USE OF HEAT PUMPS FOR HEATING AND COOLING AGRICULTURAL GREENHOUSES

Heat pump, the new surprisingly old way to heat a greenhouse

With the increasing trend of environmentally friendly efforts, heat pumps have successfully replaced old oil-fired boilers. The air energy heat pumps are now used in various fields as a part of energy conservation and emission reduction policies.

Now, not only thousands of households but agricultural specialists also do heat pump greenhouse heating to get effective results. So, what is a heat pump, and how can you use it to enhance your vegetation experience? Let’s find out.

What Is a Heat Pump?

A heat pump is a device that helps greenhouse growers transform low-temperature energy into higher-temperature energy, just like your refrigerator. This technology has been around for more than a century now.

It is believed that ground-source heating systems save up to four times more energy than oil or gas boilers. This is because these devices move naturally-available energy rather than producing emissions that lead to global warming.

Greenhouse heating with low enthalpy geothermal heat

Types of Heat Pumps

There are four types of pumps for environmentally-friendly greenhouse heating:

• Ground Source Heat Pumps. These draw the heat energy from the Earth to your greenhouse.
• Air Source Heat Pumps. These are the ideal pumps for greenhouse growers, providing a versatile system that efficiently heats and cools the greenhouse.
• Water Source Heat Pumps. These pumps draw heat from the water and transform it into heat for your greenhouse.
• Heating & Cooling Systems. These all-in-one pumping systems serve as an air-conditioner, a boiler, and a radiator to offer underfloor heating and fan coil services.

Is an Air Source Heat Pump the Ideal Choice for a Greenhouse?

An air-source heat pump absorbs heat from the outside environment and releases it into the greenhouse’s interior. These devices are of two types depending on their functions:

• Air-to-Air Systems. They deliver warm air that heats the entire greenhouse.
• Air-to-Water Systems. They heat water that provides both heating and hot water for the greenhouse through radiators or an underfloor heating system.

How Does Air Source Heat pump Greenhouse Heating Work?

The heat pump has three parts that ensure adequate heating to your greenhouse, including:

• Evaporator Coil. It draws heat from the outside.
• Compressor. It then pumps a refrigerant type with the heat pump and compresses the gaseous form to the ideal temperature for heat distribution.
• Heat Exchanger. This part then transfers the heat energy obtained from the refrigerant to air or water.

The air-to-air and air-to-water systems then transfer this heat to the greenhouse for effective and energy-sufficient vegetation.

PV-powered geothermal heat pump system for greenhouses

The PV-powered geothermal heat pump system was tested for a year, with measurements taken by Researchers at the University of Nottingham. The UK group showed it was able to maintain an indoor temperature over 16 oC in all seasons, except for winter.

The system was tested in a greenhouse located in Loughborough, Leicestershire, England.

Researchers at the University of Nottingham have investigated the performance of a shallow geothermal heat pump coupled to a PV system for the heating and cooling of a greenhouse.

Described in the paper Experimental investigation of a ground-source heat pump system for greenhouse heating–cooling, published in the International Journal of Low Carbon Technologies, the proposed system comprises a PV system, a 200 l water tank, a 5 kW commercial heat pump, a seasonal soil-based thermal energy storage system (SSBTESS), and a fan coil unit (FCU).

According to the scientists, the SSBTESS unit, which has 24 single vertical type heat exchangers (VHXs) and is placed at a depth of 0.2 m, is the critical component of the entire system. The heat pump serves as a buffer between the collection loop, the SSBTESS itself, and the fan coil subsystem. Thick insulations were also deployed to reduce soil heat loss and the radiant floor was covered by concrete, which acts as an underground heat extractor or injector unit in the greenhouse.

During high-temperature periods, the system absorbs heat from the greenhouse and brings it to the ground through the VHXs, while in low-temperature periods, the heat is taken from the pump’s buffer tank through circulation pipes to the fan coil unit.

This configuration was tested in a greenhouse with an area of 46.94 m² and a volume of 142.87 m³ located in Loughborough, Leicestershire, England.  “The building gross wall area was 98.22 m², with the roof area being 77.01 m²,” the scientists said. “It was covered with double glazing semi-transparent PV rooftop and opaque PV on all sidewalls.”

Advantages of Greenhouse Heating using Heat Pump Technology

Greenhouse heating technology, employing methods such as heat pumps and sunlight, is widely applied to the cultivation of flowers and vegetables. The increasing preference for this method among growers is associated with the following advantages:

1. Safety and Environmental Friendliness: Heat pump technology eliminates the need for traditional devices like heating stoves, which require fuel. This reduces the risk of fire and minimizes the emission of pollutants.
2. Efficiency, Stability, and Cost Savings: Air-source heat pumps, not relying on burning any substances, are highly efficient and stable. This method significantly reduces energy consumption and costs. The heat pump’s efficiency surpasses traditional coal stoves by over 200%.
3. Intelligent Management: With the integration of automation control technology, heat pump technology exemplifies intelligent control. It can scientifically and intelligently regulate indoor temperatures based on the growth characteristics of crops and day-night temperature differences, reducing labor costs and enhancing crop profitability.
4. Stable Heating, Achieving Terminal Flexibility: The use of heat pump technology ensures stable heating. By adopting automatic adjustment during usage, growers can customize temperature settings for different crops, minimizing heat loss and ensuring crops’ canopy temperature is maintained with minimal energy consumption.
5. Uniform Heating, Faster Heating Speed: Heat pump technology ensures uniform heating and faster heating speeds. The benefits of intelligent control not only maintain a constant temperature but also contribute to the better cultivation of vegetables and flowers, especially in rapidly urbanizing areas.