Greenhouses allow gardeners to grow plants year-round by creating a warmer environment protected from the outside weather. However, heating a greenhouse can be expensive, especially in colder climates. Using solar energy to heat a greenhouse is an eco-friendly and cost-effective alternative to traditional heating methods.
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How does solar heating work for a greenhouse?
There are two main ways solar energy can be used to heat a greenhouse:
- Passive solar heating – This relies on collecting heat from the sun’s rays through the greenhouse glazing (glass or plastic sheeting). The greenhouse itself acts as a solar collector.
- Active solar heating – This uses solar collectors to absorb heat from the sun’s rays and transfer it to the greenhouse space via fans or pumps.
In both methods, the greenhouse acts as a thermal mass, absorbing heat from the sun during the day and releasing it slowly at night. Glazing materials like glass or polycarbonate panels allow sunlight to enter the greenhouse but reduce heat loss.
Passive solar greenhouse heating
Passive solar heating takes advantage of basic greenhouse design. The sun’s rays pass through the glazing and hit objects inside the greenhouse, heating them up. These objects then radiate heat and warm the air. For passive solar:
- Locate greenhouse to maximize winter sun exposure – facing south (northern hemisphere) or north (southern hemisphere)
- Use antireflective glazing to maximize sunlight capture
- Add thermal mass (e.g. concrete slab floor, water containers) to absorb and radiate heat
- Insulate well to reduce heat loss
- Seal carefully to prevent air leaks
- Ventilate to regulate temperatures – open vents allow excess heat to escape
Passive solar greenhouses require proper siting and glazing to optimize solar gain. The right amount of thermal mass regulates temperatures – retaining just enough heat while preventing overheating.
Active solar heating systems
Active solar heating uses solar collectors designed to capture heat from the sun’s rays. Common types are:
- Flat plate collectors – thin rectangular boxes with dark absorptions plates and tubes or passages underneath. Enclosed in a glass or plastic cover.
- Evacuated tube collectors – rows of parallel glass tubes, each containing a glass outer tube and inner black absorption tube connected to a header.
The collectors are mounted facing the sun, either on the greenhouse or nearby. A pump or fan circulates a heat-transfer fluid (usually water or diluted antifreeze) through the collectors to absorb solar heat. This heated liquid is then piped to a heat exchanger inside the greenhouse, where fans blow the warmth into the growing space.
Active systems provide more direct and controlled heating, but require pumps, controllers and piping. They are more complex than passive solar. Active systems can also overheat in summer – requiring shading or a heat dump.
What factors affect greenhouse solar heating efficiency?
Several factors impact how efficiently solar energy can heat a greenhouse. These include:
Climate and weather
- Amount of direct sunlight and solar radiation
- Ambient air temperature
- Wind speed
- Humidity
- Cloud cover
More sun exposure equals more solar gain. Colder and windier conditions increase heating demands. Cloudy weather reduces solar heating potential.
Greenhouse design and glazing materials
- Glazing type – glass, plastic sheet, polycarbonate, acrylic panels
- Glazing layers – double or triple layer improves insulation
- Vents/doors – minimize air leaks
- Insulation – in walls and roof
- Thermal mass
Glazing materials that transmit light while trapping heat work best. Proper insulation reduces nighttime heat loss. Thermal mass balances day/night temperatures.
Solar collector efficiency
- Collector type
- Collector tilt angle
- Piping insulation
- Heat exchanger performance
Solar collectors optimized for winter sun anglesmaximize heat capture. Proper tilt toward sun and insulated pipes reduce heat losses.
Control system
- Automatic operation of pumps, vents, etc. based on conditions
- Prevents overheating/freezing
- Allows differential heating day vs. night
Automated controls regulate solar heating efficiency by operating equipment based on temperature sensors.
What are the costs of solar greenhouse heating?
Solar greenhouse heating involves the following costs:
Passive solar heating costs
- Constructing an optimally oriented and insulated greenhouse structure
- Glazing materials – glass, polycarbonate, plastic sheeting
- Thermal mass materials – concrete, water containers
- Ventilation equipment – vents, exhaust fans
- Insulation materials
For new greenhouse construction, orienting the structure properly is free. Glazing, insulation and thermal mass make up most passive solar expenses.
Active solar heating system costs
- Solar collectors
- Mounting equipment
- Pumps, piping, insulation
- Heat exchanger
- Control equipment
- Labor costs for installation
Active solar system costs vary based on type and size, but $3-$5 per square foot for flat plate and $6-$10 per tube for evacuated tube collectors is typical. Controls, pumps and other equipment add several thousand dollars in costs. Professional installation is recommended.
Sample solar greenhouse heating system costs
| System | Size | Cost |
|---|---|---|
| Passive solar greenhouse | 24′ x 36′ (864 sqft) | $5,000 – $15,000 |
| Flat plate collector system | 648 sqft collector | $25,000 – $30,000 |
| Evacuated tube system | 60 tubes | $10,000 – $15,000 |
Passive solar greenhouses are the lowest cost option, with active systems costing anywhere from two to ten times more depending on size.
What are the benefits of solar greenhouse heating?
Solar heated greenhouses provide numerous benefits:
Low operating costs
Solar heating reduces fossil fuel use for heating by 25-100%, cutting heating bills significantly.
Grow year-round
Adequate solar heat allows extending the growing season and growing summer crops year-round in some climates.
Protects plants
Prevents cold damage to plants on freezing nights.
Earlier planting/transplanting
Warm greenhouse allows starting seedlings sooner for transplanting outside.
Increase yields
Warmer greenhouse temperature can increase yields for heat-loving crops like tomatoes, peppers and cucumbers.
Environmentally sustainable
Solar heating reduces greenhouse gas emissions from fossil fuel use.
What are the limitations of solar greenhouse heating?
Some limitations and drawbacks of solar heating include:
Upfront costs
Solar heating systems have significant upfront costs, particularly for active systems. Payback periods may take several years.
Climate variability
Solar heat capture depends on sunny weather. Overcast periods limit solar gain.
Nighttime temperature maintenance
Passive solar greenhouses especially can get quite cold at night as stored solar heat dissipates. Back-up heating may be needed.
Space requirements
Solar collectors take up additional space outside the greenhouse. Active solar systems require mechanical rooms.
Complexity
Active solar systems are more complex. They require maintenance of pumps, controls and piping.
Summer overheating
Solar gain continues in summer. Preventing overheating requires ventilation, shading and/or diverting excess heat.
Conclusion
Solar energy can be a very effective method for heating greenhouses. Passive solar designs optimize greenhouse orientation, glazing and insulation to maximize solar gain at lower cost. Active solar systems with collectors, pumps and controls provide additional heating capacity for larger greenhouse complexes. With proper design and controls, solar greenhouse heating can significantly expand the growing season and yield for gardeners in many climates.