Select Page

Nonresidential Thermally Driven Cooling Model

2016 Title 24 CASE Topics

Measure Overview

This code change proposal would add a new compliance option for applicants that wish to comply with Title 24 through the performance approach (whole building simulation energy trade-off). Specifically, the proposed code change recommends revisions to the ruleset for the compliance software. This will allow applicants to take credit for the energy benefits of cooling spaces with a thermally driven cooling system to demonstrate compliance with Title 24. This proposed code change affects new construction or major retrofits in nonresidential buildings or high-rise residential buildings that use thermally driven cooling in which the cooling effect is driven by heat rather than mechanical compressors. The heat source could be a boiler, solar thermal, waste heat, or a combination. The waste heat could be process heat, cogeneration, or other sources.

The proposed code change would enable applicants to model the impacts of a thermally driven cooling system that uses either an absorption chiller, adsorption chiller, or desiccant system. Absorption chillers are the most common form of thermally driven cooling systems and have been available for over 50 years. A number of manufacturers currently offer a variety of absorption chillers. Adsorption chillers are also available but they are less common. Another form of cooling that makes use of waste heat is desiccant cooling. Desiccants are used to remove water vapor out of an air stream and waste heat is used to regenerate the desiccant so it is ready to absorb more water vapor. The desiccant system operates mostly on heat energy rather than mechanical energy. Its electric demand is around 25% of a vapor-compression air conditioner. This system is especially effective at dehumidifying air and most of the cooling is latent heat transfer.

Figure 1 presents a psychrometric chart that illustrates the desiccant cooling process for a typical system. This unit includes a direct expansion (DX) pre-cooling coil, a desiccant wheel, and an integrated condensing package that reactivates the desiccant. At 6000 cfm airflow, for example, entering at 95°F db and 75°F wb, this unit uses 30 tons of compressor cooling. A conventional vapor compression DX unit uses 50 tons of compressor cooling to deliver the same conditions.


Figure 1: Psychrometric Chart for Desiccant Cooling Process

This change allows an applicant to model the energy impacts of cooling spaces with a thermally driven cooling system to demonstrate compliance with Title 24 through the performance (whole building simulation energy trade-off) approach. When on-site renewable energy or site recovered thermal energy is used to provide space cooling, the energy consumption of the proposed cooling system includes all energy consumption required to extract the heat and to convert this into cooling energy and all energy required to deliver the cooling to spaces within the building. If thermal storage is used any heat losses and equipment energy usage is also accounted for. The solar energy or thermal energy recovered from a process is treated as free and thus is not included in the energy budget. There is no credit for cooling energy created that is not used for space conditioning on site.

The proposed code change is neither a prescriptive requirement nor a mandatory requirement and does not affect the base case budget used to verify compliance with the performance approach.

Stakeholder Meeting

Several stakeholder meetings and CEC staff workshops were held regarding the Nonresidential HVAC Economizer Modifications building energy efficiency topic. The list of past meetings and hearings can be found here. If you would like to receive email updates on this CASE topic, click the link below.


  1. Final CASE Report