Compost Heat Recovery: The Extraction Rate at Which Cooling a Pile Slows Its Own Decomposition

Compost is a heat source that destroys itself if it is used too hard. This paper models a 20,000 kg wet compost pile in a covered vessel, follows its core temperature over 60 days under microbial heat generation, surface loss and a constant water-loop extraction, and derives the trade bound: the largest extraction rate that preserves a thermophilic treatment window and at least 80 per cent of the decomposition achieved with no extraction. SIMULATED, NOT MEASURED. With no extraction the pile peaks at 71.43 °C and holds at or above 55 °C for 35.75 days. The bound is 3.75 kW. At 3.75 kW the pile peaks at 68.45 °C, holds at or above 55 °C for 21.54 days, retains 0.9951 of the no-extraction decomposition index, and delivers 5,400 kWh of heat over 60 days — enough to preheat 3,096 litres of water per day from 10 °C to 35 °C, or to cover 0.469 of the 8 kW peak heating demand of a stated 100 m² greenhouse. The bound is sharp, not gradual: at 6 kW the pile never reaches 55 °C at all (peak 50.93 °C, zero days at or above 55 °C), and at 8 kW it does not self-heat. Heat recovery from compost is real but small, and its useful size is set by the biology, not by the heat exchanger.