Reducing Greenhouse Energy Costs – Independent, Off the Grid

The economic viability of a conventional commercial greenhouse operation is, in northern latitudes, questionable, at best. Conservative estimates suggest that the cost of producing vegetables in a greenhouse in Canada is roughly four times that of field-grown produce in the southern USA. Even allowing for transportation costs from the USA, the pricing differential makes competing directly with field produce in winter months unviable.

Some greenhouse operators choose to compete indirectly, positioning their produce as fresher, organic or higher in flavour. This seldom justifies the higher cost.

In order to be competitive on price, substantial innovations are required in greenhouse operation and greenhouse construction.

While one may think that the initial capital cost is a one-time expense, this is far from true. Aside from ongoing costs of financing capital expenses, the design of the structure and equipment will also impact significantly on the operating expense, year upon year.

This is why exploring innovations in design and finding the best alternatives, capitalized over fifteen to thirty years, will allow northern latitude greenhouses to operate and price their products competitively.

In addition to design, cost efficiencies in transportation, market development and reach, and ongoing supplier, labour and client relations advantages over imported goods must be aggressively explored and developed.

The purpose of this article, however, is to explore how to design a facility to minimize energy costs, since many estimates place the ratio of energy outlay to total expense to be as much as 30% of a traditional greenhouse’s operating cost (Proquest, 2020).

The Province of Manitoba (citation) calculates the annual cost in 2021 of operating a 492 sq. m greenhouse (approx. 5,000 sq. ft.) for various conventional heating sources. The breakdown is as follows: a) Electricity: $17,355, b) LNG: $22,126 and c) Natural gas: $10,200.

To reduce the per unit cost of energy, owners must reduce the outlay of expense or increase the number of units of production. A more intense growing regimen may dramatically reduce the percent of energy cost per sale, but it does not reduce the absolute cost.

Alternative systems designs and/or alternative fuels and energy patterns will reduce the gross cost and, in turn, the per-unit expense.

Energy costs consist of lighting (grow lights), heat, storage, ventilation, transportation, and water heating. Of these, greenhouse heat is the most significant cost. However, water heating is a consideration, since many tender plants react badly to extremely cold water. Lighting options may contribute 10-18% to overall energy costs in a conventional system, while ventilation in winter months is more costly than in summer. Transportation costs of equipment and even product can be fairly easily mitigated.

First, let us deal with lighting. Modern LED systems actually can not only reduce costs greatly over older fluorescent or incandescent and heat-generating lighting, but can produce a better quality of artificial light. As well, capital costs and ongoing replacement costs are much lower than in older systems.

Storage costs consist of product storage and bedding plant nursery areas. This cost can be managed in conjunction with overall greenhouse operations. By strategically sizing and locating these venues, costs also can be managed effectively. Storage costs form only a modest amount of the overall budget, though.

Ventilation systems that include passive systems operating in conjunction with the heating process offer more efficiencies than simple barn fans. Using solar power (not necessarily photovoltaic) to move air in larger buildings is inexpensive and makes use of latent heat/cold exchange for air flow.

Water heating costs are best managed by using storage for water, rather than direct from well. Other options include using residual water from co-located livestock operations, or heat recovery from nearby biodigesters.

Transportation costs focus on fuel like diesel or gasoline. Where greenhouse facilities are co-located with a livestock operation, production of both biogas (methane) for heat and biodiesel from waste oilseed (heated, sprouted, etc.) may offset increased fuel expenses. Biogas is readily produced from manure, but also from decaying green vegetation, such as plant stalks, offering a closed-loop heating option for greenhouses.

The last component of energy use in a greenhouse is the cost of heating in winter. Here is where the greatest savings may be achieved, making a greenhouse operation much more viable in Canada.

Author and grower John Bartok says, “The typical greenhouse uses between 1 and 2 kilowatt hours of electricity per square foot of floor area per year.” Therefore, a 5,000 sq. ft. conventional greenhouse would use 5,000 to 10,000 kwh per year.

The four ways heat is lost in a building are conduction, convection, infiltration, and radiation. Infiltration through cracks, openings and ventilation occurs in every building. In greenhouses, most heat is lost through conduction, where heat transmits through the thin, uninsulated glass or plastic exterior.

Most greenhouses have four walls and a roof that allow for sunlight to infiltrate (and some heat), but that allow generated heat to escape to the outside.

There are a few ways to minimize this loss. First, use insulated tarpaulins o cover the building during extreme cold periods and nighttime. These can be rolled up to allow sunlight in during warm winter days.

Second, using a bermed north wall made of rammed earth or bales of straw forms a great insulating barrier that also acts as a heat sink to gather daytime summer heat, when the wall is coated dark. In late autumn, early spring and winter, additional bales can be stacked on the east and west side to protect against heat loss.

Doubling the plastic clear walls with an air gap between layers creates a mild insulated pocket to reduce energy loss.

Next is the consideration of heat sources. Mixed use farms generally have access to straw bales which can be used either in biomass burners or in gasification units. Manure can be converted to biogas, with remnants burned as fuel, along with wood. These options are very cost effective.

Other options for heating include geothermal systems (horizontal or vertical loop or well-to-well). And photovoltaic panels.

A PV system will generate from 10 to 35 kWh/square feet per year. If you operate 10,000 square feet of greenhouse space that uses 1 kWh/square foot per year, and have a collector system that provides 25 kWh/sq ft-yr you would need 27 3-feet by 5-feet solar panels to supply your electricity needs. (https://www.greenhousemag.com/article/photovoltaic-solar-electricity-for-greenhouses/).

Utilizing innovative energy generation systems and designing a co-located greenhouse offer the best options for developing a competitive greenhouse by producing vegetables with lower operating costs. A further measure is to develop markets that further reduce costs. This will be discussed in an upcoming article.

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