The Effect of Piping Heat Losses on the Efficiency of Solar Thermal Water Heating in a Net-Zero Energy Home | NIST

This study evaluates the thermal performance of a dual-tank domestic hot water (DHW) system designed for a net-zero home at the National Institute of Standards and Technology, Gaithersburg Maryland campus. Hot water demand for a virtual family of four in this facility is met with an active, indirect solar thermal water heater and a heat pump water heater that serves as the auxiliary heating method. Testing under a representative water-use schedule between February 2014 and June 2014 has shown that heat losses in interconnective plumbing between the two hot water storage tanks can significantly diminish solar fractions at times when solar thermal water heater output is high. For June, the monthly solar fraction of the DHW system was 86 %, while a 95 % solar fraction would have been achieved without heat losses. This highlights one known drawback of dual-tank hot water storage systems, where constant heating of interconnecting plumbing results in energy loss to the ambient environment and should be considered part of the total hot water storage volume. INTRODUCTION Residential buildings account for 22 % of all energy consumed in the U.S. (U.S. DOE 2011). The concept of achieving net-zero energy performance of buildings has received much attention in recent years as a means to reduce this energy expenditure. The United States’ Department of Energy Building Technologies Program, for instance, has planned to support research and development activities in new residential construction that will lead to “marketable zero energy homes in 2020” (U.S. DOE 2008). Net-zero cannot be achieved without the use of renewable energy and highly energy-efficient technologies, and without mitigating associated inefficiencies. A Net-Zero Energy Residential Test Facility (NZERTF) constructed on the Gaithersburg, Maryland campus of the National Institute of Standards and Technology (NIST) was built for the evaluation of such technologies. Details on the facility can be found in Fanney et al. (2014) and Pettit et al. (2014). The focus of the current study is to describe one such inefficiency associated with the dual-tank domestic hot water (DHW) system of the facility, which was designed to reduce the electrical demand compared to that of conventional water heating equipment. According to Hiller (1996), dual-tank systems generally have lower efficiencies than single-tank storage water heaters of similar capacity. One reason is that interconnecting plumbing is hot for a majority of the time resulting in heat loss to the ambient environment if not heavily insulated. The results of this paper highlight this type of heat loss as it negatively impacted the solar energy utilization of the DHW system over the course of a 5-month period. Tania Ullah is a mechanical engineer and William M. Healy, PhD is mechanical engineer and Group Leader at the Engineering Laboratory, National Institute of Standards and Technology, Gaithersburg, MD. BACKGROUND During the first year of building operation, the NZERTF was used to demonstrate that a home similar in size and amenities to those in the surrounding community could generate as much energy through onsite renewable sources as used by a typical family of four. The family was in fact a virtual family whose water-use and electricity-use behaviors were automated according to a weekly schedule derived from the Building America Research Benchmark Definition (Hendron et al. 2008). Each week, an average of 2570 L (680 gal) of mixed hot and cold water was utilized at sinks, showers, and baths, and by the clothes washer and dishwasher. The hot water for these activities was supplied by a solar thermal water preheating system and a heat pump water heater downstream. Description of Water Heating Systems Solar Thermal Water Heater (SWH). The residential test facility is currently using an active, closed-loop solar thermal water heating system, Figure 1. The system utilizes two Solar Rating & Certification CorporationTM (SRCC) OG100 certified single-glazed flat plate solar thermal collectors, with individual aperture dimensions of 1.1 m x 2.0 m (3.8 ft x 6.6 ft). The array is located on the porch roof and faces true south at an 18.4 tilt. The thermal energy absorbed by the collectors is transferred to a 303 L (80 gal) water storage tank, located in the house basement, with its 4500 W auxiliary heating element disabled. An insulated cross-flow heat exchanger external to the tank contains two pumps, one of which circulates a 50 % by volume propylene glycol in water solution (heat transfer fluid) used in the solar thermal collector fluid loops. The second pump circulates potable water from the storage tank through the heat exchanger. The controller turns the circulating pumps on when the temperature differential between the glycol solution leaving the collectors and the water in the lower portion of the storage tank exceeds 10 C (18 F) and turns them off when the differential is below 3 C (5 F) or when the storage tank temperature sensor exceeds a reading of 71 C (160 F). Downstream of the solar storage tank and before the piping enters the heat pump water heater, a thermostatic mixing valve is installed and set at 49 C (120 F). The cold water inlet into the thermostatic mixing valve was manually shut and not opened until June 2014 when mixing valve operation was required. Heat Pump Water Heater (HPWH). The heat pump water heater provides hot water in the event that the solar thermal water heating system cannot meet the demand. The unit consists of a 189 L (50 gal) storage tank with an integrated air source heat pump. The system is operated in the “Hybrid” mode with a temperature set-point of 49 C (120 F). The “Hybrid” mode ensures that the heat pump provides a majority of the hot water load while its 3800 W electric element only energizes when the heat pump cannot meet the hot water demand. In the “Hybrid” mode, under test conditions of 57 C (135 F) set-point temperature and 20 C (67.5 F) ambient temperature, the manufacturer-reported Energy Factor (EF), Coefficient of Performance (COP), and standby loss of the unit are 2.5, 2.6, and 0.20 C/h (0.36 F/h), respectively. Solar Storage Tank: 303 liters Heat Pump Water Heater: 189 liters F T