Moving heat is hard, even (especially?) when it comes from manure

Compost is useful as fertilizer but it also generates heat, and heat can be useful. Therefore, compost can be doubly useful.

That bit of reasoning has been in play at UNH’s Durham campus since 2013. At the university’s Organic Dairy Research Farm, compost manure gets turned into fertilizer so researchers can capture some of the warmth that is generated when stuff rots. Now, UNH’s Woodman Horticultural Research Farm is trying to expand on the idea so that small farms can do the same.

“Our goal is to make it farmer friendly, make it as easy as possible. Don’t mess around with all that high-tech stuff… just aerate the compost with a regular conventional blower,” said Md Sazan Rahman, the assistant professor at UNH who is overseeing this project at the N.H. Agriculture Experiment Station. “We’re testing it in a real greenhouse, in a larger scale, to see whether it will work or not in actual farming practice.”

The problem Rahman’s team wants to solve is straightforward: Cold.

Even though our winters aren’t as potent as they used to be and will be getting less so with time, New Hampshire’s growing season still pales in comparison to those further south. Greenhouses have long been the method used by farmers and gardeners to extend growing seasons for vegetables and other plants.

The problem is greenhouses have to be heated, which is expensive. Figuring out how to use the heat from compost – especially if you’re a farm with animals that produce manure – seems like an obvious way to reduce that cost.

This idea fits into a much bigger issue, that of food security. Climate change and resulting environmental changes are going to scramble the world’s food supply, so the more we can produce locally the better.

A coalition of organizations across New England is exploring ways for the region to produce 30% of the food we consume by 2030. That seems a stretch – New Hampshire’s Local Food Count estimated that just 3.1% of the food Granite Staters ate in 2022 was produced in-state. But if reaching that goal is at all possible, extending the growing season for small farmers will be part of the solution.

If you’re not a compost fan, you may not realize how much heat can be generated by the microbes and bacteria that feed on the glop shoved into the pile. If it has a good mix of what are known as greens and browns (sources of carbon and nitrogen), is kept moist and is turned frequently to provide the little beasties with oxygen, a home compost pile can pretty easily get to 160 degrees Fahrenheit while industrial piles can make it to 300 degrees or so.

The tricky part is getting those temperatures out of the pile to someplace more useful. As the familiar slogan goes, “Heat is not easy to move.”

The project at Woodman Horticultural Research Farm is testing two different systems while growing lettuce hydroponically: one moving heat with water and one moving it with air. They’ll analyze the impacts on lettuce quality and quantity.

One system leverages heat from composting manure piles to warm a water reservoir, which then distributes heat throughout the greenhouse via a heat pump. The second system blows hot air from the compost into the greenhouse using a biofilter to reduce bacteria and pollutants within the manure.

The research is set to last through 2027 testing lots of variables including the size of storage bays, the design of pipes and fans and whether heat can be stored in water tanks for later use. Since the budget is at most $80,000 over a period of three years, part of the approach is keeping costs low by building walls with lumber and not concrete, for example. Data logging is a big part of it, too; this is science, after all.

If the results are good, then comes the long process of getting the word out to farmers and perhaps helping them put this tactic into practice.

Rahman views this project as a stepping stone toward fully sustainable greenhouses. Add in some solar and maybe geothermal energy and you’ve got a greenhouse system that can be a model for protected agriculture across cold regions.

“This is just the beginning,” he said.