Battery Rooms from HVAC POV

1. Article Purpose

This article aims to explore the air conditioning considerations specific to battery rooms. This topic has received limited attention in the field, largely due to its application being restricted to a few industry sectors.

2. Introduction

In my experience over the past 7 months working closely with the Oil & Gas / Power Generation Sector, I have encountered various design approaches for battery rooms by both local and international consultant firms. The necessity for 24/7 air conditioning with total fresh air intake in these rooms, despite serving a "Power Generation" plant, prompted me to delve deeper into this issue. I sought to clarify if there are any code or standard requirements for such specifications and to explore alternative methods for reducing energy consumption.

3. Basis of Design

ASHRAE Handbook Applications 2019 – Chapter 28: Power Plants. Provided that Battery Room pressure shall be Negative/ Neutral but referred to ASHRAE Guideline 21- 2012 for the indoor design conditions & ventilation rates.

Hence, the following indoor design conditions are based on ASHRAE Guideline 21-2012. The outdoor design conditions are based on ASHRAE meteo (Handbook Fundamentals weather data) for Cairo.

• Outdoor: Summer DB/ WB (38.8 / 21) °C.
• Outdoor: Winter DB (9.2) °C.
• Indoor: Summer DB/RH (22 °C / 35% - 65%).
• Indoor: Winter DB/RH (20 °C / 35% -65%).

Simulation and HVAC Load calculations carried out in this article are based on Carrier HAP V5.1. The designated battery room will be the one used for sample calculation at ASHRAE Guideline 21-2012 Annex B & C.

• 1600 A Load
• 3 Parallel Strings of batteries (24 Lead-Calcium, 1.215 s.g. cells per string, 4000 Ah)
• 16x200 A rectifier to feed the load and charge/ recharge the batteries.
• 30m x 30m x 4m room.

Only information of relevance is listed above. For the full data refer to ASHRAE Guideline 21.

4. Design Approach

In the following sections, I will present various design approaches that I have encountered, followed by an analysis of the energy consumption by the HVAC equipment required.

For the purposes of this analysis, factors such as wall construction, roof materials, windows, and other related data that could impact the overall load are omitted. These examples are provided purely for comparative purposes, and therefore, the peak load values presented do not need to be accurate or compliant with specific building envelope design or code requirements.

4.1. (6 ACH) Total Fresh Air.

Most of the Battery Rooms I have encountered assume the need for 6 Air Changes Per Hour of total fresh air to ensure safety and Hydrogen dilution.

In our scenario, the ventilation required would reach 6,000 L/s.

• Peak Cooling Coil Load: 173.2 kW in August.
• Annual Energy Consumption: 101,890 kWh.

Of course, such values can be reduced by using economizers, and heat recovery wheels but that shall be discussed later.

4.2. NFPA Default Calc.

A direct approach offered by NFPA 1 Chapter 52 is to determine the ventilation rate based on 1 CFM/ft2.

In our scenario, the ventilation required would reach 4,590 L/s.

• Peak Cooling Coil Load: 144.5 kW in July.
• Annual Energy Consumption: 89,415 kWh.

That’s almost 12.3% energy savings compared to the 6-ACH approach.

4.3. NFPA Worst Case Calculation

Under the same section. NFPA 1 Chapter 52 offers an alternative way in case more data is available for Hydrogen generation. The code states that limiting the Hydrogen concentrations at 25% of the Lower Flammability level at “Boosting” shall be enough.

Referring to ASHRAE Handbook Fundamentals Chapter 11: Air Contaminants. ?LFL of hydrogen is 4%. Utilizing equation 32, Chapter 16: Ventilation and Infiltration, the ventilation required for hydrogen dilution to 1% can be simplified to [ Source Heat Generation divided by the design concentration].

In our scenario, the ventilation required would reach, 10 L/s. However, the results below are based on 810 L/s. to satisfy ASHRAE Standard 62.1 ventilation per area requirement.

• Peak Cooling Coil Load: 74.1 kW in July.
• Annual Energy Consumption: 54,618 kWh.

That’s equal to a 46.4% reduction compared to that of the 6-ACH approach.

Notes:

• ASHRAE Guideline 21 included different Hydrogen generation rates according to the operation mode of the battery room. Yet, the boosting mode is recognized as the extreme mode, especially at elevated temp.
• ASHRAE Guideline 21 conducted the calculation to limit the Hydrogen concentration to 2% which contradicts with NFPA 1.
• ASHRAE Standard 62.1 may not directly apply to battery rooms; however, it is utilized because it aligns with NFPA guidelines and offers the added benefit of ensuring air circulation within the space.

4.4. ASHRAE Guideline 21 - IEEE Approach

ASHRAE Guideline 21 adopts a different worst-case calculation methodology that is more conservative and provides an additional layer of protection compared to the NFPA standards.

The Guideline assumes failure of the rectifiers due to short circuits or other reasons leading to overcharging the batteries. For such a scenario, the AC of 1600 A shall be redirected towards the 3 strings of parallel batteries. i.e. 533 A/string.

Once again, the hydrogen generation rate will be evaluated based on the corresponding charging current and the batteries’ capacity. The ventilation requirement will be calculated as outlined in the previous section.

In our scenario, the ventilation required would reach, 500 L/s. However, the results below are based on 810 L/s. to satisfy ASHRAE Standard 62.1 ventilation per area requirement.

[Monthly Energy consumption here matches that of the previous section]

4.5. British Safety Code Approach.

BS EN IEC 62485-2:2018: Safety requirements for secondary batteries and battery installations. Provides an easy-to-use equation with an assumed boost current that ends up demanding 400 L/s of fresh air. Almost equal to that of ASHRAE Guideline 21 approach.

[Monthly Energy consumption here matches that of the previous section]

5. Conclusion

Comparing the results, we can see a wide gap between “best practice” and the requirements of NFPA, ASHRAE, IEEE, British Standard! Hence, it is recommended to perform calculations based on Hydrogen gas generation whenever possible to help cut down the HVAC initial and running costs.

Another strategy followed is installing hydrogen detectors within the space to be interlocked with variable-speed fans. This may be of higher initial cost but way better performance in addition to being safer.

In the event of using the 6 ACH approach due to local code requirements or whatever the reasons. It’s highly advised to implement a heat recovery method and an Economizer (Depending on the weather profile). Doing so in our case cut down the total annual energy consumption to 79,838 kWh. Resulting in a 12.5% reduction (almost like the NFPA Default Calc. Approach).

Disclaimers:

• Manufacturer-estimated Hydrogen generation shall be used instead of ASHRAE Guideline 21 if available.
• All listed calculations are based on Vented Lead Acid Batteries, which is an old type of high hydrogen generation. Other types shall be studied separately.
• Investigating the presence of Hydrogen Pockets is a must. Keep in mind that the flow rate to be admitted to the room must prevent the formation of any Hydrogen pockets.
• Internal Heat Dissipation wasn't listed in this article, yet it was used in performing all the calculations above. ASHRAE guideline 21 provides equations that help calculating the overall heat dissipation in kWh. The exact heat release rate is to be determined according to the estimated battery operation time under the same current. Discharge mode is considered the extreme condition in our scenario.
• It is advisable to conduct a life-cycle analysis to determine the financial viability of installing HVAC equipment and operating it in the first place. For advanced battery types like Nickel Cadmium, the degradation rate increases by 50% for every 30 °C increase above 25 °C. Therefore, cooling the batteries to room conditions of 22 or 23 degrees Celsius throughout their lifetime may be more expensive than cooling them to 30 or 35 degrees Celsius and replacing them midway.