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How to calculate the annual energy consumption of a battery dry room dehumidification system

It’s the annual consumption that matters to the electricity bill. But how do we measure it, and how can we compare across different solutions?

Usually, a dry room project gets specified on peak values, like summer or winter conditions. That does make sense for dimensioning the dehumidifiers but not for comparing energy consumption. These peak values do not reflect a balanced operation of the dehumidifier.  

The average state of the dehumidifier is far below the summer conditions or far above the winter condition. The annual energy consumption is closer to the average conditions multiplied by the annual running hours rather than summer peak conditions multiplied by the annual running hours. So, what is important is to find a measure for the average condition. Below we will process how to get a realistic measure of the annual consumption. 

If we look at the purpose of the dehumidifier, it would be to handle moisture loads from two sources: 

  1. Operational moisture load
    Operational moisture load originates from the operation inside the battery dry room, and the respiration of working personnel often dominates this. The more personnel present, the more moisture in operation.  
  2. Fresh air moisture load
    Fresh air moisture load stems from the fresh air supply used to compensate for leakage in the dry room or the air removed from the dry room by exhaust systems. 

The operational moisture load would be the same regardless of the year.  

However, the fresh air moisture load is defined by the external ambient air, leading to sessional fluctuations and dependencies. The performance of the dehumidifier unit is, therefore, mainly related to the ambient air condition.   

The following figure shows a sketch of the different air streams. The fresh ambient air's temperature and humidity affect the dehumidifier's performance.  

Exergic Dehumidification airflows-min

Dehumidifier consumption 

To understand the calculation of the dehumidifier consumption, the first step is to look at the purpose and result of the pre-treatment coils.  

The fresh air gets pretreated in either frost protection or a cooling coil before being processed in the dehumidifier. 

Airflow Exergic-min-cropped

Frost Protection Coil 
The role of the frost protection coil is to increase the temperature of the incoming air to avoid problems with hoar frost and to avoid having extreme cold surfaces in the dehumidifier unit.  

Typically, the setpoint after the frost protection coil is in the range of 2-6°C. This frost protection coil is followed by a cooling coil.  

Cooling Coil 
The role of the cooling coil is to condense moisture, so it is actually the first dehumidification step. The set point of the cooling coil depends on the available cooling water and is typically in the range of 6-12°C, but for some applications, even higher. 

3 basic conditions 

After the coils, the following three conditions for ambient temperatures could therefore be defined based on the average weather condition datasets:  




If the temperature is below the frost setpoint... 

...then the frost coil heats to setpoint, and the cold coil has no action 

If the temperature is between coil setpoints...

...then both coils do nothing 

If the temperature is above the cooling coil setpoint... 

...then frost coil has no action, and cold coil cools to setpoint 


To assess the performance of the two initial coils and the subsequent performance of the dehumidifier, each of the ambient conditions is put into the three states as shown above. The average of the states of temperature and humidity is calculated for each of the three states, with the average fulfilling the same conditions and requiring the actions specified in the table. 

Based on the number of conditions in the different states, the load of the frost protection coil and the cooling coil is calculated, respectively. 


From the dehumidifier's perspective, it will now only recognise three air conditions in three states (let's assume frost protection setpoint at 5°C and initial cooling coil temperature at 10°C): 

  1. State No.1 
    Temperatures at 5°C with different absolution humidity and dew points below 5°C correspond to a max. 5.4 g/kg.  
  2. State No.2 
    Temperatures between 5 and 10°C and dewpoints in the same range 
  3. State No.3 
    Temperatures of 10°C dominate these states with a dewpoint of 10°C or close to 10°C due to the nature of the cooling coil. 

The dehumidifier's performance can be calculated based on the three states above. The trick is to categorise realistic ambient weather conditions, so they fit the 3 states.  


Weather data

Cotes imports weather data from a trusted weather service provider called Meteonorm weather service. Please see more here (https://meteonorm.com/en). The dataset is from the weather station closest to the site. The dataset contains average temperature and humidity data, measured every hour for all days in a year and averaged over ten years. 

The procedure follows the description above, where every hour is sorted into bins/states for the pretreatment – frost protection coil and initial cooling coil – performance is calculated as the average output. Furthermore, the result of the pretreatment is processed in the dehumidifier calculation software. The total energy consumption is obtained by adding the pretreatment with the dehumidifier consumption. 


How big is the difference between the average and peak?  

It is hard to give any specific numbers to answer this question, it mainly depends on how extreme the peak state is. We often see temperatures +5°C of the highest average temperature and +3 g/kg relative to absolute humidity, even though the highest average temperature usually only occurs less than 10 hours per year. Furthermore, the difference depends on fresh air rates, temperature set for coils, and the scattered weather data. 

For decent cooling water and not too extreme peak conditions, we see that the cooling demands on average are between ¼ to ½ times lower than the peak. The average energy used for running the dehumidifier is up to 30 % lower for the average state compared to the peak state. Energy used for frost protection is often negligible compared to other energy consumers.  


The realistic annual energy consumption is closer to the average conditions multiplied by the annual running hours rather than the summer peak multiplied by the annual running hours. 

To calculate accurate energy consumption for the entire battery dry room dehumidification system, you need to understand the following: 

  1. Internal moisture load [number of people inside the dry room at the same time] 
  2. Fresh air requirements (for positive pressure and extraction if applicable) 
  3. Room operation conditions (humidity and temperature setpoints) 
  4. Peak ambient conditions for design specifications 
  5. Average ambient conditions defined in three states for calculating energy consumption
  6. Available utilities on-site location (electricity, cooling water and heating water) 

Cotes, the right way to dry your battery dry room.