Can you heat a greenhouse with solar power?

Heating a greenhouse can be challenging, especially during colder months when plants need consistent warmth to thrive. Many greenhouse owners use fossil fuel-based heating systems like natural gas or propane to provide heat. However, these systems can be expensive to operate and are not environmentally friendly. An alternative option is using solar power to heat your greenhouse. Solar energy is a clean, renewable resource that can be harnessed in several ways to provide greenhouse heating.

How do greenhouses lose heat?

Greenhouses lose heat through conduction, convection, radiation, and infiltration.

Conduction is the transfer of heat through solid materials, like the greenhouse glazing and frame. The greenhouse structure conducts heat to the outdoors. Using insulation and materials like polycarbonate glazing panels can help reduce conductive heat loss.

Convection is the movement of heat through liquids and gases, like the air inside the greenhouse. As the warm air rises and escapes out open vents or cracks, cooler air enters in a continuous cycle. Proper ventilation is important, but too much increases convective heat loss.

Radiation is the emission of infrared radiation from warm objects. The greenhouse components like the plants, soil, surfaces, and glazing radiate heat that can be lost, especially at night when there is a bigger temperature difference between the interior and exterior. Installing a thermal screen can help reduce radiative heat loss.

Infiltration is the uncontrolled air leakage into and out of the greenhouse. Gaps or cracks allow warm interior air to escape and cold exterior air to enter. Caulking and weatherstripping seals can minimize infiltration.

Ways to heat a greenhouse with solar power

There are several solar technologies that can be used to heat a greenhouse:

Passive solar heating

Passive solar heating takes advantage of the greenhouse’s glazing to naturally collect and store solar radiation during the day. The greenhouse should be oriented properly to maximize winter sun exposure. Materials like stone, concrete, or water barrels can absorb and slowly release heat. Insulation helps retain the heat inside the greenhouse overnight. While passive solar isn’t enough to fully heat a greenhouse in cold climates, it can reduce the extent of additional heating needed.

Solar air collectors

Solar air collectors, often wall-mounted panels, contain a dark absorbent surface that heats up when exposed to sunlight. Fans push air through the collector, which warms the air and releases it into the greenhouse through vents and conduits. This direct air heating system needs full sun exposure to function well. Solar air collectors work best in milder climates as a primary or supplementary heating source.

Solar hydronic heating

Solar hydronic systems use solar thermal collectors to heat a liquid like water or glycol. This heated liquid circulates through pipes or hoses to a heat exchanger inside the greenhouse, which transfers warmth to the interior air. Solar hydronic heating can utilize large collectors mounted outside the greenhouse or smaller rooftop collectors. The stored solar heat can supplement a boiler and distribution system. Solar hydronic heating provides consistent greenhouse warmth but requires more extensive installation.

Solar electric heating

Photovoltaic (PV) solar panels convert sunlight into electricity which powers electric heaters and fans to warm the greenhouse. PV systems provide flexibility since excess electricity can be sent to the grid. Electric heat sources like radiant heaters, convection heaters, and heat pumps can be tailored to the specific greenhouse heating needs. While solar electric systems have a higher upfront cost, the fuel-free operation can pay off long-term.

What factors determine what solar heating system to use?

There are several factors to consider when selecting the optimal solar heating system:

Climate conditions

The local climate impacts how much heat is needed to maintain ideal greenhouse temperatures. Passive solar, solar air collectors, or small hydronic systems may sufficiently heat greenhouses in milder climates, while colder regions may require extensive solar hydronic or electric systems with thermal storage to provide enough heating.

Greenhouse size and use

The scale and functionality of the greenhouse affect heating needs. Large commercial greenhouses, especially those used year-round, need more heating than a small backyard greenhouse used to extend the growing season. The crops grown also influence temperature requirements.

Sun exposure

Most solar heating options require direct, unshaded sun exposure at least 6 hours daily. Obstructions like trees and buildings can limit solar collection. A south-facing site with limited landscape shading optimizes solar gain.

Budget

While long term operating costs are lower, solar heating systems can have high upfront costs. Passive solar and solar air collectors are generally the lowest cost options. An investment in a more extensive system may pay off over decades of fuel-free operation.

DIY vs. professional installation

Passive solar techniques and solar air collectors are DIY-friendly. Solar hydronic and electric systems are complex and usually require professional design and installation.

What are the steps to install solar heating in a greenhouse?

The key steps to installing solar heating in a greenhouse are:

Evaluate heating needs

Determine the size of your greenhouse, typical temperature setpoints through the year, and heating needs during coldest conditions. A heat loss calculation helps estimate the heating system size. Factor in the climate and whether the greenhouse will be used in winter.

Select a solar heating system

Choose the type of solar heating that best matches heating needs and budget. Passive solar or solar air collectors may sufficiently heat smaller greenhouses. Larger operations usually need active solar hydronic or electric systems, potentially coupled with a boiler backup.

Design and size the system

Properly size the system components like collector area, heat exchanger, pumps, piping, ventilation, controls, and storage tank (if needed). Consider existing systems, fuel sources, and integration needs. Model the performance to optimize efficiency and meet heating demands.

Obtain permits and incentives

Check on building codes and permit needs. Take advantage of solar tax credits and rebates to offset costs. The payback period can be under ten years with incentives.

Install components

Solar collectors need unshaded south orientation, either roof mount or ground mount. Install insulation, seal the greenhouse, and integrate ventilation components. Mount electric panels in a sunny location if installing PV. Run piping and conduit as needed.

Complete the system setup

Connect components, charge hydronic fluid, test electric generating and heating equipment, program controls and automation. Do a final check that all equipment is properly functioning.

Perform regular maintenance

Inspect and service components annually. Check piping insulation, seals, glazing, and collectors. Repair leaks, clean panels, and replace worn parts. Tune up the heating system for optimal efficiency.

What are the costs of solar greenhouse heating?

The upfront and operating costs for solar heating include:

Passive solar heating

– Glazing upgrades: $3-$8 per sq ft
– Insulation: $1-$3 per sq ft
– Thermal mass materials: $5-$20 per sq ft
– Total upfront cost: $1000-$8000
– Operating cost: $0 fuel cost

Solar air collector system

– Collector panels: $200-$500 each
– Fans, vents, ductwork: $500-$2000
– Total upfront cost: $2000-$10,000
– Operating cost: $0 fuel cost

Solar hydronic heating system

– Collectors: $2000-$4000
– Heat exchanger, pumps: $500-$1500
– Piping, valves, insulation: $1000-$4000
– Controls, sensors: $500-$1500
– Storage tank: $1000-$2000 each
– Total upfront cost: $10,000-$25,000
– Operating cost: $0 fuel cost

Solar electric heating system

– PV panels: $1-$3 per watt
– Mounts, inverter, wiring: $0.50-$1 per watt
– Electric heat sources: $2000-$6000
– Total upfront cost: $8000-$30,000
– Operating cost: $0 fuel cost

Incentives and solar tax credits can offset 25%-40% of the system cost.

What are the best solar heating options for small backyard greenhouses?

For small hobby greenhouses under 300 sq ft, the best solar heating options are:

Passive solar

Optimizing glazing, insulation, tightness, and thermal mass for solar gain.

Solar air heating collector

Wall mounted systems are easy to install and good for mild climates.

Small solar hydronic system

A simple system with 1-2 collectors and gas water boiler backup.

Small solar electric heater

A grid-tied PV panel system powering a space heater.

Combination system

A hybrid approach such as thermal collector and PV panel(s).

Simple DIY-friendly options like passive solar collectors work well for small hobby greenhouses. Solar electric and hydronic systems are great long-term investments if you can afford the upfront cost.

What are the best solar heating options for large commercial greenhouses?

For large greenhouses over 1000 square feet, especially in colder climates, the best solar heating options are:

Extensive solar hydronic

A large system with multiple collectors, significant heat storage, and backup boilers.

Large solar electric

A robust grid-tied PV array powering multiple electric heaters and heat pumps.

Combination system

Coupled solar hydronic collectors and a large PV array for extra direct electric heat.

Geothermal heat pump

Ground source heat powered by on-site solar PV.

Commercial greenhouses have substantial heating loads requiring robust solar systems, thermal storage, and integration with existing heating sources. Solar hydronic and electric systems can be sized to meet all or most heating needs with minimal supplemental heat.

What are the best practices in operating and maintaining a solar greenhouse heating system?

To keep a solar greenhouse heating system performing optimally:

Inspect collectors

Check collector glazing for broken glass or accumulated dirt. Clean panels regularly to maintain solar absorption.

Prevent leaks

Check piping, valves, and heat exchanger for leaks. Replace worn seals. Insulate all hydronic piping.

Maintain thermal screens

Repair any holes or gaps in night insulation curtains. Ensure tight closure and raise/retract mechanisms are working.

Check temperature settings

Recalibrate thermostats and sensors annually. Monitor conditions to ensure ideal temperatures.

Clear snow

Keep collectors and PV panels clear of snow for winter solar gain.

Tune heating distribution

Balance hydronic systems and calibrate heaters and ventilation for even heating and efficiency.

Prevent shading

Prune any new growth that may shade solar collectors. Alter greenhouse orientation if needed.

Check controls

Inspect controllers and automation systems for correct operation. Update software and settings as needed.

What are the main advantages of using solar power for greenhouse heating?

The major benefits of solar greenhouse heating include:

Lower operating costs

Solar heating eliminates perpetual fuel payments and provides decades of free heating after initial equipment investment.

Energy independence

On-site solar generation means greenhouse heating is separate from utility prices and fuel delivery schedules.

Sustainability

Solar heating reduces greenhouse emissions for an environmentally-friendly operation.

Federal tax credits

The 26% federal ITC incentive significantly offsets the cost of solar heating equipment and installation.

Improved plant growth

Consistent warmth from solar energy can enhance the health and yield of heat-loving plants.

Compatibility with ventilation

Solar heating works well with greenhouse ventilation needs, unlike some fossil fuel heaters that deplete interior air.

Low maintenance

Solar heating systems have minimal components and maintenance requirements.

Increased property value

Installing solar collectors or a PV system can increase the resale value of the property.

Conclusion

Solar greenhouse heating provides an economical, efficient, and sustainable method to maintain ideal temperatures. Passive solar techniques can preheat smaller hobby greenhouses at very low cost. Larger operations are best served by active solar hydronic or electric systems properly sized for the heating demand. With wise investment and regular maintenance, a solar heating system can provide free supplemental or total heat for decades. Capturing the sun’s radiation for greenhouse warming both cuts costs and emissions. The advanced solar collectors and materials available today make greenhouse solar heating an achievable strategy even in cool climates. With the efficiency, incentives, and environmental benefits, solar power is put to great use heating productive greenhouse spaces.