Waste Heat Recovery RTU and Hot Water System Field Installation Report
Compared to all commercial buildings types, food service has the highest energy intensity (energy consumed per square footage), and restaurants consume three times more energy per square foot compared to the average commercial building. Water heating and cooling constitute about a quarter of the energy consumed at the site (U.S. DOE, 2012). This high-level of energy consumption means there is significant opportunity for energy savings.
Both restaurant owners and customers consider energy a part of the dining experience. According to National Restaurant Association (NRA), 58 percent of restaurant customers say they are likely to make a restaurant choice based on its eco-friendly practices, with this percentage increasing to 73 percent of customers in family dining. Additionally, 96 percent of foodservice operators indicated that Gas/Energy/Utility costs were either a significant or moderate challenge in 2013, and 46 percent thought they would continue to be more of a challenge in the future (NRA, 2014). As such, high-efficiency equipment can be key components to a restaurant’s business model.
The Waste Heat Recovery (WHR) Packaged Rooftop Unit (RTU) and Hot Water System by taking the heat removed from the air conditioned space — which would normally be rejected into the atmosphere — and uses it to pre-heat hot water. Vendors, namely Rheem, are providing a solution in this space. The packaged RTU system is a conventional rooftop air conditioning unit with the ability to remove the heat from a conditioned space and transfer it to incoming water, by switching from an air-cooled to a water-cooled condenser. This is accomplished through exclusive Integration Technology advanced controls. Ultimately the packaged RTU system system can provide up to 100% of a building’s air conditioning requirements, while simultaneously pre-heating water to as high as 125° F.
The WHR components are fully integrated into the RTU (meaning it is not an add-on accessory) and therefore the refrigerant transfer valve, water pump, controls and unions are all assembled and installed at the factory and fully run tested before shipment. This pre-engineered, preinstalled feature greatly minimizes installation errors. The refrigerant-to-water heat exchanger is a 50-layer double-walled brazed-plate with copper on the refrigerant side, and stainless steel on the water site.
The goal of this study was to quantify the performance and energy savings of the Waste Heat Recovery RTU under actual operating conditions in the field. With low cost-to-savings paybacks, and extremely positive feedback from the contractors and host sites, the Waste Heat Recovery RTU is clearly a technology that can save significant energy at sites that are 4700 ft2 / 1500 GPD or larger, use the 15 ton system, and are located in a climate with 1800+ Cooling Degree Days per year.
The field demonstration site is a full-service national chain restaurant located in Laguna Hills, CA in San Diego Gas & Electric’s Service Territory, the Waste Heat Recovery RTU was installed and the performance was monitored. The site is owned by T-Bird Restaurant Group who operates approximately 62 chain restaurants. The facility is 6,000 ft2, in operation seven days a week, from 8 am to midnight, and serves approximately 475 meals per day.
The Waste Heat Recovery RTU System provided significant natural gas savings compared to the baseline water heating system. Ambient weather conditions have the largest impact on system performance, which significantly increases in hot and humid conditions. The average hot water load saved was 37.3 percent, and averaged the hot water savings of 32.8 percent. This equates to 1009 annual therms with a baseline condensing tankless heater, or 1384 annual therms with a baseline tank heater. On the cooling side, ambient conditions had the largest impact on cooling EER with the system
peaking to 17.77 Btu/Wh during the summer months, and dipping during the winter months. To understand the effect the WHR system had on the cooling performance, the data was filtered for only times when the system was in WHR mode, and compared it to the baseline system performance. During the summer and shoulder months, the system is more efficient during baseline mode, which led to an annual increase of 1,481 kWh more electricity in WHR mode; which at $.13 / kWh equates to about $190 per year.
In order to get a payback of less than 5 years with the 10 ton equipment at this load and climate, gas prices would need to increase and the host site would need a tank water heater as the baseline case.
Table 13 provides a comparative economic performance analysis, and compares different hot water usage loads to different California climates and baseline water heaters. In the high water usage, 2285 GPD, in a hot-dry or marine climate, the technology makes economic sense, regardless of the baseline water heating equipment. In a cold climate, even with a high hot water load, a user would have a less than five year payback only if they were using a higher-priced electric product. If the hot water load is reduced to a 3,000 ft2 restaurant, or 775 GPD, the product only begins to make economic sense in a hot-dry or marine climate when gas prices are higher.