The Importance of Lighting Design in a Cooling Dominate Climate
The cooling dominate climate of South Florida and the tropics benefits from a well-designed interior lighting plan more than other climate regions. Addressing the lighting design as early as possible in the design phase is key. If trying to reach a target energy savings, having a good lighting design in place early on can save time and money in the long run. Ultra high efficiency fixtures and high efficacy bulbs can contribute to a good design, but many times a smart layout is all it takes.
Interior lighting consistently accounts for 20%-30% of a newly designed commercial building’s annual energy consumption, (15%-25% if the building is of hotel or condo type), and 15%-20% of the cooling load. Any reduction in lighting power density results in a direct savings on the overall energy consumption, i.e. a 20% reduction in lighting power density results in a 4%-6% minimum savings on the utility bill for a typical office space.
Then, there is the additional savings associated with the cooling equipment. This additional savings is two-fold. First, the utility costs associated with cooling are lower as the cooling demand is reduced due to the smaller internal loads. Secondly, the cost of purchasing the cooling equipment is lower as the peak cooling demand is lowered.
In a cooler climate the savings might not be as appreciable or may not even exist, depending on HDD vs. CDD of the climate and peak cooling and heating loads, (i.e. the heat generated by lighting also provides heat during heating hours). But, as the heat generated inside a building is reduced, (through lower lighting power), the demand for cooling is also reduced. Therefore, in a cooling only (mostly) climate, the savings on the HVAC side can be recognized year round. The savings does not have to come from LED bulbs, but can come from optimizing the layout, using efficient fixtures, and coordination between the owner-engineer-lighting consultant. Obviously high efficiency and high efficacy lighting can play a big part in reducing lighting power inside a building, but sometimes simply moving or eliminating bulbs and fixtures can have as big of an impact.
Although all aspects of a project should be addressed to improve energy efficiency, when time and resources are limited lighting should be analyzed before expensive upgrades are brought to the table.
As a further example, take a building with the following typical cooling coil load profile (for a densely occupied 2-story office with a 30% window-wall ratio and ASHRAE 90.1 envelope characteristics);
- Roof 3%/2% (peak / off peak)
- Wall 6%/4%
- Window Conduction 4%/3%
- Glass Solar 10%/4%
- Lighting 14%/20%
- People 20%/29%
- Plug 9%/13%
- Ventilation 30%/20%
- Supply Fan 4%/5%
Using a typical enduse breakdown of; 25% lighting, 43% HVAC, 22% plug, 5% DHW, 5% site, and less than 1% heating, then a 20% reduction in lighting results in a 3% reduction in peak load and equipment size.
Viewing off peak loads from milder temperatures or even cloud cover, one can see how the 20% LPD reduction will add a further 2% reduction in total annual energy consumption, off of the cooling energy. This brings the total reduction in annual energy consumption to 7%, or roughly $0.15/sqft annually. To achieve 7% savings through envelope improvements alone would require roughly a 17% reduction in cooling loads. The savings is not available with envelope improvements alone, unless windows were replaced with well insulated walls. (The percentages off of percentages savings are not 100% scalar due to design, fan consumption, and part load operation, but it is a good estimate).
In our typical building, a 20% reduction in the lighting power density reduces the overall cooling load by more than if we doubled our wall insulation value, and more than if we effectively eliminated all heat transfer through the roof with a green roof.
All numbers are averages from building models performed and visited by the author.