Energy costs in operating a greenhouse

If you plan to operate an aquaponic system or hydroponic system all year round, energy costs play a major role. Since greenhouses have very little thermal insulation, precise calculations must be made here. In addition, the energy gained through solar radiation cannot be predicted. To calculate the necessary energy requirements, the Board of Trustees for Technology and Construction in Agriculture (KTBL) has published a specialist article that significantly simplifies the calculation . 

Before you decide on a type of energy, you must consider the following factors, among others, and include them in your calculation. There are many uncertainties here.

• Location: sunshine duration, angle of incidence, ambient temperature, etc
• Legislation: energy tax, delivery costs, basic costs, etc.
• Operating costs: maintenance, spare parts, running time, etc.
• Climate zone: Temperature progression throughout the year and its 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 requires 4.183 KJ of energy (1 Kcal). 1J equals one Ws - i.e. 1kJ equals 1kWs and 3,600 kWs equals 1 kWh. To heat 1000l of water by 10 degrees it takes 4,183 (4.183*1000*10) kWs, which corresponds to around 10.16 kWh (/3600). At 20° warming, twice as much.

Air

A greenhouse facility with 1000 m² (standing wall height 3 m) of floor space at the Hanover location with a single glass roof and energy shield insert designed for an internal temperature of 20 °C (and a minimum of -14 °C external temperature ~ ∆T = 34 K) has an energy requirement of around 350 kWh.  Example calculation see here.

As a rule of thumb, with a heat demand coefficient (Ucs) of 6.1 W/(m2 K), you can calculate around 35 kWh per 100 square meters. Please consider all factors when calculating your greenhouse. Here is a brief overview of the insulation:

Ucs values ​​for calculating heat requirements

1) Energy saving effect only half taken into account.

You can find a list of insulation materials here.

Here is a brief overview of the different energy sources, prices, conversions and meanings of technical terms

Butane has the disadvantage that it is no longer in gaseous form at temperatures below 0 degrees and therefore no longer comes out of the "bottle".

Compare energy characteristics - Calculation example:

Consumption example:  Heater output: 20.00 KW
Propane heating output: 12.87 KWh/kg = Propane consumption: 1.55 kg/h.

What is the difference between calorific value and calorific value?

A condensing boiler also extracts thermal energy from the exhaust gases from burning oil or gas, which is fed into the heating circuit. On the other hand, a boiler that does not have this technology simply uses the energy content of the fuel. This allows valuable energy to escape unused through your chimney. It is precisely this difference in efficiency that can be quantified using the key figures calorific value and calorific value.

So: The calorific value describes the energy content of a substance that can be used as heat simply by burning it. According to the calorific value definition, the value indicates how much heat energy a modern heating system can generate if it also extracts energy from the combustion exhaust gases.

Price history as of 2022-02

When deciding how and with what to heat, the conversion costs must be taken into account. A change from a supply medium, forced by price developments or legal rule changes, should always be kept in mind. Please also remember that funding in the Federal Republic of Germany always depends on current politics. Many companies that relied on government funding (solar cells, wind power, etc.) have already fallen by the wayside.

Here is a case study from: https://www.fh-swf.de/media/neu_np/fb_aw_2/dozentinnen/professorinnen_2/lorleberg/projekte_masterstudiengang/Report_Planung_Aquaponik-Demonstrationsanlage_2015.pdf

Example Calculation

Energy assessment
In the following part, the heat requirement for the parts of the building used is calculated. For this purpose, all relevant walls and the floor area of ​​the rooms for hydroponics and fish farming were measured and calculated. In the
next step, the heat losses that escape through the parts of the building were calculated. It should be noted that the loss or heat calculation depends on a whole range of factors. A precise calculation is anything but trivial, as the solar gains from solar radiation, the ventilation rates and the efficiency of the heating system can only be estimated. In addition, the rooms used are located within the entire building complex, and it can be expected that the rooms used will be heated indirectly through heat penetrating from the other greenhouse rooms. Another important factor is the outside temperatures in winter and spring. In order to be able to carry out a precise calculation of the likely costs, future, exact temperature curve values ​​would be needed, which would then flow into a heat requirement determination. 

It remains to be noted that the following values ​​could only be approximately calculated due to their complexity.

Greenhouse – areas and volume calculation

The following part contains the calculations of the wall and roof areas as well as the room volumes.
Calculations for the “hydroponics side” 

Floor area of ​​the room (without taking into account the internal concrete upstand):
8.60m x 4.80m = 41.3 m²

Volume of the room:
41.3m² x 2.6m (to eaves) = 107.38 m³
41.3m² x 0.9m /2 (roof space) = 18.58 m³

107.38+ 18.58= 125, 96m³ room volume

First external wall:
8.60m x 2.6m (to eaves) = 22.4m²

Second external wall:
4.8 m x 2.6 m (to eaves) = 12.5 m²

First interior wall:
4.8 m x 2.6 m (to eaves) = 12.5 m²

Second inner wall:
8.60m x 2.6m (to eaves) = 22.4m²

Roof area:
Panel length from eaves to ridge: 2.56 m
2.56 x 8.60 x 2 = 43.8m² roof area

Calculations for the “Fish Farming Page”

Floor area of ​​the room:
4.8m x 4.3 m = 20.6 m²
Volume of the room:
20.6m² x 2.6m (up to eaves) = 53.56m³
20.6m² x 0.9m /2 (roof space) = 9 .27m³
53.56m³+ 9.27m³= 62.83 m³ room volume

First external wall
4.8 x 2.6m = 12.5m²

Three internal walls
4.3 m x 2.6m = 11.2m²
4.6 m x 2.6m = 11.2 m²
4.8 x 2.6m = 12.5m²

Roof area:
Panel length from eaves to ridge: 2.56 m
2.56 m x 4.3 m = 22 m² roof area

Determination of heat requirements for the greenhouse

In order to determine the heat requirement of the two parts of the room, especially in the colder winter months, you can use the so-called U-value to calculate the heat transfer coefficient (formerly the k-value). If the U value for the building envelope is known, the next step can be to determine the required heating output. The following shows schematically how
the U-value can be calculated (PLAG 2014).

Calculate transmission heat losses using the heat transfer coefficient:

First step: Determine U-values ​​for the two parts of the room Second step: Determine ventilation losses for both parts of the room
Third step: Taking thermal bridges into account: Add 0.05 W/m²K to all calculated U-values

The U-value and its height depend on the materials used and their insulation capacity. In order to be able to estimate the properties of the greenhouse envelope, the following material combination was used for the calculation:

Building envelope:
Building envelope made of 5 mm combined double-wall sheets (plastic)

Insulation material:
Linitherm PAL SIL

Insulation element for internal wall insulation
PU rigid foam according to DIN EN 13165
Aluminum top coated on both sides,
special edge connection for mechanical fastening
with 6 mm silicate board laminated on the room side,
building material class B2
format 2500 * 1200 mm

The U-value per m² area of ​​the outer shell was calculated from the product combinations mentioned. As stated above, a ventilation surcharge of 0.05 watts per m² area is assumed. For the material combination, the heat transfer coefficient (U-value) is 0.44 W/m² + 0.05 W/m² ventilation surcharge is 0.49 W/m² (W= Watt) (PLAG 2014).
Calculation of heating output 

Assumption :

First of all, only the outer walls and the roof area are considered as loss areas; the inner wall losses clearly depend on how warm the greenhouse is heated in the other rooms! In addition, the assumption is made that the outside temperature should be on average - 6 degrees and the internal air temperature in the fish farming and hydroponics room should be 24 degrees: 

The heat flow through the individual surfaces is obtained by multiplying the U-value by the area (in m²) and the temperature difference:

Transmission heat loss power :
P = U-value * area * temperature difference The
advantage of using the formula is the freely “selectable” temperature difference.
This means that the energy cost determination can be calculated dynamically depending on the outside temperature (PLAG 2014). In the following case, the values ​​mentioned above were used as an example:

The following transmission heat losses are calculated for the hydroponic side:
External walls: 0.49W/m²K x (22.4+12.5)m² x 30 K = 513.03 watts
Roof: 0.49W/m²K x 43.8m² x 30K = 643, 86 watts

The sum of both values ​​is 1156.89 watts!

The determined wattage indicates the heat loss per hour for the building envelope under the above-mentioned conditions. It is known that electricity costs amount to €0.21 per kilowatt hour (electricity price for businesses, as of 2022). Gas costs are 5.5 cents per kilowatt hour (electricity price for businesses, as of 2022) . To calculate the heat loss and the resulting costs, the following procedure was followed:

Heat loss for 24 h=
1156.89 x 24/1000= 27.76 kW for the hydroponic side per day

Costs at 100% efficiency through electricity/gas heating:
Electricity: 27.76 kW/hx €0.21 = €5.83 per day
Gas: 27.76 kW/hx €0.055 = €1.52 per day

Costs at 80% Efficiency through electricity/gas heating:
Electricity: €7.28 per day
Gas: €1.9 per day

When determining this cost, it is assumed that the costs would arise if the heat transfer efficiency were 100% or 80% and there were no solar gains from solar radiation or internal gains from lighting heat.

The following transmission heat losses are calculated for the fish farming side:
Walls: 0.49W/m²K x 12.5m² x 30K = 183.75 watts
Roof: 0.49W/m²K x 22m² x 30K = 323.4 watts
The total is 507.15 watts!

Heat loss for 24 hours

507.15x 24/1000= 12.17 kW for the fish side per day

Costs with 100% efficiency through electricity/gas heating:
Electricity: 12.17 kW/hx €0.21 = €2.55 per day
Gas: 12.17 kW/hx €0.055 = €0.66 per day

Costs at 80% efficiency through electricity/gas heating:
Electricity: €3.18 per day
Gas: €0.825 per day

Calculation of the heating of the fish tanks

The following calculates how much energy is required to heat the fish tanks to the desired temperature (24 degrees Celsius). Two different operating processes are assumed. The first case
describes the operational process with 3% water loss per day and the second case describes the raining of water, for example when the nutrient concentrations of the water become too high for the fish being kept.

General conditions:
4500 liters for the fish tanks
8 degrees water temperature “ACTUAL”
27 degrees water temperature “TARGET”

Heat requirement:
1J= 1 watt second
You need 4.19 kilojoules to heat 1 liter of water by 1 degree Celsius!

Rule of thumb:
1.16 watt hours x temperature difference) /100= kW/h required per liter of water

Calculation:

One-time heating using electricity/gas at 100% efficiency:
Formula 1.16 Wh x 27 degrees Celsius - 8 degrees Celsius = 0.022 kW/h
Electricity: 0.022 kW/h x €0.21 x 4500 liters = €20.79
Gas: 0.022 kW7h x €0.055 x 4500 liters = €5.44

One-time heating with electricity/gas at 80% efficiency:
Electricity: €25.98
Gas: €6.80

Heating of the lost water that has to be replaced continuously every day

Assumption “Rained”:
400 liters of water per day (without the circulation system) are  lost because the amount of water is rained on:

At 100% efficiency:
Electricity: 0.022 kW/hx 0.21€ x 400 liters = 1.84€ per day or 8.8 kW/h
Gas: 0.022 kW/hx 0.055€ x 400 liters = 0.48€ per day or 8.8 kW/h

At 80% efficiency:
Electricity: €2.3 per day
Gas: €0.6 per day

Assumption of “circulatory system”
Assumption of 3% water loss per day results in 4500 liters of water: 135 liters

At 100% efficiency:
Electricity: 0.022 kW/hx 0.21€ x 135 liters = 0.623€ per day or 2.97 kW/h
Gas: 0.022 kW/hx 0.055€ x 135 liters = 0.16€ per day or 2 .97 kW/h

At 80% efficiency:
Electricity: €0.77 per day

Gas: €0.2 per day

Lighting concept

The growth of plants can be controlled by different artificial light sources. Different types of lamps can be used to increase or decrease growth or to specifically induce flowering in plants. In the following section, two different light sources are presented and compared 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 (NDL for short) are used for general lighting and artificial lighting of plants. Sodium vapor lamps have various advantages and are therefore the most commonly used lighting means in commercial horticulture. The connected load for the lamps is low and the luminous efficacy is high (up to 150 lumens per watt). After switching on, NDL need a few minutes to reach full brightness. The service life is 25-30,000 operating hours. However, if the lamps are switched on frequently, their lifespan can be significantly shortened. When using NDL lamps, a
ballast is absolutely necessary. This regulates the ignition process and keeps the electrical current at a constant strength. The heat output of the lamps is around 90%, which means that most of the electrical power is converted into heat (LICHT 2014; OSRAM 2014). When operating, sodium vapor lamps develop temperatures of up to 1000
degrees Celsius in the burner and external temperatures of up to 300 degrees Celsius can be reached on the lamp glass. This high level of heat radiation should be taken into account when evaluating the economic efficiency of the lamps. If the hydroponic plants are additionally exposed to light in winter, the lamps would provide additional heating energy. Conversely, the heat development in summer when the outside temperatures are high also represents a disadvantage for the climate control in the greenhouse. The economic efficiency of the sodium vapor lamps is present for the ongoing aquaponics project in that it is no longer necessary to purchase new lamps, as the lamps are already in place before the building is converted were installed. It will be necessary to examine for which crops artificial lighting, for example in winter, makes sense and, above all, is economical. However, if you look at current developments in the lamp and lighting industry, LED technology in particular is the one that is already in wider use in commercial horticulture and, thanks to ongoing developments, is very likely to be the lighting technology of the future. In the following section, the LED lighting concept is presented in more detail with the 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 through lighting in the winter months, thereby reducing the cultivation time. This also makes it possible to produce plants of better quality (TANTAU 2014 p.11). According to SPRINGER (2012), the exposure of plant crops has long been an important topic in horticulture. The aim is to produce higher yields, shorter cultivation times and better quality (SPRINGER 2014). However, this measure, which is used across the board, can be described as expensive due to the high energy costs involved (TANTAU 2014 p. 11).

The high-pressure sodium vapor lamp is the most widely used in gardening practice (MÜLLER 2011 p. 11).
In the discussion about saving energy when lighting greenhouse crops, “light -emitting diodes” (LED) are repeatedly coming to the fore (TANTAU 2014 p. 11). When it comes to LED lights, some speak of future-oriented technology, but others are skeptical due to technical difficulties and the limits of performance (SPRINGER 2012). 

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