Features Business Efficiency Energy Trends
Waste not, want not

Perspective is everything. A broad view of waste heat (WH), according to Wikipedia, is that it “is heat that is produced by a machine, or other process that uses energy, as a byproduct of doing work.” Traditionally, the WH process is generally described as “heat that is either lost through the flue stack of an industrial operation, or which is rejected from a power generation station to improve the thermodynamic efficiency of the cycle.”¹ A flexible perspective is that WH is actually not ‘waste’ but a ubiquitous, adaptable, ‘renewable’ resource with most of its applications yet to be discovered. 

There are numerous examples of WH applications. In Bavaria a cement plant has reduced both its electricity use and its CO₂ emissions by using its own WH.² In Vancouver’s False Creek neighbourhood 6,210 apartments are heated by renewable energy resources that include sewage.³ Data centres, which account for “1% to 1.5% of global electricity use” and are rich sources of WH, are taking advantage of WH in France, the U.S., and Sweden for greenhouse applications.⁴

Combining energy efficiency with sustainability
SaskPower’s Shand Power Station, located just outside of Estevan, Saskatchewan provides up to 276 megawatts of coal-fired electricity. “Equipment at the power station is cooled via a closed-loop, zero-discharge system, meaning no water escapes to the environment except through evaporation in a cooling tower,” states Shelley Heidinger, Environmental Consultant (SaskPower). “The cooling tower reduces the heat in the cooling water before it returns to the power station to continue cooling equipment.” 

The heating/cooling process is circular. “The greenhouse accesses the cooling water supply via a pipe that links into the cooling water line prior to it reaching the cooling tower. It runs underground to behind the greenhouse where the line connects to each of three greenhouse bays,” notes Heidinger. Each bay is 500 sq. m., totaling 1,560 sq. m. under glass. 

“We grow two crops per year, producing up to 600,000 native or mostly native coniferous and deciduous tree and shrub species,” Heidinger explains. Over 14.9-million seedlings of native species used to create shelter belts and conservation initiatives have been distributed. The shrubs and trees offer a natural climate solution that removes CO₂ from the atmosphere.⁵ 

Heidinger notes that each bay has four heat exchangers. “The cooling water runs through tubes in the heat exchangers where fans move warm air off the tubes and distribute it out to the greenhouse via 76.2 cm diameter plastic poly vents. These vents run along the length of the greenhouse and have holes at regular intervals that release warm air. Once the water runs through each heat exchanger, it returns to the power station and is reused in the cooling process.”

 “We utilize an Argus control system to monitor bay temperatures, and to control heating and cooling. The waste heat system is the primary heating system and is used to its fullest capacity before supplementing with an electric hot water boiler as a secondary heating source. In winter, we use both systems during the coldest periods. In the spring and fall nearly all the heating required comes from the waste heat system,” Heidinger comments. “We’re unable to get detailed use information from the system.” The “water temperature at the plant is roughly 32-35oC.”

The future
“As we move to electricity as a heat source there will be a phase out of gas heat in favour of heat pumps, which are more efficient—heat pumps are the future,” observes Dr. Sanjeev Chandra, Department of Mechanical & Industrial Engineering, University of Toronto. This opinion is echoed in the International Energy Agency report The Future of Heat Pumps, “heat pumps, are the key technology to make heating more secure and sustainable.”

A heat pump is a “device that can increase the temperature of a waste-heat source to a temperature where the waste heat becomes useful. The waste heat can then replace purchased energy and reduce energy costs. … The goal is to design a system in which the benefits of using the heat-pumped waste heat exceed the cost of driving the heat pump.”⁶

When considering a heat pump growers should take into account “energy conservation” by “reducing infiltration, installing energy curtains, insulating sidewalls and the foundation perimeter, making good use of growing space and installing electronic controls.” This will establish the system’s parameters.⁷

Greenhouses, Dr. Chandra notes, “need to be able to convey the waste heat to the cold side of the heat pump to increase the temperature at which the heat pump takes in heat. The higher this temperature the less the electrical energy required to supply a given amount of heat. This requires the installation of a heat exchanger between the waste heat source and the heat pump. Ideally a heated fluid, either liquid or gas. This should be non-corrosive and at sufficiently high pressure to drive it through a heat exchanger.” 

Chandra cautions that “up-front installation costs can be high, depending on the heat pump’s size. Maintenance costs are relatively low.” The advantages of heat pumps in greenhouses are that they replace gas or oil heaters, minimizing CO₂ emissions.” Chandra notes, that “they use energy much more efficiently than combustion-based heaters, which reduces operating costs.”

“In the future, it is very likely there will be limits placed on CO₂ emissions from agricultural operations. Electrification of greenhouse heating will become necessary to meet these standards, which will be possible through the use of heat pumps,” Chandra says.  

Sources

1. R. Andrews & J. Pearce, Environmental and economic assessment of a greenhouse waste heat exchange, Journal of Cleaner Production 19, 1446-1454, 2011. http://dx.doi.org/10.1016/j.jclepro.2011.04.016
2. N. Jones, Waste heat: Innovators turn to an overlooked renewable resource, Yale Environment 360, (2018). https://e360.yale.edu/features/waste-heat-innovators-turn-to-an-overlooked-renewable-resource
3. A. Turns, The neglected clean heat we flush down the drains, 2024, BBC.com, https://www.bbc.com/future/article/20240103-sewage-a-low-cost-low-carbon-way-to-warm-homes
4. R. Pallardy, Reusing waste heat from data centers to make things grow, 2024, Information Week, https://www.informationweek.com/sustainability/reusing-waste-heat-from-data-centers-to-make-things-grow#close-modal
5. https://www.saskpower.com/Our-Power-Future/Our-Environmental-Commitment/Shand-Greenhouse ; https://www.saskpower.com/Our-Power-Future/Creating-A-Cleaner-Power-Future/Emissions
6. U.S. Dept. of Energy, Industrial heat pumps for steam and fuel savings, 2003, https://www.energy.gov/eere/amo/articles/industrial-heat-pumps-steam-and-fuel-savings#:~:text=A%20heat%20pump%20is%20a,is%20not%20achieved%20without%20cost.
7. J. Bartok, Jr., Center for Agriculture, Food, and the Environment, University of Massachusetts at Amherst, Geothermal heat for greenhouses, 2008, https://ag.umass.edu/greenhouse-floriculture/fact-sheets/geothermal-heat-for-greenhouses.