Warehouse Energy Simulations: Finding the Right Balance Between Skylights, Lighting Layout and Cooling Demand

Introduction

Warehouses are often seen as simple industrial buildings, but their energy performance can be challenging to optimise. Their large roof areas, high internal volumes, long operating hours and lighting requirements can lead to significant electricity consumption, especially for warehouses that require air-conditioning in warm and humid climates such as

Malaysia.

Two design questions are particularly important for air-conditioned warehouse projects: whether skylights should be included, and how the electric lighting layout should be coordinated with the storage rack layout. Skylights can bring daylight into the building and reduce artificial lighting needs, but they can also increase solar heat gains and cooling demand. Similarly, a poorly coordinated electric lighting layout can increase the installed lighting power required to achieve the target illuminance.

This article presents a simulation study on warehouse energy performance, focusing on the balance between daylight use, artificial lighting efficiency and cooling load reduction. The aim is to identify practical design strategies that can help reduce annual electricity consumption and reduce the peak cooling demand in warehouse buildings.

Figure 1: Warehouse model in IES VE building simulation software

Key Warehouse Design Parameters

Areas & setpoints

Roof construction

The roof is insulated (85 mm PU foam) and was modelled with an albedo of 30%.

Material	Thickness (mm)	Conductivity W/(m·K)	Density (kg/m³)	Specific Heat Capacity J/(kg·K)	Resistance (m²K/W)
[MD1] Metal Deck (ASHRAE)	0.5	47.6	2800	896	0
[STD_PHF3] Insulation	85	0.033	40	1450	2.58
[MD] Metal Deck (ASHRAE)	0.5	47.6	2800	896	0

Roof U-Value: 0.360 W/m² K

External wall construction

The walls are insulated (81 mm PU foam) and were modelled with an albedo of 30%

Material	Thickness(mm)	Conductivity W/(m·K)	Density (kg/m³)	Specific Heat CapacityJ/(kg·K)	Resistance (m²K/W)
[STD_SM1] Rainscreen	3	50	7800	450	0.0001
Cavity	50	-	-	-	0.13
[STD_EPS] Insulation	81.4	0.025	20	1030	3.256
[STD_USP] Cement bonded particle board	12	0.23	1100	1000	0.0522
Cavity	50	-	-	-	0.18
[STD_US1] Plasterboard	12.5	0.21	700	1000	0.0595

Wall U-value: 0.261 W/m² K

Roof skylight

Material	Thick-ness (mm)	Conductivity W/(m·K)	Solar Heat Gain Coefficient (SHGC)	Visible Light Transmissivity (VLT)	Outside Emissivity	Inside Emissivity
[STD_RF1] Semi-translucent polycarbonate sheet	6.0	50	0.72	0.8	0.837	0.837

Simulation approach

The simulation workflow combined energy modelling and lighting simulation. The IES VE software was used to evaluate the annual energy consumption and maximum cooling demand of the warehouse under different design configurations. The DIAlux evo software was used to assess the lighting performance of alternative luminaire layouts in the center of racking aisle (Zone 6) area where the target illuminance was 200 lux.

The IES VE model included the main warehouse geometry, roof construction, external wall construction and skylight construction. The indoor air temperature in the lower occupied zone was controlled close to the cooling setpoint of 24°C. The cooling system efficiency was set to a COP of 3.125. As the warehouse is a large-volume building, the vertical distribution of heat can influence cooling demand. For this reason, a thermal stratification case was also included, with stratification considered at 12 m height, namely the air-conditioned zone (below 12 meters height) and the non-air-conditioned zone (above 12 meters height).

The study compared several design variables, including lighting power density, skylight configuration, daytime electric lighting dimming, thermal stratification and a low-emissivity aluminium ceiling option. The simulations were structured progressively so that the impact of each design measure could be observed in relation to the previous case.

Lighting layout and LPD 

Lighting power density (LPD) is one of the key parameters influencing warehouse energy performance. It directly affects lighting electricity consumption, but also contributes to cooling demand, since electric lighting releases heat into the space. In an air-conditioned warehouse, unnecessary lighting power therefore becomes an additional internal heat gain that must be removed by the cooling system.

The DIAlux simulations compared two luminaire layouts in the warehouse racking area. In the first configuration, luminaires were positioned above the aisles, where a light level of 200 lux is required to reach the working plane between racks (80 cm above the floor). In the second configuration, luminaires were positioned above the racks and represented a realistic worst-case scenario, since the ceiling lights are included in the base building design, while the racks are installed by the warehouse tenants and may not align with the ceiling lighting grid. Both simulations were carried out without daylight contribution, allowing the artificial lighting performance of each layout to be compared directly.

Figure 2: 3D views of the warehouse racking area used for the DIAlux simulations

The surface reflectance values used in the DIAlux simulations were defined for the floor, ceiling, walls and racks. These values influence how light is reflected within the space and therefore affect the resulting average illuminance.

Light reflectance values

The required LPD was estimated from the DIAlux simulation results using a linear extrapolation based on the target illuminance of 200 lux in Zone 6 between the racks.

Required LPD for achieving 200 lux in Zone 6

The results show that the different electric luminaire layouts have a strong impact on the LPD required to achieve the 200 lux target in Zone 6. When luminaires are aligned with the aisles, light is distributed more effectively towards the working zones. When luminaires are located above the racks, a significant part of the light is blocked or poorly distributed, requiring almost 14 times more lighting power to reach the same illuminance of 200 lux.

For the energy simulation, this difference was represented by two LPD values: 2.79 W/m² for the efficient aisle-based layout and 38.55 W/m² for the less efficient rack-based layout. This highlights that coordinating the lighting design with the storage rack layout from the beginning of the project can significantly reduce the required installed lighting power.

Skylights: Daylight Benefit versus Solar Heat Gain

Skylights are a common design consideration in Malaysian warehouse projects because they can bring daylight into the building and reduce the need for artificial lighting during daytime. However, they can also increase solar heat gains through the roof, which raises the cooling demand in air-conditioned warehouses. Their overall energy impact therefore depends on the balance between lighting electricity savings and additional cooling load.

The simulations compared three skylight conditions: full skylight configuration, reduced skylight configuration and no skylight. In this study, the 100% skylight case refers to the full skylight configuration available in the model, corresponding to 2,785 m² of roof openings, or 10.3% of the roof area. The 50% skylight case refers to the reduced configuration, corresponding to 1,517 m², or 5.6% of the roof area. The no-skylight case removes the daylight contribution and therefore also removes the benefit of daylight responsive dimming the electrical lighting system.

Different areas of skylight simulated

This last point is important. In the no-skylight cases, daytime lighting dimming was removed because dimming was used to represent the reduction of artificial lighting due to daylight from the skylights. Without skylights, this daylight contribution is no longer available, so the luminaires are assumed to operate without daylight dimming. This creates a realistic trade-off: removing skylights reduces solar heat gains, but it also removes lighting savings from daylight dimming.

Therefore, the impact of skylights cannot be evaluated only from a daylight perspective. It must be assessed together with the lighting power density and the cooling demand.

Thermal stratification and material improvements

Thermal stratification is relevant in high-volume warehouse buildings because the occupied zone and the upper air volume do not always behave in the same way. Heat can accumulate at higher levels, while the lower occupied zone is maintained close to the cooling setpoint. Representing stratification allows the simulation to better reflect the thermal behaviour of a tall warehouse.

In this study, stratification was considered at 12 m height. The purpose was to examine whether separating the thermal behaviour of the upper zone from the occupied zone could influence cooling demand, particularly maximum cooling demand, which is important for cooling system sizing.

Thermal stratification allowing a non-air-conditioned hot heat pocket under the warehouse roof

Material improvements were also examined through a low-emissivity aluminium ceiling option. This measure was included to assess whether improved material properties could support cooling load reduction. In practice, material improvements should be seen as complementary measures, alongside lighting optimisation and skylight design.

The annual indoor temperature profile was also checked to verify that the lower occupied zone remained close to the cooling setpoint. This verification is important because the comparison between cases would not be meaningful if some cases saved energy simply by allowing higher indoor temperatures.

Figure 3: Indoor temperature control verification for warehouse in Malaysia

Results and discussion

The simulation results confirm that warehouse energy performance is strongly influenced by the interaction between lighting power density, skylight configuration and cooling demand. The results should therefore be read as a combined lighting and cooling analysis, rather than as separate design decisions.

Summary of simulated cases and key energy results

The annual energy consumption results show that LPD has a major impact on total electricity use. The case with the higher LPD leads to a much higher annual energy consumption, because the lighting system uses more electricity and also adds internal heat gains to the warehouse. This confirms that reducing LPD is one of the most effective strategies for improving warehouse energy performance.

Daytime lighting dimming also has a clear effect when skylights are present. By reducing the artificial lighting load during daylight hours, dimming helps lower annual electricity consumption. However, this benefit only exists when skylights provide a daylight contribution. In the no-skylight cases, daylight dimming was removed, which means that the lighting system is assumed to operate without daylight-related reduction.

The annual energy consumption graph compares the different simulation cases and shows how each design measure affects the total electricity use of the warehouse.

Figure 4: Annual energy consumption by simulation case

The highest annual energy consumption occurs in Case 2, where the LPD is high, the skylight area is 100%, and no daytime lighting dimming is applied. This shows that high LPD strongly increases energy use, both through lighting electricity and additional internal heat gains. When daytime dimming is introduced in Case 3, annual energy consumption decreases significantly, while thermal stratification in Case 4 provides a smaller additional reduction.

Reducing the skylight area from 100% to 50% further lowers annual energy consumption by limiting solar heat gains through the roof. The low-emissivity aluminium ceiling option provides a smaller improvement, showing that material upgrades can support energy reduction but are not the main driver. Overall, the final cases show that combining low LPD, stratification, material improvement and skylight optimisation leads to much lower energy consumption, with the final no-skylight case achieving the lowest annual energy use.

Figure 5: Maximum cooling demand results by simulation case

The maximum cooling demand decreases when solar heat gains are limited. Reducing or removing skylights lowers the cooling load because less solar radiation enters through the roof, which explains why the no-skylight cases perform well in terms of peak cooling demand. Thermal stratification and the low-emissivity aluminium ceiling option provide additional reductions, although their impact is smaller than the effect of LPD and skylight configuration.

Overall, the final no-skylight case gives the lowest annual energy consumption and maximum cooling demand from an energy perspective. However, if daylight contribution, visual connection to the outdoors or operational preference is required, the reduced skylight configuration with daytime dimming remains a relevant compromise. The main finding is therefore that the best strategy depends on the balance between LPD, daylight dimming and cooling impact from solar gains.

Additional case: Alternative daylight dimming profile

An additional case was analysed to assess the influence of a different daylight dimming profile on the energy performance of the skylight configurations. This supplementary analysis was added to better understand how the assumed lighting control strategy can change the comparison between no skylight, 50% skylight and 100% skylight options.

Figure 6: Daylight dimming profiles used for the additional scenario

In this additional case, the 100% skylight configuration uses a more favourable daytime dimming profile, with a stronger reduction of artificial lighting during daylight hours. This increases the lighting energy savings associated with the full skylight configuration.

Figure 7: Annual energy consumption for different skylight configurations in the additional scenario

The annual energy consumption graph shows that the preferred skylight strategy changes depending on the LPD. At low LPD, the no-skylight option gives the lowest annual energy consumption because the lighting penalty remains limited and solar heat gains are avoided. However, as LPD increases, the 100% skylight configuration becomes more favourable. This is because the stronger daylight dimming profile reduces artificial lighting electricity enough to compensate for the additional solar heat gains.

Figure 8: Maximum cooling demand for different skylight configurations in the additional scenario

The maximum cooling demand graph shows a different trend. The no-skylight option always gives the lowest maximum cooling demand because no solar radiation enters through roof openings. By contrast, the 100% skylight configuration has the highest cooling demand, even though it can perform better in terms of annual energy consumption at high LPD. This shows that the best option depends on whether the priority is annual electricity reduction or peak cooling demand reduction.

Overall, this additional case confirms that skylight performance depends strongly on the daylight dimming profile. A full skylight configuration can become more favourable at high LPD if the lighting control strategy provides sufficient daytime dimming. However, this must still be balanced against the increase in maximum cooling demand.

Practical design recommendations

The main recommendation for air-conditioned warehouses in tropical climates is to coordinate the lighting layout with the storage rack layout from the early design stage. Luminaires should be located above aisles wherever possible, so that light reaches the working zones instead of being blocked by racks. This reduces the LPD required to achieve the target illuminance and helps lower both lighting electricity consumption and cooling demand. Minimising LPD should therefore be considered before relying on daylight strategies, as an efficient lighting system improves performance regardless of whether skylights are included.

Skylight design should be assessed together with daylight dimming and cooling demand. Daylight dimming should only be applied when a real daylight contribution is available, and skylight area should be carefully controlled because more skylight area does not necessarily mean better energy performance. Thermal stratification and low-emissivity material improvements can also support cooling load reduction, but they should be considered as complementary measures rather than the main energy-saving strategies.

Design issue	Recommended approach	Expected energy impact
Lighting layout	Place luminaires above aisles and coordinate with rack layout	Lower LPD, lower lighting electricity use and lower internal heat gains
Skylight area	Avoid excessive skylight area; compare reduced and no-skylight cases	Lower solar heat gains and lower cooling demand
Daylight dimming	Apply only when skylights/daylight are present	Reduces lighting electricity when daylight is available
Thermal stratification	Represent high-volume behaviour and control the lower occupied zone	Can reduce peak cooling demand
Material improvements	Use low-emissivity/reflective ceiling options where appropriate	Helps to reduce cooling load

Conclusion

This warehouse simulation study shows that air-conditioned warehouse energy performance depends on the interaction between lighting design, skylight configuration and cooling demand. Skylights can reduce lighting electricity through daylight dimming, but they can also increase cooling load through solar heat gains. Similarly, a poorly coordinated lighting layout can increase the required LPD and therefore raise both lighting electricity consumption and cooling demand.

The DIAlux analysis showed that placing luminaires above aisles is a much more efficient strategy for achieving the 200 lux light level target in the racking zone. This highlights the importance of coordinating the lighting layout with the storage rack layout from the early design stage.

The results also show that there is no universal answer to whether skylights should be kept or removed. At low LPD, the no-skylight option provided the lowest energy consumption and cooling demand. However, when daylight contribution or visual comfort is required, a reduced skylight configuration with daytime dimming remains a practical compromise.

Overall, the most robust strategy is to first optimise the lighting layout and minimise LPD, then assess skylight area using integrated lighting and cooling simulations. This approach can support lower electricity consumption and more efficient cooling system sizing in future warehouse projects.

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Acknowledgements

I would like to thank Gregers Reimann from IEN Consultants for giving me the opportunity to work on this topic and for his guidance and support throughout the project.