Firstly, we can reduce the carbon energy input. There’s a whole bunch of “alternative energy” options: oil, propane, coal, wood pellets, wood chips, cord wood, beetle infected wood and dried corn kernels. Unfortunately these are all carbon sources, so we’re not really reducing our carbon input. Other possible fuel sources, while also (ultimately) carbon, make use of otherwise “waste” products (although pine beetle infected furniture is selling for a good price as designer “Denim Pine”). These include used vegetable oil (a great alternative to power diesel engines), methane from landfill (removing this aggressive “greenhouse gas” from the equation), on-farm bio-diesel production, bio-digestion (a number of examples now exist across the country), and even getting heat from the composting process using long lengths of flexible pipe winding its way through the compost pile.
Using waste heat from another industry is a great way to utilize (probably carbon-generated) waste, but of course depends on a suitable location and a partnership (e.g. the new SunSelect project). “More and more greenhouses that use residual heat and CO2 from a nearby power plant have developed lately, such as the 7.6-hectare Red Tomatoes Productions nursery of Austrian grower Zeiler, located near Vienna International Airport.”1
So, what else is there? Geothermal has been much in the news lately (e.g. see Greenhouse Canada, July 2015). But like waste factory heat, it also depends on choosing a location that specifically works for this energy source.
Perhaps passive solar greenhouses are an option? Of course, traditional glasshouses and poly-houses are “passive” in that they rely on heat from the sun to heat them during the daytime. But we’re talking here about many of the greenhouses in rural China that store that heat for use once the sun goes down. They also (typically) have a brick or stone north-facing wall to absorb the daytime energy, and often also have large amounts of water (in plastic bottles or tanks under the floor of the greenhouse) to also store that captured energy. Such passive houses rely on non-carbon energy (solar), and so can be seen as “green.” But they may not be viable for our familiar large-scale commercial greenhouse businesses of today.
Solar panels on the roof of a passive solar house can also provide both warm water and electricity, so this may be a useful option.
Wind may be another option. While most likely used to generate electricity (which of course may be used to heat underground cables, for example) it’s less likely to be suitable purely for heating greenhouses. And again, selection of a windy enough location is critical.
Perhaps a simple option is to use multiple greenhouse layers, as per Eliot Coleman’s technique at Four Season Farm in Maine. Using low poly-tunnels to cover crops within a larger poly-house, Coleman says each layer of plastic cover equates to “moving” the crop 500 miles south in terms of warmth units. Yeah, OK…but it’s not going to provide sweet cherry tomatoes in quantity in Ontario in February.
Perhaps we shouldn’t look for a silver bullet, a one-size-fits-all solution. But maybe we could consider a mix of all options (which is what passive solar greenhouse designers are doing). After all, this superficial review has revealed around 20 options.
Or, perhaps, the future is not to use a glasshouse at all. Perhaps the ubiquitous news items about “vertical” or “container” systems are not a coincidence? Having an insulated shell obviously reduces heat loss, so that’s not a bad thing.
Or, maybe, there is no one solution. Perhaps there is actually room for us all, if we are each conscious of our (carbon) energy use. Which is, after all, where we started this conversation.
- www.HortiDaily.com, March 16, 2016.