Different types of domestic heating systems with wood

Tiled stoves

Tiled stoves were the most efficient traditional way of burning logwood. Recent developments have led to significant improvements in design, enhancing the efficiency and lowering emissions. Operating comfort could be improved by features such as automatic electric ignition. Tiled stoves are the most popular device for additional heating in Austria and an excellent heating solution for low energy houses. Their price varies strongly according to the fanciness of design. Simple designs can be cheaper than logwood boilers.

Kontakt: Association of tiled stove producers

Pellet boilers and pellet stoves

Wood pellets are a homogeneous fuel with high energy density which allows for simpler and cheaper boilers. Requirements for storage are smaller and heating comfort is equal to oil boilers. This segment is the fastest growing segment of biomass boilers at present. Pellets can be used for stoves, too, which are significantly cheaper than boilers and can provide comfortable and cheap additional heating, e.g. to an existing electric or fossil fuel based heating system.

The price of pellet boilers is about 7500 ECU, the price of pellet stoves is around 2500 ECU. Pellets cost about 30 ECU/MWh in Austria. Recently boilers have been designed that can be fuelled both with pellets and with logwood.

Logwood boilers

Figure 2 shows the scheme of a new generation logwood boiler. Wood combustion takes place in a two-stage process: gasification in the first stage and high temperature combustion in a specially designed chamber in the second stage.

In order to ensure clean combustion, most producers nowadays use lambda sensors to measure the remaining oxygen in the flue gas so that an optimal quantity of secondary air for the combustion process can be supplied via adjustable valves.

The boiler is generally operated at full power and heats up a hot water storage tank for continuous heat retrieval. The storage tank can serve as a solar boiler in summer if thermal solar collectors are installed. The latest models of logwood boilers do not need a storage tank as they can run at 30% of full power without significantly higher emissions than at full power.

Fig. 2: Scheme of a logwood boiler QueSource: Energy from Biomass - R&D in Austria (Ministry of Science and Transport)

The average price of a unit including storage tank for a single family house is about 6000 ECU. Logwood costs about 25 ECU/MWh. It can be stored outdoors.

Woodchip boiler

The advantage of woodchip boilers is their automatic feeding system for fuel which allows for full automatic operation and similar user-friendliness as oil or gas fired boilers. State of the art woodchip boilers are equipped with continuous power control and do not need a heat storage tank. A disadvantage of woodchips is their relatively large space requirement for indoor storage. Due to possible variations in fuel humidity, woodchip boilers need advanced electronic control equipment. The possible variation of woodchip size and humidity requires rather robust (and expensive) feeding and control mechanisms. Thus woodchip boilers are significantly more expensive but also more comfortable than logwood boilers. They are a good solution when heat requirements are so high that the effort of logwood handling is unacceptable. The average price of a unit for a single family house is 11 000 ECU. Woodchips cost 20 - 25 ECU/kWh.

Fig. 3: Scheme of an automatic feeding system of a woodchip boiler Source: Praktischer Ratgeber Hackgutfeuerungen (RES Regionalenergie Steiermark)

Woodchips are of particular interest if the user has direct access to cheap fuel; this is why they are the fuel of choice for farmers as well as for the timber and woodworking industry.

Other types of wood boilers

Apart from the above mentioned types there are other centralised heating systems which are especially designed for smaller or low energy houses:

• tiled stoves with water pipes;
• tiled kitchen stoves (for cooking, as well as space heating [also of adjoining rooms] and water heating);
• pellet stoves (pellet-fuelled stove, storage tank for up to 90 hours continuous operation, capacity up to 10 kw);
• hypocaust heating systems (heating system distributing heat through flues which can be integrated into the structure of new buildings);
• hot air heating systems (heat is released directly into the room through discharge flaps).

Furthermore, wood-based systems are increasingly being used for additional heating in flats and houses as people appreciate the cosy warmth of wood fires. In the interseasonal period these systems can also be used to heat the entire residence.

• fireplace heating system (a closed fireplace from which hot air can be discharged into the room where it is installed or into the rooms above);
• freestanding stove (a further development of the classic cast iron stove).

District heating with biomass - advanced technologies for maximum convenience of use

The introduction of small scale biomass district heating plants is a success story for renewable energy in Austria. Between 1980 and 1998 more than 350 plants were built to provide heating predominantly for rural villages. Recently around 50 new plants have been established yearly. Dedicated political support, the active role of provincial energy agencies and agricultural chambers and substantial subsidies (30-50% of investment costs) have been vital for this success.

Fig. 4: Annually installed capacity of biomass district heating systems in Austria

Biomass district heating plants are fuelled with industrial wood wastes and wood from forestry. Most of the plants are operated by agricultural cooperatives, as many farmers are forest owners seeking to create a market for low-grade wood from thinnings. Dedicated R&D efforts have contributed to the success of biomass district heating by introducing substantial improvements of plant technology.

Recent research has focused on different approaches to minimise the environmental impacts of biomass combustion by minimising NOx emissions and emissions of heavy metals. Cost-effective technologies to minimise fine dust emissions have been developed and new approaches to maximise energy gains from flue gas condensation are under investigation.

Contact: BIOS

In addition to these small-scale district heating networks, however, so-called micro networks present a viable option for less densely populated areas where district heating is uneconomical. These networks are frequently operated under contracting schemes or by groups of farmers running heating plants in the low capacity range.

Wood-based energy contracting: How does it work?

In this type of contracting project, a group of farmers invest in the biomass installation and the required construction work and rent space in the customer's cellar. The operators are responsible for the functioning, maintenance and any repairs of the heating system. As in the case of district heating networks, the customer pays a connection charge and the price for the actual heat intake, which is determined by means of a metre.

The customer is not involved in the organisational side of running the heating system. If the coefficient of performance is favourable, surrounding buildings can also be supplied (directly, or indirectly via heat exchanger). The intention behind wood-based energy contracting schemes is to facilitate the use of biomass heating systems for multi-storey residential buildings, municipal buildings and trade and business establishments without requiring potential users to bear the investment costs. In effect a kind of micro-district heating plant is set up: customers are supplied with heat without having to deal with the installation, operation or financing of the system.

The success of wood-based energy contracting projects

The dynamic development of this type of contracting project and the great number of projects realised demonstrate the success of this model. The first installation of this kind came on stream in 1995; by early 1998, 34 of them (representing a total capacity of about 3,000 kw) were operating smoothly and successfully. By October of that year, definite plans had been made for 10 more projects with an overall capacity of just over 1,000 kw.

Whereas the customers in such projects have tended to be public-sector institutions (town halls, kindergartens, schools) or privately owned houses, there is now a marked trend towards supplying multi-storey residential buildings. The initially sceptical housing associations have come to see the advantages, too.

Costs of a wood-based energy contracting project

The current price situation for such schemes is as follows:

The operators (farmers) must come up with approximately 10 to 15% of the capital requirement; the rest can be financed from connection charges and subsidies available under investment promotion schemes (currently about 30 to 50%). The operators can expect to achieve a price of ATS 250 (excluding VAT) per cubic metre (bulk volume) of soft woodchips; maintenance and service work can be charged at the hourly rates set by ÖKL /1/ (approximately ATS 100 per hour).

For the customers, this works out to standard biomass district heating rates:

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Footnote

/1/ Österreichisches Kuratorium für Landtechnik und Landentwicklung (Austrian Council for Agricultural Engineering and Rural Development)