Integrating solar panels with a greenhouse can make it off-grid, but takes careful consideration of your goals and the best strategy for doing so.
It is one of the top requests we get: “I want a greenhouse with solar panels.” This conversation usually begins with definitions. The word solar (related to the sun) applies to many systems: Solar photovoltaic (PV) panels generate renewable electricity; solar hot water systems heat water, and passive solar heating provides space heating for buildings. While solar panels are a good option to power a building, a much wiser use of the sun’s energy for heating is through passive solar design: designing a building to maximize solar gain and reduce heat loss. This is because electric space heaters are extremely energy-intensive devices. Converting the sun’s energy to electricity and then converting electricity into hot air is not efficient. A much more effective use of the sun’s energy to use the hot air already present in the greenhouse during the day, taking advantage of the natural greenhouse effect. Ceres’ greenhouses do this by storing heat in the soil underground through a Ground to Air Heat Transfer system. A re-cap:
- Passive solar design refers to building principles that maximize solar gain and minimize heat loss. Since the 1970’s, the term ‘solar greenhouse’ has normally been used as shorthand for a greenhouse designed with passive solar design.
- Solar panels produce electricity to power electric equipment in the greenhouse like fans, pumps or lights, and ‘solar-powered’ conventionally refers to solar PV systems.
What happens when you combine these two – adding solar panels to a passive solar greenhouse? The short answer is you get the most sustainable, energy-efficient greenhouse possible. Integrating solar panels allows an energy-efficient greenhouse to be net-zero energy, or possibly off-grid. You can create an abundant source of food that is entirely self-powered and self-heated.
While that is an enticing option, adding solar panels to a greenhouse requires careful consideration of your goals and the costs. There are often better strategies using solar panels than directly on the greenhouse. Here are the first five things to consider before you invest:
1. Know Your Goals
Growers have different reasons for wanting to integrate solar panels into their greenhouse. They include :
- Reducing carbon emissions
- Providing a reliable source of power (if your electric grid experiences frequent outages)
- Being independent of the electric grid, and creating an off-grid greenhouse. This can stem from a moral drive to be more self-reliant, or a practical drive to avoid the costs of connecting to the grid if there is no grid-tie available at your site.
There is no wrong reason to invest in solar power, but it is crucial to understand your motivations for doing so, as this will greatly influence the best system type, whether grid-tied, or an off-grid with battery back-up system. For more on the the pros / cons of different system types see our book, The Year-Round Solar Greenhouse, and stay tuned for future blogs.
2. Evaluate the Best Site: Your Greenhouse or Your Home?
When a residential greenhouse grower comes to us and tells us they want a greenhouse with solar panels, the first question we ask is: “Do you have solar on your house?” Surprisingly, many people say no, they haven’t considered solar for their home even though the home is a much larger energy-user. If your goal is to reduce carbon emissions, it doesn’t matter if the panels are on your home or the greenhouse. Furthermore, a home-integrated system has several advantages:
- There is usually more room on the home’s roof, enabling you to install a larger system that powers both the home and greenhouse. This is usually more cost-effective as well, allowing you to get more power generation out of your investment. The greenhouse will be wired to your home and will then inherently be solar-powered as well.
- Putting solar panels on the greenhouse roof can block light needed for growing or heating. If a greenhouse is large enough, like the 7,000 sq. ft. greenhouse at The Golden Hoof Farm pictured below, there can be enough roof space to accommodate solar panels. Smaller residential greenhouses will want to consider utilizing their home’s roof or a ground-mounted system.
- If you want to finance the solar panels (instead of purchasing outright), generally a conventional home system is easier to finance with loans or leasing programs.
3. Evaluate the Power Demand
This is one of the make or breaks questions for investing in a greenhouse PV system: what is the total electric demand (also called the electric load) of all equipment in the greenhouse when running? This varies hugely by greenhouse depending on how you plan to grow. For example, a small residential greenhouse may have only one exhaust fan. This small electric load can be powered by a small solar PV system, possibly even one that runs on direct current. That avoids the cost of an inverter (which converts DC to AC power) and can keep the total cost of the system quite low. On the other end of the spectrum, a large aquaponics greenhouse will have aerators, water heaters, water pumps, and possibly space heaters or cooling equipment. Multiple systems create a sizable electric load and necessitate a much larger solar PV system that is drastically more expensive. You can find the power demand of your greenhouse by adding up the wattage of all electric components you plan on using. For example, the power usage of a fan should be provided in Watts under the product specifications. Once you know the total electric load, you can talk to a solar PV installer, or begin to size your PV system and get an idea of costs. We provide a step-by-step guide for sizing a greenhouse PV system in our book, The Year-Round Solar Greenhouse. Online calculators are also available, like PV Watts provided by NREL.
NOTE: The total electric load only tells you the demand. There are subsequent steps to determine the system size required to supply that demand. This is influenced by factors like the amount of sunshine at your location and the system orientation. Sizing a PV system requires calculators to get a general estimate, or talking to a professional to get an accurate one.
4. Invest in Efficiency First
A key strategy whenever you invest in solar power is to first make your greenhouse is as efficient as possible so that you do not have to rely on electrical systems to provide the heating and cooling. If you reduce the electric demand, you reduce the size and thus the cost of your PV system. Energy efficiency measures, like using sufficient insulation and designing the greenhouse with passive solar design, are always a better investment than relying on a large PV system to operate your greenhouse.Particularly, you want to do everything possible to avoid relying on electric heaters. Electric space heaters are extremely power intensive pieces of equipment and can make the total electric load of the greenhouse (and thus system cost) skyrocket. Thus, we always suggest first investing in a thermal storage solution, like a GAHT system or Phase Change Material, which stores the heat of the greenhouse in the soil underground or a material built into the wall. Use the free heat provided by passive solar greenhouse design as much as possible before relying on solar panels!
5. Evaluate Battery Back-Up and Costs
Many people are interested in building an off-grid greenhouse, one that can grow year-round without reliance on the power grid. It is an admirable goal, but also one that can drives up costs dramatically. Consider that the new battery from Tesla – the 7 kWh Tesla Powerwall – costs an estimated $7,000 with installation (for the battery alone, not including the PV system). While cheaper battery systems are possible, batteries are always a huge increase in the cost of for a solar-powered greenhouse. To evaluate whether that investment is worth it, go back to the reasons why you want to invest in solar in the first place. If one of your major goals is to be independent of the power grid for moral reasons, or if there is absolutely no access to the grid at your site, then an off-grid greenhouse is a good choice. Similarly, if you have a very small system (e.g. powering only one or two fans), an off-grid system may be simple and low cost. On the other hand, if you have a normal size solar panel system and can connect to the grid, a grid-tied system is more cost-effective. We provide a more thorough comparison of the different strategies for integrating solar panels in our book, The Year-Round Solar Greenhouse. Is a solar-powered greenhouse right for you? We’d love to help you find out. Email us at firstname.lastname@example.org