How to Design District Cooling Systems
From eSociety, June 2019
District cooling (DC) could be a solution to efficient energy use. According to UN Environment, district energy systems are a best practice solution to provide a local, affordable and low-carbon energy supply to help cities scale up energy-efficiency and renewable energy use.
Modern, affordable district energy systems could reduce greenhouse gas emissions while serving the primary energy demand, according to a UN Environment report.
To help design professionals implement this cooling strategy, ASHRAE updated its District Cooling Guide and published the first edition of the Owner's Guide for Buildings Served by District Cooling. The publications fulfill a worldwide need for a modern and comprehensive complete design guidance for district cooling systems.
The forthcoming guides are written for consulting engineers with campus specialization, utility engineers, district system operating engineers, central plant design engineers and owners and designers of buildings served by district cooling. The guides provide in-depth coverage and case studies on the design, operation and maintenance of district cooling systems. They include the following:
- Comprehensive coverage of district cooling system design
- Detailed coverage of piping systems for chilled-water distribution
- Guidance on avoiding low delta-T syndrome
The principal investigator and author of these district cooling publications—Gary Phetteplace, Ph.D., P.E., Fellow/Life Member ASHRAE and one of the coauthors, Steve Tredinnick, P.E., CEM, Member ASHRAE—discuss the significance of these updated resources.
1. What is the significance of this topic?
Growth in air-conditioning, which already represents 20% of total electric use in buildings, is expected to increase rapidly due to the economic and demographic growth in the hotter regions of the world. Thus, increased efficiency of the air-conditioning process is essential to curb the expected demand growth, and district cooling systems can play a large role in achieving increased efficiency.
Without increases in air-conditioning efficiency, energy demand from air-conditioners will more than triple by 2050, according to International Energy Agency predictions. (IEA, 2018).
2. Why is it important to explore this topic now?
Global warming can be expected to result in increased demand for space cooling, even with reductions in CO2 emissions. District cooling is posed to play a large role both in increasing air-conditioning efficiency in countries where comfort cooling is commonplace now and where demand will grow, as well in areas where comfort cooling becomes increasingly necessary.
Recognizing this, the United Nations Environment Programme (UNEP, 2015) has endorsed district cooling for its potential role in achieving energy efficiency and renewable energy use in cities.
3. Why was the District Cooling Guide updated now?
District cooling continues to see increasing interest worldwide, and significant developments have occurred since the first edition of the District Cooling Guide was published about five years ago. The second edition of the District Cooling Guide has expanded coverage in all areas contained in the first edition.
4. What lessons, facts and/or guidance can an engineer working in the field take away from this topic?
DC systems are not inordinately complex, but there are a few differences from the more conventional design of in-building cooling systems, which require special attention if the resulting DC system is to be efficient and cost-effective.
5. What do ASHRAE members need to know about district cooling systems?
Design of these systems requires a few special practices not found in design of in-building cooling plants. Designers should consult these two new publications, and where doubt remains, seek assistance from an experienced designer of DC systems. As noted earlier, these systems are not overly complex, but their scale causes them to be very cost-intensive, and failure to exercise due diligence in design can result in very significant excess cost, unplanned outages and large efficiency penalties.
6. What are some common challenges engineers face while designing, installing and operating these systems?
HVAC designers will normally not be familiar with design of complex piping systems that are miles in length with many branches and loops. Pipe diameters from 2 feet to 6 feet are common, and a detailed hydraulic analysis is required for all DC piping systems, except the simplest systems. In addition, heat gains to the system should be calculated to verify their magnitude and operating cost impact, as well as the potential for unacceptable temperature gain in the supply piping. Designers should not assume that ground temperatures are close enough to supply water temperature to cause heat gains to be of negligible consequence. The District Cooling Guide contains complete coverage on the heat gain calculation procedures with examples.
7. Why was the Owner’s Guide for Buildings Served by District Cooling created?
It has become increasingly apparent to those in the DC field that the preponderance of inefficiencies in DC systems comes about due to design and operational errors within the interconnected buildings. These design errors still have impacts when buildings are not served by DC; however, the impacts are normally fewer, and they go undetected due to lack of metering, a feature of almost all DC systems.
8. How will the information in the Owner’s Guide for Buildings Served by District Cooling help building owners and their engineers?
In writing the guide we had two main focuses.
The first was to help building owners understand what DC is, what is different about buildings that are served by DC and why they should consider DC when deciding how to meet the cooling needs of their buildings.
Secondly, for the designer, we provided guidance on designing buildings for service with DC, as well as references that the designer may refer to for more detailed information.
In addition, dealing with the issues of retrofitting buildings previously not served by DC service is also covered.
REFERENCES
IEA. 2018. The Future of Cooling, Opportunities for energy-efficient air conditioning, International Energy Agency, Paris.
UNEP. 2015. District Energy in Cities, Unlocking the Potential for Energy Efficiency and Renewable Energy, United Nations Environmental Programme, Paris.