How do you keep a greenhouse warm with solar panels?

Keeping a greenhouse warm in colder climates can be challenging, especially during the winter months when temperatures can drop well below freezing. Using solar panels to generate heat for a greenhouse provides an environmentally friendly and cost-effective solution for maintaining warmer temperatures inside the structure.

What are the heating requirements for a greenhouse?

Greenhouses require supplemental heat during the winter months when outside temperatures fall below the optimal range for plant growth. The specific heating requirements depend on several factors:

• The size of the greenhouse – Larger structures will require more heat.
• The types of plants being grown – Some plants need warmer temperatures than others.
• The climate – Colder regions require more heating to maintain temperatures.
• The quality of greenhouse glazing – Double or triple pane glass or polycarbonate retains more heat.
• The time of year – More heat is needed in mid-winter than in the fall and spring.

As a general guideline, greenhouses require maintaining an inside temperature of at least 45-60°F during the winter, though the optimal temperature range depends on the specific plants. Tropical plants may require minimum temperatures of 55-65°F whereas cool weather crops like spinach and lettuce can tolerate temperatures around 45-55°F.

How can solar panels provide heat for a greenhouse?

Solar panels can be used to produce electricity to power electric heaters inside the greenhouse. But solar thermal panels, also known as solar hot water panels, provide a more efficient way of directly heating the greenhouse. Solar thermal panels work by circulating a heat-transfer fluid through panels where solar energy heats the fluid. The hot fluid is then pumped through pipes in a radiant floor heating system or a hydronic heating system with fans to circulate warmed air throughout the greenhouse.

Key components of a solar thermal heating system:

• Solar collectors – Flat plate or evacuated tube collectors absorb solar energy and transfer heat to the fluid inside the panels.
• Pipes – Insulated pipes circulate the heated fluid from the panels into the greenhouse.
• Heat exchanger – Transfers heat from the fluid to the greenhouse air or floor.
• Pumps – Circulate the heat transfer fluid between the solar collectors and heat exchanger.
• Controller – Controls the pumps and regulates the system.
• Storage tank – Stores excess solar heat for use at night or on cloudy days.

What factors should be considered when installing a solar thermal system?

Properly designing and installing a solar heating system for a greenhouse involves considering several key factors:

• Climate – Colder climates require more solar collector area to provide sufficient heat in winter.
• Greenhouse size – The number and size of solar collectors needs to match the heating demands of the specific greenhouse.
• Glazing materials – Using double or triple-pane glass or polycarbonate reduces heat loss.
• Insulation – Adding insulation to walls,foundation, and roof containing the heat.
• Supplemental heat sources – Most solar systems require a backup heating system for extended cold periods.
• Solar exposure – Solar collectors should be tilted to optimal angle and unshaded for maximum winter solar gain.
• Local building codes – Permits and inspections may be required for larger systems.

What are the main types of solar collectors used in greenhouse applications?

The two main types of solar collectors used for greenhouse heating are:

Flat Plate Solar Collectors

• Less expensive than evacuated tube collectors
• Absorb solar energy into copper plates with attached heat transfer tubing
• Lighter weight than tubes
• Good for moderate climate heating needs

Evacuated Tube Solar Collectors

• More efficient than flat plates in cold climates
• Copper heat pipes surrounded by glass vacuum tubes
• Minimal heat loss due to vacuum
• Recommended for cold winter climates

In milder climates, using flat plate collectors can reduce the system cost. But in regions with freezing winter temperatures, evacuated tube collectors provide the highest efficiencies as they suffer less heat loss on cold days.

How can the solar heating system be integrated into the greenhouse?

There are two main ways the solar heating system can be integrated into the greenhouse:

Active Radiant Floor Heating

• Tubes or electric cables embedded in the floor or gravel bed
• Heated fluid from solar loop circulates through floor system
• Gradual floor heat keeps plants warm from roots up

Forced Air Heating

• Air or hydronic handler with a fan and heat exchanger
• Heated air from solar loop distributed through greenhouse
• Ducts or perforated poly tubes to direct hot air

Radiant floor heating provides a very even distribution of heat without drying the air. Fan-forced heat can distribute heat quickly to meet sudden cold snaps or temperature drops. Most solar heating systems combine both radiant and forced-air elements for optimal greenhouse heating.

What factors determine the number and size of solar collector panels needed?

The solar collector system needs to be properly sized to match the heating demands of the specific greenhouse design and location. The key factors determining the number and size of panels include:

Factor	Impact on Solar Collector Sizing
Greenhouse square footage	More area requires more collector capacity
Climate and winter temperatures	Colder climates need greater collector capacity
Supplementary heating sources	Reduces solar system size
Glazing and insulation levels	Better materials reduce heat load
Solar exposure	Optimal exposure increases solar gain
Plant temperature needs	Warmer crops require more heating

A general rule of thumb is to have 1-2 square feet of solar collector area for each square foot of greenhouse floor space in moderate climates. In very cold climates, the ratio may be as high as 3:1. Working with an experienced solar heating contractor is recommended to properly size the system.

What are the best ways to store solar heat for nighttime use?

Solar energy can only be collected during daylight hours, so heat storage is important for maintaining greenhouse temperatures at night or during extended cloudy periods. There are two main options for storing heat:

Water Tanks

• Excess heat transferred to large tanks of water
• Water retains heat well and releases it gradually
• Allow 30-50 gallons of storage per square foot of collectors
• Can be integrated into hydronic heating systems

Rock Beds

• Gravel or rocks below the floor absorb excess heat
• Release heat slowly through floor heating system
• Less expensive than water tanks
• Require excavating deep bed (3-4 feet) below floor

Thermal mass storage like water or rocks accumulates excess heat during the day when the sun is shining and releases it gradually at night. This allows the greenhouse to stay warm even when the sun isn’t actively heating the collectors.

What are the costs associated with installing and running a solar heating system?

When considering adding solar heating to a greenhouse, it’s important to look at both the upfront installation costs and the ongoing operating costs. Typical costs include:

Installation Costs

• Collectors – $165-$350 per collector panel depending on type
• Mounting equipment – $200-$500 for racking, hardware
• Plumbing lines and insulation – $5-$10 per square foot
• Heat exchangers and pumps – $1200-$3000
• Excavation and rock bed – $6-$12 per square foot
• Labor and contracting – $1200-$6000 depending on system size

Total installed costs often range from $4000-$8000 for a small hobby greenhouse up to $15,000-$30,000 for larger commercial structures. Tax credits, rebates, and/or renewable energy incentives can offset 25-35% of the initial investment.

Operating Costs

• Little to no fuel costs
• Minimal electricity use for pumps and controls
• System maintenance and repairs as needed

While solar power eliminates fuel expenses, maintenance and repairs may be needed over time. Thermal collectors have lifespans of 15-25 years. Overall operating costs are very low compared to fossil fuel heating.

What are some key factors when selecting a solar heating installer?

Choosing the right solar heating contractor is key to getting a properly designed and installed system. Here are some tips for selecting an installer:

• Look for experience with greenhouse solar installations specifically
• Request photos/examples of past greenhouse projects
• Ask about licensing, insurance, and bonding
• Get at least 3 quotes to compare costs
• Look for customized panel sizing for your specific needs
• Choose a local contractor for accessibility and service
• Review solar incentives and rebates they can assist with
• Evaluate options for integrating solar with existing systems
• Inquire about warranties and expected lifespan of equipment

Taking the time to carefully evaluate potential installers will help ensure your greenhouse solar heating project is set up for optimal performance and cost-effectiveness.

Conclusion

Heating a greenhouse with solar thermal energy is an efficient, cost-effective way to maintain suitable growing temperatures during colder weather. Flat plate or evacuated tube collectors can supply supplemental radiant floor and forced air heat controlled by an integrated system. Sizing the solar capacity to match the greenhouse specifics, climate, and plant needs is essential, with storage to retain heat overnight. While solar power requires an initial investment, it will pay off in free renewable heat for potentially decades of growing seasons. Working with an experienced solar heating contractor will help ensure your system keeps your greenhouse warm throughout the winter.