Different heating technologies in Danish ecovillages

When building a sustainable house there are many different heating techniques to choose from. In this blog I dive a little deeper into some heating solutions chosen by members of Danish ecovillages. I have asked them for the reasons of choosing a specific technique and the benefits as well as downsides to their technologies. You will notice that many solutions currently in use are based on the incineration of wood, however, most community members agree or speculate that this was a technology chosen 30-10 years ago and the future lies more in electric / geothermal solutions. Before I elaborate on some of the heating technologies, I want to dedicate a paragraph to passive heating techniques used in some Danish ecovillages, as for me: sustainable heating starts with lowering the demand for heating and harvesting all the heat that naturally enters a house as efficiently as possible.

Passive heating
Reducing the demand for heat in a household can be done in many ways. One often applied technique in the ecovillages I visited in Denmark is the construction of a greenhouse or glass facade to the south. Such a glass room to the south can open directly into functional rooms of the house, or act as a buffer room through which air is ventilated. At Dyssekilde ecovillage they wanted to stimulate this passive heating technique so they made a rule that every house could extend their footprint with a greenhouse and the m2 for this do not have to be paid for. Another technique is that of a mass floor or other big mass in a building which is heated by the sunlight that falls into a house during the day. This heat is then slowly emitted back to the house over the course of the night, keeping the temperature in house more stable. Off course good insulation and proper glass are other techniques to reduce heat demand. Lastly, in windy areas, it can be very effective to make sure the house is closed off to wind as a little wind can take away a lot of heat from a house.

One house in ecovillage Fri og Fro with a big facade to the south, harvesting solar heat during the day (with some solar boilers in front).
One of the many greenhouse extensions in Dyssekilde ecovillage. Some do not even have a floor so that the plants growing there can have extensive root systems.

Wood
I think it is save to say that the majority of houses in the Danish ecovillages I visited get their heat from the incineration of wood in one way or another. However, many different types of stoves are used. For all types it matters a lot where the wood comes from how renewable or sustainable the heating can be called. I visited only one ecovillage with a large enough, sustainably managed forest to be completely self sufficient in wood chips. I will explain a few of the options, before elaborating on other heating techniques that are applied which do not make use of wood.

Wood pellet stoves
A common stove used in several of the houses is a wood pellet stove. This is a type of stove which very efficiently burns prefabricated wood pellets to produce heat. The wood pellets are made of highly compressed wood chips and have to be bought from a company. The combustion is very efficient with almost no leftover residue. The pellet stove produces heat for both the hot tap water and the room heating. Although in some cases, the pellet stove is completely turned off in summer time, during which time hot tap water is provided through solar boilers. The heat generated in a pellet stove is around 76 degrees Celsius in some of the stoves I saw and is distributed to the rooms in different ways. For storing hot tap water a wood pellet stove is always combined with a highly insulated tank with capacity of around 100 litres depending on the water demand. Should the hot water run out, which according to Jesper never happens even in his household of five, the pellet stove can refill the tank in less than half an hour.

The pellet stove for a four person household of the brand windhager. The left side is the resevoir for pellets and the right side is where incineration takes place.

Most houses with a pellet stove have floor heating, the heat is in this case distributed through a network of pipes in the floor at a temperature of around 25 degrees. A big plus of floor heating is that the moisture level in the rooms stays a lot more stable compared to rooms with high temperature radiators. Rooms become less dry in the experience of inhabitants. Also, the temperature stays a lot more stable as the system takes a long time to cool down. However, the system also takes a long time to start so if the heating has been completely turned off for days, then it takes several hours to heat the rooms up again. Another commonly mentioned benefit of floor heating is the space it leaves to use all the walls in a room.

A pellet stove can also be connected to radiators which emit a higher temperature heat of about 60-70 degrees Celcius. This is often used on second floors, as it can be more difficult or expensive to install floor heating there. A big advantage of this is also for existing houses currently using for example gas boilers: The existing radiators and pipes can be connected to pellet stove and not much changes need to be made to the house.

The resevoir of a pellet stove with pellets inside.

In some cases the wood pellet stove is combined with solar boilers on the roof. This is a bigger demand on space inside the house, since the solar boilers need to be connected to a reservoir of around 300 to 1000 litre, depending on the heat demand of the household. In the farmhouse at Hallingelille ecovillage they have done this: they use their wood pellet stove solely in winter time, and in summer it is turned off completely and replaced with heat from their solar boilers. They do have a spare electric heater in place, in case the solar boilers do not provide enough heat in summer.

Solar boilers on the roof of the old farm house in Hallingelille ecovillage.

A pellet stove will use much less wood compared to a stove which burns briquettes or logs of wood because of the efficient incineration. Jesper’s household of 186 m2 uses around 2.5 tons of pellet each year. This costs him only around 500 euro per year (for both heating and hot tapwater). He is however a bit concerned for the future prices of the pellets. The downside of a pellet stove is that you need to feed it specially made pellets from a company. It can run on woodchips according to some community members, but not nearly as efficiently. Jesper sees now a trend of larger energy companies switching to pellets as fuel when the government phases out coal and gas sources. He is afraid this will drive the prices of the pellets up. Or it might also, because of the higher demand for pellets, lead to unsustainable sourcing of the wood. It would be possible to make the pellets themselves in Hallingelille community but then they would have to invest in a machine that can do this, and their forest is probably not big enough for this.

A final concern, which counts for all wood fired heating solutions, is the issue of local air pollution and CO2 emission, however small it is. And off course the issue of making sure that the wood is sustainably sourced.

Mass ovens
There are quite some other houses in the ecovillages I visited which are heated with mass ovens. In this type of wood stove, a big mass is build around the stove which is heated by the fire insight. Because of the large mass the heat is stored for a long time and slowly emitted to the rooms of the household during the day. The mass can be made from many materials, for example bricks or clay. Some design the mass creatively, or incorporate the mass oven in their staircase. The communal house at Hallingelille ecovillage has a mass oven of about 1x2x1 m made out of bricks. Making one fire in the morning is enough to keep the entire building warm for the whole day (outside temperature around 10 degrees). They do have a back-up electrical heat pump for quick heating of their gym space.

A mass oven in a cob house in Dyssekilde.
The brick mass oven in the communal house at Hallingelille ecovillage.

A big plus to this type of oven that you can simply use normal firewood and do not need a machine to produce wood chips or buy prefabricated wood pellets. Also, it creates a very nice heart of a home for gathering around like in traditional houses. A downside is that it is much less efficient incineration than in for example a pellet of rocket stove, therefor it is in most cases more expensive due to higher use of wood. (depending off course on how you get the wood).

Highly efficient stoves / rocket stoves
In Fri og Fro ecovillage I came across a stove which makes double use of the heat from the wood incineration. Wooden logs are burned in the stove which heat the rooms, but the chimney is then closed off which forces the heat down to a chamber under the fire where temperatures up to 600 degrees Celsius are reached. Here, the heat is transferred to water which is then stored in a big tank. This water is used as tap water and floor heating.

Efficient stove at Fri og Fro ecovillage with two chambers.

This concept is similar to more commonly known rocket stoves, which also force the heat of a fire back down creating very high temperatures. The heat is then guided through a long pipe which is surrounded by a lot of mass. This mass is heated up and slowly emits the temperature to the room. With both these techniques less wood is needed compared to a regular wood stove because of efficient use of the high temperature generated by the incineration.

The head of a rocket stove at Makvaerket collective, with an explanatory sign on the left.
Another rocketstove with a nice warm bench at Makvaerket collective.

A downside mentioned to both the mass oven and these types of stoves is the time they take to heat up a room, as well as the time that is needed for starting the fire. Some community members mentioned that in their busy lives with fulltime jobs and children they simply do not have the time to get up extra early to make a fire which will take an hour to heat up the house. Compared to this, a pellet stove workes completely automatically and does not take up this time.

Communal solution: wood chip incinerator with small distribution network
In two of the bigger communities I visited the households are heated through a communal wood chip incinerator. In these cases the community owns one large incinerator which heats water to a temperature of around 90 degree Celcius. This water is then distributed to the households via a small distribution network. In the households the rooms are heated with radiators radiating a heat of around 70 degrees. It is a low maintenance solution which can be cheap and easy for a large community. However, the sourcing of wood chips should be done very carefully. And off course, the issue of air pollution remains. Also, hot radiators cause rooms to be very dry and temperatures to fluctuate a lot in the rooms.

The communal wood chip incinerator at Munksogard community.

Heat pumps
An alternative to heating with wood is heating with electricity. For this, again many different techniques are applied in the ecovillages. Ranging from air-to-air heat pumps (basically inverted air conditioners), to geothermal systems that only use a little heat pump. The sustainability of electric systems depends a lot on the type of electricity used. Also the affordability of the alternatives is dependent on the source of electricity. For example, an air to air heat pump may be completely renewable and for free when you have installed enough solar panels or wind power to power the heat pump. But it can be very expensive to heat an entire house with a heat pump if you buy in electricity from the grid.

Several houses or blocks I visited have combined geothermal heat with a heat pump. An advantage of such a system is that the source temperature fed into the heat pump is more constant and, especially in winter time, a lot higher compared to an air to air heat pump. This saves a lot in electricity usage. In these cases a long distance of pipes is laid at a distance of abot 1 m underground, water is pumped through these pipes. Because the temperature in the soil at this depth remains constanat at about 12 degrees, the water fed into heat pumps has this stable temperature. Especially in winter time this saves a lot of electricity compared to air to air heat pumps. Geothermal systems like this one can be installed for individual households, or blocks of houses. In Dyssekilde one street collectively owns such a system, at Hallingelille one commune of 6 appartments uses it.

Heat pumps are by several community members I spoke seen as the alternative in the future. They expect the efficiency of heat pumps to go up drastically and with that the prices, and noise disturbance down. They also expect that renewable electricity will be much more widely and affordably available in the future. But for now, it has to be said that the electricity costs of all households using heat pumps are very high, making the yearly costs for heating a lot higher than the wood alternatives. However, communities producing enough of their own renewable electricity are different.

With the geothermal system the main problem is that when the system is not working properly it is difficult and expensive to solve as there is many metres of pipes underground.

Low tech solution
In Hallingelille they have also realised an example of a low tech ventilation system which heats the house. They built a system made of recycled soda cans. These have been sprayed black and put in rows inside a glass box. This box is attached on the facade and two holes are made in the bottom and top of the box connecting the air in the box with the room inside. When the sun falls on te black cans, the air inside is heated to high temperatures. This generates a natural airflow with which cold heat from the room is sucked into the box at the bottom, this rises when it is heated by the sun and enters the room again at the top of the box.

Low tech natural ventilation system heating the air. Made out of recycled cans.

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