that defines all the energy stored in organic matter thanks to the photosynthesis process, which becomes fuel when burned or transformed.

The most common and oldest form used by humans is the combustion of wood. Today, biomass is proposed as a renewable alternative to fossil fuels derived from oil and coal, but it is not free of emissions because the combustion of organic matter consumes oxygen and generates CO₂.

Pros and cons of building a house with biomass

CO₂ emissions in biomass combustion

When we talk about renewable energies, we usually assume they are clean energies, but the reality is that renewable only refers to their unlimited availability; neither the generation process nor their emissions are typically analyzed.

This deeper analysis is beginning to be carried out through the life cycle assessment of construction products and systems, something that will start to gain momentum with the new directives regarding life cycle analysis and the circular economy being prepared by the European Union.

In the case of biomass, we can defend as an advantage that the final carbon footprint calculation is zero, since the absorption of CO₂ during the growth process of the tree or shrub is equal to the emissions from its combustion. Here we enter into a certain ethical questioning regarding the sense of felling a forest—which represents a complex ecosystem housing habitats and living beings of all kinds—for the sole purpose of energy production and with the argument of having absorbed the CO₂ we are going to emit.

Production

At this point, we might think it would be more positive to take advantage of organic waste to give it a second life, so it will be important to provide the appropriate treatment to these residues, which are sometimes polluting.

One option is forest cleaning through undergrowth management, thereby limiting the spread of fires. However, it is a form of raw material extraction that is expensive and has little tradition in our country.

Another option is energy crops for biomass production, i.e., tree plantations intended to generate fuel. These achieve—although proper management is necessary—the reforestation of land, increasing water retention and decreasing soil degradation and erosion.

Within ecological forests, there are two types: short-rotation forestry plantations and the selection of old specimens. In the first case, monoculture is discarded by definition, combining intensive agricultural crops where trees have been planted to be cut in a period ranging from 15-20 years. In the case of natural forests, specimens of 100-150 years are selected following very controlled felling so as not to destroy the forest.

Depending on the type of waste, some prior treatment may be needed at the extraction site itself, at an intermediate conditioning plant, or at the biomass plant facilities. These treatments include natural or forced drying, crushing (chipping), homogenization (grinding), and densification (pelleting or briquetting).

Transport

Transport is a key point in assessing the total ecological footprint of biomass fuel, as the fact that production takes place near the point of consumption is not always possible when we talk about a plant-based fuel. Unlike solar or wind energy, which is available at any time and place, although the latter cannot be stored like biomass.

Within the carbon footprint calculation, we must take into account the distance the fuel travels to reach its destination, which in some cases involves traveling between continents.

We must keep in mind at all times that obtaining more energy-efficient systems should not imply a consequent increase in energy expenditure. Our initial goal is to reduce consumption as much as possible, so we must understand that changing the type of energy also implies a change in the logic of production and distribution.

Power

A point in favor of building a house with biomass is its high power, which allows for a large amount of energy to be obtained in a short time.

In this sense, unlike other energy production solutions currently on the rise, such as geothermal or aerothermal, with biomass we reach higher temperatures. While this may or may not be an advantage beyond the comfort temperature of domestic hot water, the water temperature is directly related to the heat distribution system. If we evaluate low-temperature radiant heat systems such as radiant walls or floors, having water at too high a temperature is inefficient.

As we explain in the article differences between geothermal and aerothermal, both energies heat at lower temperatures.

Building a house with biomass

Systems

Before building a house with biomass or any other renewable energy system, we must understand the difference between production systems and distribution systems.

In the case of biomass, the production system is the wood chip or pellet boiler, or the thermo-fireplace where wood combustion occurs. The distribution system can be both domestic hot water pipes and heating ducts, whether through convection or radiation systems.

Biomass boiler

These can be individual or collective boilers, but they have in common that they will provide energy to conventional central heating. As we mentioned before, biomass provides such power that it allows the demand of an entire home to be met with a single boiler.

This boiler is usually combined with a storage tank to heat water, which transfers it to a distribution system via DHW, whether radiators or underfloor heating. It can also be distributed through air splits, but this is a less common system and not optimal for our health, as it dries out mucous membranes, affecting our defenses.

The water is heated by a heat exchanger and transferred to the radiators via a recirculation pump. It is normally placed in a utility space in the home, which also houses a hopper to be refilled with bags or a pellet tanker. In this way, the boiler feeds itself automatically, without requiring manual effort from the user.

Although many European countries use specific boilers that can only use certain solid biofuels, in Spain, given the heterogeneity of biomass resources, boilers are sought that can use most available solid biofuels. The final use of the energy (thermal or electrical) does not influence the choice of biofuel type, although its price can be a limiting factor, especially for electrical uses.

Air stoves or hydro-stoves

Within distribution systems, we can replace conventional radiators directly with stoves, very similar to traditional wood stoves but with significant technological improvements.

There are two types of stoves: water or air. The air stove directly heats the air through combustion, which is expelled into the room, thus acting as both a production and localized distribution system. For its part, the water stove or hydro-stove heats water that passes through a circuit near the combustion chamber, to then transfer it to one or more adjacent radiators.

Within this second type, there are stove models with an integrated water storage tank. Their operation is based on taking part of the heat from combustion and integrating it into the heating system through the intermediate storage system, the accumulator. In this way, in case of low thermal energy demand, the stove in combination with a solar energy system can ensure the supply of heating and DHW. This combination of biomass with other renewable energies is a very interesting option in Spain within the domestic sphere.

Pellet fireplaces and cookers

These are two appliances inherited from tradition that resolve a specific demand for a particular action or room. In the case of the insertable fireplace, it operates the same as an air stove but is built into an old fireplace. On the other hand, the pellet cooker features a wood oven and burners for cooking while simultaneously heating the kitchen. Both products must be fed with pellets manually as they are localized systems.

Suppliers and available formats

At the Spanish level, the main industry association is the Spanish Association for the Energy Valorization of Biomass (AVEBIOM), whose purpose is to promote the implementation of this system in housing and industry, as well as the creation of certifications for management control.

All new plants whose main activity is energy recovery or the handling and transformation of biomass must submit an environmental impact study which, among other issues, confirms the characteristics of the environment where it will be located, the project analysis, the forecast of alterations and corrective measures, residual impacts, and the monitoring plan.

Another way to ensure the sustainable nature of biomass production is through the ENplus A1 certification, which speaks to product quality and good practices throughout the entire supply chain.

At the European level, pellets have a greater reach in Germany. However, in Spain, we have managed to position ourselves successfully in the top 5, according to statistics from the German Pellet Institute, with 85% of Spanish pellets being certified.

The chain is established following the line of production, distribution, and service provider. This last link, the service to the final consumer, is carried out in several formats: 15kg bags, big-bags (1,000 or 1,250 kg), or in bulk via tanker trucks.

Storage can be done in large containers made of reinforced plastic, fiberglass, or fabric that can be placed inside or outside the home. Combustion will produce ash that will need to be removed manually from time to time.

A good example of good practice in Spain is Ecowarm, a company in Galicia dedicated to transforming residual industrial by-products into pellets, advocating for low emissions and proximity.

Amortization

An important aspect when assessing the viability of building a house with biomass is the possibility of amortizing the initial investment, which at first is not as high as aerothermal and geothermal systems.

In the case of biomass, we can meet the DHW and heating demand of a home with a 16-25 kW boiler, with a budget of €2,000 to €3,000. Similarly, in Spain, individual 7-10 kW stoves are sold for about €800. However, it is true that this is a system where a monthly fuel price must be taken into account.

The power of pellets is calculated with an equivalence of 2 kg of pellets to one liter of diesel. Furthermore, we must keep in mind that the price trend for pellets is downward, contrary to that of crude oil or gas, as they are fossil fuels.

The calorific value of the pellet is 4100 kcal/kg (4.76 kWh/kg), so in winter in an average home, a 15 kg bag lasts about 2-3 days, amounting to an approximate expenditure of €50 per month during peak demand periods.

Thus, while aerothermal and geothermal are calculated with an amortization period of about 7-10 years, the biomass calculation is more complex and we should evaluate the total monthly fuel maintenance. However, as this expenditure is much lower than the cost of grid energy and the installation is so affordable, it can be easily amortized in a medium term of 2 to 3 years.

Health

Regarding human health, industrial biomass production near residential areas can be very harmful, as biomass combustion releases a significant amount of particles invisible to the human eye. This is not the case in domestic installations where boilers are designed to prevent any harmful leaks.

Furthermore, it is essential that the fuel we buy is free of any type of glue or binder, especially in the case of industrial waste.