Cooling Load Calculation Methods: ASHRAE RTS vs GB vs SHASE
A comprehensive comparison of cooling load calculation methodologies across ASHRAE Radiant Time Series (RTS), China GB 50736 cooling load index method, and Japan SHASE detailed simulation approach. Understand how each method handles solar gain, occupancy diversity, and thermal storage.
Overview
Cooling load calculation is significantly more complex than heating load calculation due to the time-varying nature of solar radiation, internal heat gains, and building thermal storage effects. Three distinct methodological approaches dominate the industry: ASHRAE's Radiant Time Series (RTS) method, China's GB 50736 cooling load index method, and Japan's SHASE detailed simulation approach. Each reflects a different trade-off between calculation simplicity and accuracy.
ASHRAE's RTS method is the current recommended practice for manual and spreadsheet-based cooling load calculations. GB 50736 uses a simplified index approach suitable for quick sizing. SHASE-S 101 recommends detailed hourly simulation for high-performance buildings, especially those targeting ZEH and ZEB standards.
Parameter Comparison
| Parameter | ASHRAE (RTS) | GB 50736 | SHASE |
|---|---|---|---|
| Methodology | Radiant Time Series | Cooling load index | Detailed simulation |
| Base index — residential | 90 W/m² | 100 W/m² | 90 W/m² |
| Base index — commercial office | 160 W/m² | 180 W/m² | 160 W/m² |
| Base index — retail | 200 W/m² | 220 W/m² | 200 W/m² |
| Solar gain handling | Hourly RTS factors | Simplified zone factor | Hourly simulation |
| Occupancy diversity | Profile-based | Fixed factor | Profile-based |
| Thermal storage | Radiant time factors | Not included | Full dynamic model |
| Latent load method | Design wet-bulb + occupancy | Climate zone index | Hourly psychrometric |
| Design outdoor condition | 0.4% / 1% dry-bulb + MCWB | Summer design DB/WB | Region-specific design day |
Formula Differences
The ASHRAE RTS method separates total cooling load into convective and radiant portions. Convective gains become instantaneous cooling loads, while radiant gains are distributed over time using radiant time factors (RTFs). The total cooling load at any hour is the sum of convective gains plus the time-delayed radiant gains. The formula is Qtotal,h = Qconv,h + Σ(ri × Qrad,h-i) where ri are the radiant time factors.
GB 50736 uses a far simpler approach: Q = A × q where A is the floor area and q is the cooling load index in W/m². This index is pre-calculated for different building types and climate zones and includes envelope transmission, solar radiation, people, lighting, and equipment gains. While fast, this method cannot capture building-specific variations or thermal storage effects.
SHASE-S 101 recommends full dynamic thermal simulation using response factor methods or finite difference models. This captures the building's thermal mass, time-varying internal gains, and solar heat gain through windows with hourly angular-dependent shading coefficients. The method requires detailed building geometry and construction layer information.
Design Impact Analysis
The GB cooling load index method tends to produce conservative (higher) peak load estimates because it does not account for thermal storage diversity. For a typical office building, the GB method may yield 10-20% higher peak cooling loads compared to ASHRAE RTS, leading to oversized chillers and air handling equipment. ASHRAE RTS and SHASE methods provide more accurate sizing, reducing first cost and improving part-load performance.
The choice of method has significant implications for energy modeling accuracy. Simplified index methods cannot evaluate the impact of design choices such as external shading, high-performance glazing, or thermal mass. For projects targeting green building certifications (LEED, WELL, CASBEE), detailed methods like ASHRAE RTS or SHASE simulation are strongly recommended.
Regional Applicability
ASHRAE RTS is widely used in North America, the Middle East, and Southeast Asia. It is suitable for all building types and climate zones. GB 50736 is mandatory for Chinese projects and works well for standard building types in China's five climate zones but lacks flexibility for non-standard designs. SHASE detailed simulation is preferred in Japan, especially for buildings targeting ZEH (residential) or ZEB (commercial) certification. Many Japanese consultants also use SHASE methods for energy performance contracting and commissioning.
Frequently Asked Questions
What is the Radiant Time Series (RTS) method in ASHRAE?
The Radiant Time Series method is ASHRAE's recommended cooling load calculation method that separates radiant and convective heat gains, then applies radiant time factors to convert radiant gains into cooling loads over multiple hours. It is more accurate than the simplified CLTD/CLF method.
How does the GB 50736 cooling load index method work?
GB 50736 uses a simplified cooling load index method where the total cooling load is calculated as the floor area multiplied by a predefined index value (e.g., 100 W/m² for residential, 180 W/m² for commercial). The index includes all internal and envelope gains and is adjusted for climate zone and building characteristics.
What base cooling load index does SHASE recommend?
SHASE recommends a base cooling load index of 90 W/m² for residential buildings and 160 W/m² for commercial office buildings, with adjustments for occupancy density, equipment heat gain, and glazing ratio.
Which cooling load method is most accurate?
ASHRAE's RTS method and SHASE's detailed simulation approach both provide higher accuracy than the GB index method because they account for thermal storage, time-varying solar gains, and zone-specific diversity. The GB index method is simpler and faster but may overestimate peak loads by 10-20%.
Can I use the GB cooling load index for non-Chinese projects?
The GB 50736 cooling load indices are calibrated for Chinese climate zones and construction standards. Using them directly for other regions may produce inaccurate results. For international projects, ASHRAE RTS or SHASE methods are more appropriate.