Heat content in drain water leaving a building is generally lost energy but can be recovered through a drain water heat recovery (DWHR) system. DWHR systems are based on a heat exchanger that consists of a section of copper drain pipe with copper supply tubing wrapped around the drain pipe in close physical contact. When installed in the simplest configuration, these heat recovery systems have no moving parts, and even more complex configurations may add little more than a circulation pump and controller. The recovered energy is usually delivered as pre-heated water to the domestic water heating system.
Figure 1. Schematic view of drain water heat recovery system
A recently completed CARD grant awarded to the Energy Center of Wisconsin has characterized the performance of DWHR as an energy efficiency technology in selected commercial applications (Figure 2). While past studies have looked at DWHR performance in single family applications, the need for this study was based on the observation that commercial occupancies can have hot water loads much larger than those in single family residences, and thus larger energy savings potential.
Figure 2. This photo shows a drain water heat recovery system installed in the test site where the simplest configuration was possible.
The investigation found that identifying commercial facilities suitable for retrofit installation of DWHR systems is a major challenge. To find four sites for use in the project, over 150 facilities were screened through phone conversations, with follow-up site visits to at least 38. The most common reasons for classifying a facility as unsuitable were:
It is expected that new construction, when systems such as DWHR can be planned into the overall design, would offer many more opportunities and cost advantages for the use of the technology compared to retrofits.
The four sites selected for the field tests of DWHR systems included an apartment building, a restaurant, a self-service laundry, and a student housing facility. Several different plumbing configurations were used in the systems. In one case the heat exchanger was installed in parallel with a balance valve intended to induce flow through the system, but this did not work well and did not produce usable results. Two of the systems were installed using a pump to circulate water through the heat exchanger. The final system was the simplest configuration, with no moving parts.
The three systems that produced usable results recovered from 7.6 to 43 million Btu of useful energy annually, corresponding to about $80 to $460 in natural gas savings (Table 1). The restaurant installation recovered by far the most energy of the installed systems. This was somewhat surprising, because the main hot water load at the restaurant is a dishwasher. Other variables, however, explain the high performance. The systems at the laundry and student housing facility had somewhat diminished heat recovery performance, most likely because drain water passing through the heat exchanger included large cold water flows.
|Site 2 Restaurant||Site 3 Laundry||Site 4 Dormitory (student housing)|
|Projected annual energy recovery (Million Btu)||43.1||18.7||7.6|
|Value of energy recovered annually||$460||$200||$81|
|Estimated system-specific annual delivered water heating energy (Million Btu)||150.5||211||28.9|
|Heat recovery as percent of estimated system-specific delivered water heating energy||29%||8.9%||26%|
Table 1. Summary of performance of installed drain water heat recovery systems (see the full report for additional notes)
Short-term monitoring tested the “system effectiveness” (heat recovery as a fraction of delivered water heating energy, which includes the effects of system piping) at each site. With one exception, these tests produced values that were, as expected, somewhat lower than effectiveness values for the heat exchangers alone. Regression analysis show a fairly strong relationship between recovered energy and the combined factors of cold water temperature and measured exterior drain line temperature. Measurement of drain line temperature appears to have promise as a method for predicting the availability of recoverable energy, though there was not enough information to formulate a general method.
The report includes a list of guidelines for selecting commercial applications where DWHR systems would most likely have the highest performance. It also recommends measures that should be evaluated in conjunction with consideration of a DWHR installation to reduce hot water energy consumption in commercial buildings.
Details of the project are given in the final report, “Drain water heat recovery: A field study of commercial applications” (pdf), available on the Commerce website. In addition to the final report, the Energy Center of Wisconsin conducted a two-part webinar in July 2013 to explore results from the study. Part 1 focuses on the energy-savings potential of DWHR systems, while part 2 addresses practical DWHR system applications. The link to the recorded webinar is on the Energy Center’s website. For more information on this CARD project, contact Bruce Nelson or CARD grant program administrator Mary Sue Lobenstein with any questions concerning this project.