How To Size A Chiller For Biogas And Anaerobic Digestion Facilities

Biogas facility chiller sizing requires a different analytical approach than standard commercial or industrial cooling applications. The loads are continuous, the biological sensitivity is high, and heat recovery from engine-generators adds a layer of thermal complexity that must be modeled correctly. Undersizing means production losses; significant oversizing wastes capital and creates part-load efficiency problems.

Step 1: Identify All Cooling Loads

Biogas facilities typically have three categories of cooling load:

• Digester thermal balance: even when engine waste heat provides the primary heating source, excess heat in summer or engine downtime creates cooling load. Calculate the maximum heat surplus the digester system will experience during peak ambient conditions.
• Engine and CHP equipment cooling: engine jacket water systems and intercoolers generate heat that must be managed. In summer, waste heat recovery may not fully dispose of all engine heat, creating direct cooling demand.
• Gas processing equipment: compressors, upgrading systems, and conditioning equipment generate heat that must be removed for proper operation.

Step 2: Map the Heat Recovery Loop

Most biogas facilities with CHP use engine waste heat to warm digesters through a heat recovery loop. Understanding the full thermal model is essential:

• Engine jacket water heat output at operating temperature
• Digester heating requirement at design ambient temperature (worst case is usually coldest winter conditions)
• Heat surplus during warm months when digester heating demand drops but engine heat generation continues
• Engine downtime scenarios and how digester temperature is maintained during planned and unplanned outages

"The most common sizing error we see in biogas facilities is failing to model summer heat surplus properly. In winter, the engines are barely keeping up with digester heating demand. In summer, they're generating far more heat than the digesters need, and that excess has to go somewhere."— Andy Backer, VP of North American Sales, G&D Chillers

Step 3: Define the Chiller's Role

Based on the thermal model, the chiller may serve one or more functions:

• Summer heat rejection: removing engine waste heat that exceeds digester heating demand
• Engine backup cooling: serving as primary cooling source when engines are down for maintenance (digester temperature must be maintained regardless)
• Gas processing cooling: continuous cooling of compression and conditioning equipment independent of the heat recovery loop

Sizing Example: 500 kW CHP Biogas Facility

Load Category	Winter	Summer
Digester heating requirement	280,000 BTU/hr	80,000 BTU/hr
Engine waste heat available	320,000 BTU/hr	320,000 BTU/hr
Heat surplus / deficit	-40,000 BTU/hr (deficit — need supplement)	240,000 BTU/hr surplus (need to reject)
Compression cooling load	60,000 BTU/hr	60,000 BTU/hr
Required chiller capacity	Minimal (engine covers digester need)	300,000 BTU/hr = 25 tons minimum

Example values are illustrative. Actual heat outputs and load calculations depend on engine model, digester design, and facility-specific conditions.

Frequently Asked Questions

Do biogas facilities need redundant chiller capacity?

For facilities where engine cooling or digester temperature management is critical, yes. A single chiller failure that allows digester temperature to drift can result in days or weeks of reduced gas production. N+1 redundancy is appropriate for production-critical facilities.

Can a cooling tower handle summer heat rejection instead of a chiller?

Cooling towers can handle heat rejection for engine cooling and non-process loads. For loads that require controlled temperatures (gas conditioning equipment, precise process temperatures), a chiller provides better control than an evaporative cooling tower.

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