Energy Costs in Greenhouse Operation
If you are planning year-round operation of an aquaponics or hydroponics system, energy costs play a major role. Since greenhouses have only very low thermal insulation, calculations must be made carefully. In addition, the energy gained from solar radiation cannot be predicted. For calculating the required energy demand, the Kuratorium für Technik und Bauwesen in der Landwirtschaft e.V. (KTBL) has published a technical article that greatly simplifies the calculation.
Factors to Consider
Before deciding on a type of energy, you must take into account the following factors, among others, and include them in your calculations. There are many uncertainties involved here.
Location
Sunshine duration, angle of incidence, ambient temperature, etc.
Legislation
Energy tax, delivery costs, base fees, etc.
Operating Costs
Maintenance, spare parts, runtime, etc.
Climate Zone
Annual temperature trends and fluctuations
Example Calculation
💧 Water
The heat capacity of water is (according to Wiki) 4.183 kJ/(kg*K). This means that to heat 1 liter of water by 1 degree, 4.183 kJ of energy is required (1 kcal). 1 J corresponds to 1 Ws – i.e. 1 kJ corresponds to 1 kWs and 3,600 kWs equal 1 kWh. So to heat 1000 liters of water by 10 degrees, you need 4,183 (4.183*1000*10) kWs, which equals about 10.16 kWh (/3600). For 20°C heating, it is correspondingly twice as much.
🌪️ Air
A greenhouse facility with 1000 m² ground area (side wall height 3 m) in Hannover, with single-glass roofing and energy screen installation, designed for an inside temperature of 20 °C (and a minimum outside temperature of -14 °C ~ ∆T = 34 K), requires about 350 kWh of energy. See example calculation here.
As a rule of thumb, with a heat demand coefficient (Ucs) of 6.1 W/(m²K), you can estimate about 35 kWh per 100 square meters. Please consider all relevant factors for your own greenhouse calculation. Here is a brief overview of insulation values:
Ucs Values for Heat Demand Calculation
| Material and Insulation Measure | Ucs Value [W/(m² K)] |
|---|---|
| Single Glass | 7.6 |
| Single Glass + PE Foil (1) | 6.5 |
| Single Glass + Bubble Foil (1) | 6.2 |
| Single Glass + Energy Screen (1) | 6.1 |
| Single Foil | 7.0 |
| Double Glass | 4.7 |
| Double-Wall Polycarbonate Sheets | 4.6 |
| Double Foil | 5.1 |
1) Energy-saving effect only considered by half.
📊 Energy Sources, Prices and Conversions
1 kWh gas for new customers: 34.1 €-cents / as of 2022-08-23
1 kWh electricity for new customers: 37.3 €-cents / as of 2022-07-01
⚠️ Attention: Butane has the disadvantage that at temperatures below 0 °C it no longer exists in a gaseous state and therefore can no longer be extracted from the "bottle".
Comparing Energy Indicators – Calculation Example
The values for heating and calorific value mentioned in the following examples are taken from the internet and should be treated with caution.
One source is for example: https://www.energie-lexikon.info/
Values are given as megajoules per kilogram (MJ/kg) or kilowatt hours per kilogram (kWh/kg). If you want to compare values in different units, you can use conversion factors. By multiplying by the factor 0.27778, you convert megajoules into kilowatt hours. For the reverse operation, multiply the value by 3.6.
Example: The heating value of wood pellets is about 17.3 MJ/kg. Heating oil is about 11 kWh/kg.
Now multiply the heating oil value by the factor 3.6 and you know that heating oil has a heating value of approximately 40 MJ/kg.
This means: With the same mass, heating oil contains significantly more energy than wood pellets.
🔥 Consumption Example
Heater output: 20.00 kW
Heating capacity propane: 12.87 kWh/kg = Propane consumption: 1.55 kg/h
⚠️ Important Note on Gas Costs
Anyone who wants to know the cost of one kilowatt hour of gas should not only rely on the supplier’s quoted price for that kilowatt hour. In classic tariff models, consumers are also charged a basic fee, which is collected monthly, regardless of actual consumption.
What Is the Difference Between Heating Value and Calorific Value?
A condensing boiler extracts additional thermal energy from the exhaust gases of burning oil or gas, which is then fed back into the heating circuit. By contrast, a boiler without this technology uses only the direct energy content of the fuel. As a result, valuable energy escapes unused through your chimney. This efficiency difference can be quantified by the heating value and the calorific value.
So: the heating value describes the energy content of a substance that can be utilized as heat through simple combustion. The calorific value, by definition, indicates how much thermal energy a modern heating system can extract when it also recovers energy from the combustion exhaust gases.
Overview of Heating and Calorific Values
⚡ Liquefied Gases
| Liquefied Gases | Heating Value Hi | Calorific Value Hs |
|---|---|---|
| Propane | 25.88 kWh/m3 | 28.14 kWh/m3 |
| Butane | 34.34 kWh/m3 | 37.29 kWh/m3 |
| Propane | 12.87 kWh/kg | 14.00 kWh/kg |
| Butane | 12.69 kWh/kg | 13.77 kWh/kg |
| Propane | 6.83 kWh/liter | 7.44 kWh/liter |
| Butane | 7.36 kWh/liter | 7.99 kWh/liter |
🛢️ Other Energy Sources
| Energy Source | Heating Value Hi | Calorific Value Hs |
|---|---|---|
| Natural Gas Low | 8.80 kWh/m3 | 9.75 kWh/m3 |
| Natural Gas High | 10.36 kWh/m3 | 11.48 kWh/m3 |
| Light Heating Oil | 10.00 kWh/liter | 10.68 kWh/liter |
| Wood | approx. 4 – 5 kWh/kg | approx. 4 – 5 kWh/kg |
| Diesel | approx. 9.8 kWh/liter | approx. 11.9 kWh/liter |
| Gasoline | 8.5 kWh/liter | approx. 9.0 kWh/liter |
| Butane | 12.7 kWh/kg | 13.8 kWh/kg |
The heating and calorific values used are approximations. They vary significantly depending on the system used.
The heating value of liquefied petroleum gas (LPG) is about 46 megajoules per kilogram (MJ/kg) or around 12.5 kilowatt hours per kilogram (kWh/kg). The calorific value of LPG is about 50 MJ/kg or just under 14 kWh/kg.
Example: Price per kWh of butane: approx. 14 kWh/kg. With 13 kg bottles at a refill price of €35, one kWh (182 kWh / €35) costs about 19 cents (€0.19230) (as of 2024-12).
Price Trend
Price trend as of 2022-02
When deciding how and with what to heat, conversion costs must be taken into account. A change of energy supply, forced by price developments or legal changes, should always be kept in mind. Also consider that subsidies in Germany are always dependent on current politics. Many companies have already failed because they relied on government subsidies (solar panels, wind power, etc.).
Example Calculation
🏗️ Energy Assessment
In the following section, the heat demand for the building parts in use is calculated. All relevant walls and floor areas of the rooms for hydroponics and fish farming were measured and calculated.
In the next step, heat losses escaping through the building parts were calculated. It should be noted that the loss and heat calculation depends on a whole range of factors. An exact calculation is anything but trivial, since solar gains through radiation, ventilation rates, and the efficiency of the heating system can only be estimated. In addition, the rooms used are located within the overall building complex, so it can be assumed that indirect “co-heating” of the used rooms occurs through heat transfer from the other greenhouse areas. Another important factor is the outside temperatures in winter and spring. To perform an accurate calculation of the expected costs, one would need precise future temperature progression data, which could then be fed into a heat demand calculation.
Greenhouse – Area and Volume Calculations
The following section lists the calculations for wall and roof surfaces as well as room volumes.
📋 Assumptions for the Calculation
Only the outer walls and the roof area are considered as loss areas; heat losses through internal walls clearly depend on how warm the greenhouse is in the adjacent rooms! It is also assumed that the outside temperature averages -6 °C and that the indoor air temperature in the fish farming and hydroponics rooms is set at 24 °C:
The heat flow through the individual surfaces is obtained by multiplying the U-value by the area (in m²) and the temperature difference:
📐 Formula: Heat Transmission Loss
P = U-value × Area × Temperature Difference
The advantage of this formula is the freely "selectable" temperature difference. This means that energy cost estimation can be dynamically calculated depending on the temperature trend (PLAG 2014). In the following case, the above-mentioned values were used as an example:
🧮 Calculation Results
🌿 Hydroponics Side – Heat Transmission Losses
Calculation of Losses:
Exterior walls: 0.49 W/m²K × (22.4+12.5) m² × 30K = 513.03 W
Roof: 0.49 W/m²K × 43.8 m² × 30K = 643.86 W
The calculated wattage indicates the hourly heat loss through the building envelope under the above conditions. It is known that electricity costs €0.21 per kilowatt hour (commercial electricity price, as of 2022). Gas costs are €0.055 per kilowatt hour (commercial gas price, as of 2022).
Heat loss over 24 hours:
1156.89 × 24/1000 = 27.76 kWh per day for the hydroponics side
| Efficiency | Electricity (€/day) | Gas (€/day) |
|---|---|---|
| 100% | 5.83 | 1.52 |
| 80% | 7.28 | 1.90 |
🐟 Fish Farming Side – Heat Transmission Losses
Calculation of Losses:
Walls: 0.49 W/m²K × 12.5 m² × 30K = 183.75 W
Roof: 0.49 W/m²K × 22 m² × 30K = 323.40 W
Heat loss over 24 hours:
507.15 × 24/1000 = 12.17 kWh per day for the fish farming side
| Efficiency | Electricity (€/day) | Gas (€/day) |
|---|---|---|
| 100% | 2.55 | 0.66 |
| 80% | 3.18 | 0.83 |
🐠 Heating Calculation for Fish Tanks
The following section calculates how much energy is required to heat the fish tanks to the desired temperature (24 °C). Two different operational scenarios are assumed. The first case describes operation with 3% water loss per day, and the second case describes water replacement, for example when the nutrient concentrations of the water become too high for the fish being kept.
📊 Framework Conditions
- 4500 liters in the fish tanks
- 8 °C current water temperature
- 27 °C target water temperature
🔥 Heat Requirement
1 J = 1 watt second
It takes 4.19 kilojoules to heat 1 liter of water by 1 °C!
💡 Heating Cost Calculation
🔥 One-Time Heating
Calculation:
Formula: 1.16 Wh × (27 °C - 8 °C) = 0.022 kWh per liter
At 100% efficiency:
- Electricity: 0.022 kWh × €0.21 × 4500 liters = €20.79
- Gas: 0.022 kWh × €0.055 × 4500 liters = €5.44
At 80% efficiency:
- Electricity: €25.98
- Gas: €6.80
♻️ Continuous Heating of Replacement Water
🌧️ Scenario “Water Replacement”:
400 liters of water per day (outside the recirculation system) are lost due to replacement:
At 100% efficiency:
- Electricity: €1.84/day or 8.8 kWh
- Gas: €0.48/day or 8.8 kWh
At 80% efficiency:
- Electricity: €2.30/day
- Gas: €0.60/day
♻️ Scenario “Recirculation System”:
Assuming 3% water loss per day = 135 liters with 4500 liters total volume
At 100% efficiency:
- Electricity: €0.62/day or 2.97 kWh
- Gas: €0.16/day or 2.97 kWh
At 80% efficiency:
- Electricity: €0.77/day
- Gas: €0.20/day
💡 Lighting Concept
The growth of plants can be controlled by different artificial light sources. Various lamp types can be used to increase or decrease growth or to induce flowering in plants. The following section presents and compares two different lighting technologies with their advantages and disadvantages.
🔶 Greenhouse Lighting with Sodium Vapor Lamps
In the greenhouses of the South Westphalia University of Applied Sciences, so-called sodium vapor lamps (HPS lamps) are used for general illumination and artificial lighting of plants. Sodium vapor lamps have several advantages and are therefore still the most widely used lighting system in commercial horticulture. Their power consumption is low and their light output is high (up to 150 lumens per watt). After being switched on, HPS lamps take a few minutes to reach full brightness. Their lifespan is 25,000–30,000 operating hours. However, if the lamps are switched on and off frequently, their service life can be significantly shortened. When using HPS lamps, a ballast is essential. It regulates ignition and keeps the electrical current at a stable level. The heat output of the lamps is around 90%, meaning that most of the electrical energy is converted into heat (LICHT 2014; OSRAM 2014). During operation, sodium vapor lamps can reach up to 1000 °C inside the burner, and the glass envelope can reach outer surface temperatures of up to 300 °C. This high heat emission must be considered when evaluating their efficiency. If hydroponic plants are additionally illuminated in winter, the lamps also provide extra heating energy. On the other hand, in summer with high outside temperatures, the heat generated poses a disadvantage for climate control in the greenhouse. The economic efficiency of sodium vapor lamps in the ongoing aquaponics project is ensured in the sense that no new investment was needed, since the lamps were already installed before the building conversion. It remains to be evaluated for which crops artificial lighting is useful and economically feasible, e.g. in winter. However, looking at current developments in the lighting industry, LED technology is increasingly being adopted in commercial horticulture and, with high probability, will be the lighting technology of the future due to ongoing improvements. The next section presents the LED lighting concept in more detail with its current advantages and disadvantages (LICHT 2014; OSRAM 2014).
💚 Plant Lighting with LED – Technology of the Future
The assimilation of plants in greenhouses should be improved during the winter months through lighting, thereby reducing cultivation time. This also makes it possible to produce plants in better quality (TANTAU 2014, p.11). According to SPRINGER (2012), lighting of greenhouse crops has long been an important topic in horticulture. It is intended to achieve higher yields, shorter cultivation times, and better quality (SPRINGER 2014). However, this widely used measure is considered expensive due to the high energy costs involved (TANTAU 2014, p.11).
High-pressure sodium lamps are still the most widely used in horticultural practice (MÜLLER 2011, p.11). In the discussion about saving energy in greenhouse lighting, “light-emitting diodes” (LEDs) are increasingly coming into focus (TANTAU 2014, p.11). Regarding LED lighting, some call it pioneering technology, while others remain skeptical due to technical difficulties and limitations in performance (SPRINGER 2012).
The authors assume no liability for the timeliness, completeness, or accuracy of the provided content.
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