Biogas Heat Use: Livestock, Drying and Local Heat

Heat from a biogas combined heat and power (CHP) unit is best used directly on the farm: for the farmhouse and retirement cottage, livestock buildings and milking parlour, hot water and drying. Beyond that, a local heat network can supply neighbours, schools or businesses and bring in extra revenue. Every kilowatt hour used replaces heating oil, gas or wood and improves the plant’s profitability. The keys are an honest heat balance over the whole year, short pipes and clear contracts.

How much heat is available

A gas engine CHP unit converts part of the energy in biogas into electricity and a similar or slightly larger part into usable heat. This heat is recovered from the engine cooling water and exhaust gas, usually at a temperature of around 80 to 90 degrees Celsius.

Part of the heat is needed to keep the digester at a constant temperature for the bacteria. Demand varies strongly with the season, slurry temperature and insulation: it is high in winter and low in summer. The heating demand of houses runs exactly the other way. For planning you therefore need a month-by-month heat balance rather than an annual total.

As an illustration of scale: a plant with 75 kW rated output generates around 657,000 kWh of electricity a year. If the CHP unit delivers roughly the same amount of heat, several hundred thousand kilowatt hours remain for other uses after heating the digester. How much of that is actually used determines its contribution to profitability.

A reliable heat balance is built in five steps:

1. Determine the CHP unit’s heat output from the data sheet and the planned operating hours per month.
2. Have the digester’s monthly heat demand estimated, depending on slurry temperature, throughput and insulation.
3. List all possible consumers with their consumption in recent years, for example from heating oil or gas bills.
4. Compare generation and demand month by month, including pipe losses.
5. Check in which months heat is short and how the gap is covered, for example with an existing boiler.

The balance quickly shows whether an extra consumer with summer demand is worthwhile or whether heat storage could better match generation and use.

Use on your own farm

Using heat on your own farm is usually the most profitable option, because distances are short and no contract with third parties is needed. Typical consumers are:

• Farmhouse and retirement cottage: space heating and hot water, often via a short buried pipe.
• Piglet rearing and poultry: heat demand all year round, particularly well suited.
• Dairy farms: hot water for the milking system and cleaning, calf milk, the milking parlour.
• Workshop and office: space heating in the heating season.

Check which consumers also need heat in summer. Year-round demand matches the steady operation of a manure plant better than space heating alone.

Drying as a summer outlet

Drying uses heat precisely when heating demand is missing. Common uses are:

1. Grain and maize drying: seasonal, but with high demand over a short period.
2. Timber and wood chip drying: via ventilated floors or containers, also as a service for others.
3. Digestate drying: reduces volume and transport costs but releases ammonia, which must be treated with exhaust air cleaning.

Drying pays off if the dried product has real added value or saves costs. Drying merely to dispose of heat brings no economic benefit.

Local heat network for neighbours

A local heat network can sell heat all year round and increase the plant’s acceptance in the village. Profitability depends above all on how much heat is sold per metre of pipe. Long pipes to few customers cause high costs and heat losses. The denser the buildings and the higher their heat demand, the better.

A network usually needs:

• heat storage to decouple generation and consumption
• a peak load and backup boiler for cold days, maintenance and breakdowns
• substations at customers’ premises with heat meters
• route planning with permits and rights of way

A feasibility study can be funded through the Federal Funding for Efficient Heat Networks (BEW), and smaller networks serving a few buildings through the Federal Funding for Efficient Buildings (BEG). More in the guide to funding. Remember too that support under the Renewable Energy Sources Act (EEG) ends after 20 years. A heat network therefore needs a plan for the period afterwards; see the guide to biogas after the EEG.

A common approach is to set up a separate operating company or an energy cooperative in which customers can take a stake. This spreads investment and risk and strengthens local support. For small networks with a few neighbouring houses, a direct contract between farm and customers is often enough.

Many German municipalities are currently drawing up their municipal heat plans (kommunale Wärmeplanung). Ask your municipality whether your farm could serve as a heat source for an area.

Heat revenue and supply contracts

The heat price is usually based on the full cost a customer would have with their own heating system, minus a discount that makes switching attractive. Three price components are common:

If you supply households under standard contracts, the AVBFernwärmeV applies. Among other things it governs contract terms, termination and information duties. Price adjustment clauses must reasonably reflect both the cost of generating heat and conditions on the heat market, and they must be transparent.

The contract should also cover security of supply, backup heat, liability for outages, ownership of pipes and substations, and what happens if the operator changes or support ends. Clarify the tax treatment of heat sales with your tax adviser in advance.

Technology and operation

For heat use to work reliably, the CHP unit, storage and network need coordinated controls. Emergency coolers on the CHP unit dissipate surplus heat but should not become permanent operation. Heat meters on all outlets show where heat is actually used and form the basis for billing and optimisation.

Keep return temperatures low. The colder the water returning to the CHP unit, the more heat can be recovered from engine and exhaust and the lower the network losses. Hydraulic balancing of the connected heating systems and well-adjusted substations often bring noticeable improvements with little effort.

If the plant is to generate electricity flexibly in future, a larger heat store becomes important because the CHP unit will no longer run steadily. Specialists for CHP service and heating technology are listed in the directory under service and maintenance. You can check how heat affects your figures in the biogas calculator.