Composting : Heat Recovery from Compost: It's getting hot in here!

Have you ever stood next to a large pile of biowaste or steaming manure wondering how much heat is contained in there? And if there are commercially viable technologies to capture that heat? No? Well, others have.

Records from ancient China show the use of heat from compost piles almost 2,000 years ago. At the turn of the 20th century, Parisian farmers used the city’s horse manure in composting “hot beds” to heat glasshouses for urban vegetable production. And in the 1960s and 1970s another Frenchman, Jean Pain, built composting mounds of brushwood that produced enough thermal energy to heat a batch biodigester and provide hot potable water for his farm.

But in the 21st century heat recovery from compost is still not “a thing”, even though various projects and a few studies address the concept. A 2010 study from the University of Edinburgh analysed the potential for collection and reuse of compost heat as a source of renewable energy in a vessel tunnel composting facility in Scotland. The researchers found that “collecting the waste heat of compost through a heat exchanger is a realistic solution to contributing to energy demand”.

A more recent study from an international research team led by Moi University in Kenya came to a similar result in 2020. “Utilisation of the energy generated from the compost system is multipurpose. The heat can be used for heating water for domestic uses such as washing, bathing and heating rooms in cold weather conditions. With hybridisation with solar energy, the energy could be used for drying foodstuffs,” the researchers wrote.

About 15 years ago a livestock farm in Vermont (USA) installed the world’s first commercial-size composting facility that turns excess heat into energy for the farm. Diamond Hill Farm uses the compost aeration and heat transfer system of Vermont-based company Agrilab Technologies to capture the metabolic heat produced by microorganisms during aerobic composting through a negatively aerated fan system (read the interview here). The hot compost vapour is then blown against the heat exchange system to heat water for radiant floor heating, feed preparation and sanitation of equipment. “The only disadvantage is the potential to overcool your material, so you have to pay attention,” says Brian Jerose, co-founder of Agrilab Technologies.

The farm served as inspiration for a project at the University of New Hampshire (UNH). The university turned its Organic Dairy Research Farm (ODRF) into a model farm for energy independence and nutrient management, where the installation of a composting facility with heat recovery addresses the manure, nutrient and energy issues in an integrated approach.

The process is called aerated static pile/heat recovery composting (ASP/HRC). In contrast to the most common composting processes (static pile, passive composting systems let the materials decay slowly without being turned; passively aerated systems use windrowed piles that need to be turned frequently), the ASP approach does not require turning. This also means it reduces labour and fuel costs. Because of the aeration, the compost process is accelerated.

In an ASP/HRC system, the organic wastes are loaded onto a concrete floor with embedded perforated pipes. These are connected to a manifold system. This either draws air down through the decomposing feedstock (negative aeration) or forces air up through the material (positive aeration). In negative airflow applications such as the one at the ODRF, the process creates a contained airflow of heated vapour that can be directed through a heat exchanger and/or biofilter to reduce odour and other pollutants. This means the vapour captured in an ASP system is a potentially harvestable source of heat energy. At the ODRF it is used to heat water. About 1.4 MMBtu of heat energy is generated per tonne of material.

(Want to know more about how the technology works, read our interview with Brian Jerose here)

Apart from compost sales as a source of income, the on-farm used heat results in reduced energy costs, as Prof. John Aber, who set up and led this project, explains: “Diamond Hill Farm used the heat to warm a heifer barn and reports saving $10K on energy and earning $10K on the sale of the compost.” As for the UNH project, he sees the possibility to build a low-cost facility that can be paired with high tunnel greenhouses on campus.

For Aber, heat recovery from compost is a real alternative: “The technique is relatively new, and I think is often confused with anaerobic composting for methane, which has yet to prove financially viable, in my opinion,” he says. “There has been much federal money in the US spent on anaerobic processes, which really will only work on very large farms. Sometimes I think this aerobic composting for heat energy is too simple and too low-tech to get much attention. Financially, though, I think it can be a winner.”

Californian inventor Henry Hovakimian also sees compost as a reliable energy source. “Alternative energy options for fossil fuels and environmentally acceptable waste disposal systems are an essential necessity around the world and should be seriously considered,” he says. “Decision-makers should help to provide the opportunity to market new inventions and provide alternatives for consumers to choose from.”

He has two US Green Energy patents and with his company Compo Energy (CE) wants to provide a better solution for household and green waste disposal. His plant is designed to produce power utilising the waste heat derived from 250,000 tonnes of compost spread out over an area of 1,000 x 1,000 feet split into 12 identically sized composting parcels. It consists of a heat exchanger and a Rankine cycle as well as an optional solar trough system to increase the work output. “At CE there are no incinerators and the plant will generate clean, renewable energy from household and green waste utilising natural composting and solar heat. There will be no emissions of greenhouse gases from the burning of waste, coal, natural gas or other fossil fuels,” Hovakimian explains. “The concrete pads upon which the compost piles will be situated will be constructed with proper drainage, eliminating the possibility of groundwater contamination. Plant operation will be monitored 24/7 to satisfy the surrounding communities. All recyclable materials will be removed and the main end product will be compost.”

(Want to read more about generating electricity from compost heat, read the full interview with Henry Hovakimian here)

As for the large amount of compost his plant would produce, he says: “In California there are 43 million acres of agricultural land; 16 million acres are grazing land and 27 million acres are cropland. Compost at only 1” per acre will require 50 tonnes/acre equal to 2,150,000,000 tonnes of compost. Outside of California, Texas has 127 million acres of farms and ranches. The number goes up and up with more states considered. So, clearly, there is no lack of demand for compost.”

To help design his energy recovery plant, he contacted the California State Polytechnic University at Pomona to form a collaboration. Dr Kevin Anderson, P.E., Professor of Mechanical Engineering, joined the research team and started the long design process. In his report on the project, Professor Anderson gives it a very positive assessment: “This analysis has found that the proposed concept for a low-grade waste heat Rankine cycle power plant utilising isobutane as the working fluid is feasible.” The 250,000 tonnes of compost can generate enough to power 3,000 homes annually. The proposed plant is “entirely sustainable, renewable and environmentally friendly.”

So the next step would be to build a prototype. “I know that it cannot fail. The data is clear. I hope someone will see that and invest,” says Henry Hovakimian with conviction. “In current times you just need to think outside the box.”