Market and Performance 描述 of 商业 Rooftop Units (CLOSED)

客观的

Packaged roof-top units (RTUs) are ubiquitous on commercial buildings throughout the United States because of their low capital cost, 可靠性, and well-developed service and distribution network. 有一些轶事证据, 然而, that these systems tend to operate inefficiently and sub-optimally. 证实或驳斥这一证据, the project team conducted a multilevel field study to characterize both new and existing RTUs in Minnesota and monitor the energy performance of existing RTUs.

方法

这个项目的第一阶段, 由第七波乐队指挥, characterized both existing RTUs and the new/replacement market. The project team collected building-level and existing RTU data for 101 surveyed buildings and analyzed the new and replacement market for RTUs in Minnesota.

CEE conducted the second stage of the project, which monitored existing RTU energy performance. The team recruited buildings for RTU monitoring from those identified in the characterization study. Twenty agreed to an in-depth site visit aimed at identifying RTUs that could be monitored for a six- to nine-month period. A total of 93 RTUs were assessed and, of those, 52 were monitored. The project team used the monitored data from the test sites to model RTU annual consumption and sizing. 基于这些模型, the required heating and cooling loads at design temperatures could be calculated and compared to the installed capacity of each RTU.

结果

描述
 

现有的rtu

• There are currently 20,700 ±3,100 Minnesota buildings with RTUs. Approximately 80% of commercial buildings are served by RTUs.
• There are approximately 136,000 ±30,000 existing RTUs in Minnesota. On average, commercial buildings served by RTUs have 6 to 7 RTUs on site.
• The average age of an existing RTU in Minnesota is 13.1年.
• The total estimated cooling capacity of RTUs in Minnesota is roughly 1.300万吨，平均10吨.每RTU 7吨. 平均满负荷制冷效率为10.6 IEER while the average for part-load cooling efficiencies is 11.2.
• The total estimated heating capacity of RTUs in Minnesota is approximately 23.8 million MBH with an average of 205 MBH per RTU.

新/替代rtu

• An estimated 6,400 RTUs are shipped to Minnesota commercial buildings annually.
• The estimated sales of RTUs in Minnesota is $88 million annually.
• Upgrading to high-performance RTUs would lead to a predicted electricity savings of 1,1.83亿千瓦时(4),037 million kBtu) and natural gas savings of 28 million therms (2,8.39亿kBtu), about  $142 million in cost savings for Minnesota businesses.

监测现有rtu

结果 from the monitoring phase of the project indicate that it is extremely difficult to predict RTU energy performance. The spaces conditioned by RTUs vary greatly from site to site, making it difficult to accurately predict annual energy consumption for an RTU. 除了, RTU性能变化很大, and the factors impacting performance cannot be easily identified using traditional, 直接的审计技术. While there are factors that may indicate high and low performance, in-depth assessments are needed to determine the cause of and solutions to poor performance in RTUs. 

The sizing evaluation of RTUs at each site showed that RTUs are typically sized to meet the cooling needs of a site and this often leads to oversizing when it comes to heating, 给居住者带来严重的舒适度问题. 当一个单位是超大的加热, the space it serves can experience dramatic swings between hot and cold air; more efficient sizing could solve these types of problems. 

国际马铃薯中心的建议

Expanding the Minnesota Technical Reference Manual (TRM) to include a wider scope of RTU-related measures will aid in the development of more comprehensive RTU programs. Other efficiency options that are now available and could be added to a comprehensive RTU program include:

• 需求控制通风;
• 改进的省煤器,
• 套管绝缘,
• 高效供风机，
• 冷凝式燃气换热器;
• 能量回收通风;
• 蒸发冷却改造包，和
• Increasingly sophisticated and intelligent controls.

Utility rebates for RTUs have historically been based on exceeding a minimum level of efficiency, 但只有在能量消耗最大的时候. Since RTUs don't typically operate at maximum capacity around the clock, 然而, it is important to find efficiencies when energy use is lower too. Developing rebates based on the Integrated Energy Efficiency Ratio (IEER) — to account for efficiency when the energy load is less — would benefit utility programs whose priority is annual energy savings.