Energy efficiency, SIPS, geothermal, and solar PV used in near zero-energy house.

ABSTRACT

This paper describes the thermal performance of the third in a series of five affordable, near zero-energy house prototypes built in Lenoir City, Tennessee. This house features structural insulated panels, geothermal space heating and cooling, and a 2 kWp grid-tied photovoltaic system. The cost to conduct this research house in 2004 was around $100,000, and in 2005 it had a net daily cost for off-site energy of only $0.79 (based on the local electric rate of $0.068/kWh). This all-electric house was heated and ventilated according to ASHRAE Standard 62.2 in the winter of 2003-2004 for less than $40.

INTRODUCTION

This paper describes the thermal performance of the third in a series of five affordable, near zero-energy house (ZEH) prototypes. This Habitat for Humanity (HfH) house was designed to surpass the required energy efficiency set forth by the US Department of Energy (DOE) Building America benchmark house model by 50% (DOE 2005; Hendron et al. 2004). The house is located in Lenoir City, Tennessee, near Knoxville, which has a mixed humid climate with an average of 3900 HDD base 65[degrees]F. The design process for the series of five near zero-energy houses started at the Oak Ridge National Laboratory (ORNL). The fifth was completed in 2005 and will not be discussed in any detail in this paper.

Whole House

Hot box tests and evaluations from hundreds of different wall systems were examined to select a wall system with minimum thermal shorts and inherent airtightness potential (Christian and Kosny 1995, 1996). The motivation for locating the HVAC distribution system inside the insulating envelope came from multiyear tests of HVAC systems in test houses of similar size and identical location as the near zero-energy houses discussed in this paper (Vineyard et al. 2003).

All five of the near zero-energy test houses had a DOE Building America Consortia member on the design team. Whole house building computer simulation software was used, either Energy Gauge (UCF 2005) or Rem/Rate (AEC 2006), to run design optimization prior to selecting specific building components.

One major design constraint was that the site did not have natural gas available; therefore, these are all-electric houses. A second limitation was that the HfH International guidelines were followed relative to house size, number of bathrooms, etc. These houses are located in an all-HfH subdivision. This meant that the house amenities for the near zero-energy test houses could not be substantially different from those of the houses in the rest of the neighborhood. The limitation of eight windows was one of these constraints. ZEH2 and ZEH3 both had 6/12 pitch roofs, compared to 4/12 for all the other houses in the neighborhood. The 6/12 SIP pitch roof allowed sufficient space under the cathedral roof to design two 50 [ft.sup.2] (4.6 [m.sup.2]) children's lofts. Although the design included these features, they were not built due to the "no extra feature" limitation imposed by HfH, as well as concern about the safety of the young children in the families identified for these houses.

A final constraint was that the design team only selected technologies that researchers believe will have affordable potential installation costs for typical house buyers by 2010. This required assumptions that technological development, economics of scale with market growth, and utility, state, and federal incentives would all lead to lower homeowner costs in the near future.

An electric water heater was connected to the geothermal system and at the design stages of this project was thought to provide significant desuperheating to eliminate summertime electric energy needs for providing hot water. A solar water heater was not selected because the additional $1500-plus installation cost produced a limited projected home energy rating system (HERS) increase for this climate. The fifth in the series of these test houses does have a solar water heater with which to compare measured results against the heat pump water heater coupled to the refrigerator waste heat in ZEH1, ZEH2, and ZEH4.

The third zero-energy home (ZEH3) has monthly energy bills, after solar credits, with typical internal energy usage of around $24--a 72% reduction in the cost of energy from a HERS reference house estimated by Energy Gauge version 14 (UCF 2005). The reference house of the same size, which just meets the 2003 International Energy Conservation Code (IECC), had an annual consumption of 15,168 kWh, which would have generated a monthly energy cost of $86. This 1060 [ft.sup.2] (100 [m.sup.2]) home has been occupied by a couple and their young child and utilized from time to time as a day care center. The floor plan for ZEH3 is shown in Figure 1 and a cross section in Figure 2. Forty-nine sensors were installed around the house to monitor the performance of the energy-saving technology and solar photovoltaic (PV) system. The resulting data were used to evaluate the design of the home and improve upon the energy systems to be used for future homes. This particular home utilizes many unique design features that have proved to be extremely energy efficient. This section will describe the ZEH3 features in depth. Tables 1 and 2 summarize the ZEH3 features as well as the features of ZEH1, ZEH2, and ZEH4.

Foundation

The three-bedroom, 1060 [ft.sup.2] (100 [m.sup.2]) house sits on an unvented, 5.5 ft (1.7 m) high crawl space. A black, 6 mil polyethylene ground cover stretches across the entire crawlspace dirt floor and runs 1 ft (0.3 m) up each wall and pilaster. The foundation walls are 8 in. (0.2 m) concrete block with 2 in. (51 mm) polyisocyanurate boards lining the inside surfaces. This insulating layer gives the foundation walls a total R-value of 12 h x [ft.sup.2] x [degrees]F/Btu (2.11 [m.sup.2] x [degrees]C/W), which indicates the wall's resistance to heat flow. This crawlspace is very similar to that of ZEH2, except there are no motorized dampers opened in the wintertime (Christian et al. 2006).

The crawlspace for ZEH3 houses the geothermal compressor and desuperheater equipment, which generates a very small amount of heat into the crawlspace year-round. In the first two weeks of July 2004, the ZEH3 crawlspace average temperature was 71.6[degrees]F (22[degrees]C), which is about 3[degrees]F (1.7[degrees]C) higher than ZEH2's during the same two-week period. The average dew point was 69[degrees]F (21[degrees]C), which was 2[degrees]F (1.1[degrees]C) higher than that measured in ZEH2. The height of this space is about 6 in. (0.15 m) higher than ZEH2. A layer of 3/4 in. (19 mm) thick extruded polystyrene (XPS) boards span the surface of the crawlspace ceiling. There are four foundation vents in the crawlspace that are sealed with 2 in. (51 mm) thick pieces of XPS foam from the inside. These vents remain operable by just removing the XPS plugs.

[FIGURE 1 OMITTED]

[FIGURE 2 OMITTED]

Walls

The house walls are composed of 6.5 in. (0.165 m) structural insulated panels (SIPs). SIPs consist of a single core of rigid 1 lb/[ft.sup.3] (16 kg/[m.sup.3]) dense expanded polystyrene (EPS) foam laminated on both sides by structural oriented strand board (OSB) and pressure-treated to form a single, solid panel. These panels come prefabricated and can be cut to any desired size to enable quick on-site assembly by fitting splines through pre-routed grooves. The splines used on these walls are "structural splines," cut all the way through the insulating core of the SIPs. There are a total of thirty-one 2 x 6 in. (51 x 153 mm) structural splines in the walls that have a total perimeter length of 145 ft (44 m). This does account for the extra six 2 x 6 in. (51 x 153 mm) splines needed to support the ridge beam in both gable walls. Additional 2 x 6's are used at the perimeter of each window and door rough opening. All of the SIPs in ZEH3 are composed of 1 lb/[ft.sup.3] (16 kg/[m.sup.3]) EPS foam between 7/16 in. (11 mm) OSB panels. The wall panels are 8 ft (2.4 m) high on the eave walls with various higher lengths to fit the 6/12 pitch of the roof on the gabled walls. On each of the panel connections, black peel-and-stick tape is used to seal off any possible leakage passing through the panel seams on the inside surface. House wrap and vinyl siding protect the outer surface of the wall panels, creating a total R-value of 21 h x [ft.sup.2] x [degrees]F/Btu (3.7 [m.sup.2] x [degrees]C/W) for the walls.

Roof

The roof structure of ZEH3 is composed of 10 in. (0.25 m) SIPs held together by surface splines. The panel joints are well sealed with expanding two-part foam sprayed into drilled holes from the top surface of the roof along each panel-to-panel junction. The holes on all roof panel joints were drilled every 12 to 18 in. (0.3 to 0.46 m) right along the seam. Every other hole was foamed. After the foam cured, any holes that did not have foam showing required additional holes to determine the extent of the foam sealant and then refoaming to fill any voids. The panels in this roof consist of ten 8 ft (2.4 m) wide and 16 ft, 2 in. (4.9 m) long spans from roof eave to ridge. Two additional 4 ft (1.2 m) wide by 16 ft, 2 in. (4.9 m) long panels were needed to complete the entire roof. Above the OSB is a layer of titanium underlayment that is 100% asphalt-free and very lightweight and provides additional reflectivity. The roofing is continuous standing seam 24-gauge steel panels. The forest-green paint on the steel roof has a special pigment that makes the roof 35% more reflective than the similar roof of the second zero-energy-house (ZEH2). The steel roofing reflectivity is 23%, and the total R-value of the roof system is 35 h x [ft.sup.2] x [degrees]F/Btu (6.2 [m.sup.2] x [degrees]C/W). The standing seams on the roof allow for the attachment of the PV system without any penetration into the roof. This is advantageous because the house is designed to operate under very airtight conditions. By using a clipping mechanism, shown in Figure 3, the PV arrays were installed on the roof with no penetrations.

[FIGURE 3 OMITTED]

Mechanical, Electrical, and Plumbing

Domestic Water Heating. ZEH3 is equipped with a 50 gal (189 L) electric water heater that has a 94% efficiency (EF = 0.94), the highest available in October 2003. The water heater is located in a utility closet within the conditioned kitchen area. The occupants on average use only 28 gal (106 L) of hot water per day. This is 42% lower than the 2003 Building America benchmark definition for a three-bedroom household hot water usage. This is due in part to the fact that the occupants have only one young child who is not yet taking daily showers, which draw the largest hot water usage per day. Also the water heater is very close to the hot water outlets, which minimizes distribution losses.

[FIGURE 4 OMITTED]

The house has a desuperheater unit, which is an auxiliary heat exchanger that uses superheated gases expelled from the heat pump's compressor to heat water for the hot water supply. The desuperheater unit produced 14.2% of the hot water used by the household for the month of July in 2004. Figure 4 shows sample weekend morning hot water usage and desuperheater flow during the cooling season. The desuperheater remains at a low flow, but during the late morning hours when the compressor runs more frequently due to an increased demand for cooling, the desuperheater produces a peak flow rate of 0.485 gal (1.8 L) per 15-minute period, 6.8% of the peak hot water demand for that time period.

[FIGURE 5 OMITTED]

Space Heating and Cooling. ZEH3 has a 2 ton geothermal direct exchange heat pump to provide for space heating and cooling needs. The unit is a direct exchange system and uses the refrigerant R-417a as the working fluid through the closed-loop system. This was the first time R-417 refrigerant was used in this particular type of unit. The geothermal system works by utilizing the ground as the heat source during the heating season and as a heat sink during the cooling season.

There are three copper pipeline loops running to and from the compressor unit, located in the crawlspace, through the backyard in three 200 ft (61 m) long, 5 ft (1.5 m) deep, 20 in. (0.5 m) wide trenches (Figure 5). The bottoms of the trenches are filled with 3 in. (76 mm) of very fine gravel, followed by the copper pipelines and a water soaker hose in the first 100 ft (30 m) of each trench. The soaker hose is installed to cool the surrounding earth only in the late summer if the surrounding earth temperature is too high to allow adequate heat rejection to the ground. This can be detected by a sensor on the head pressure of the compressor. If the pressure goes above 240 psi (17 kg/[cm.sup.2]), a relay valve will open the domestic water supply to the soaker hose until the head pressure drops below the setpoint. Although not monitored, this soaker hose did not appear to be needed because the house cooling load never required continuous operation even during the peak cooling periods. The copper heat exchanger is covered with another 3 in. (76 mm) of fine limestone gravel and backfilled. The area on top of the heat exchanger field was graded and top soil capped the trenches. The entire installation required two backhoes working most of one day and two HVAC technicians installing the copper tubing. About 25 tons (1814 kg) of gravel was spread into these trenches.

In the heating season, after the heat from the ground is carried by the refrigerant to the geothermal unit, an electrically driven compressor and heat exchangers carry out a vapor compression cycle (the same principle utilized by a refrigerator and air-to-air heat pumps) to concentrate the earth's energy and produce warmer air to heat the home. In the cooling season, the process is reversed and the unit uses the ground as a heat sink, providing cooling to the inside space of the house. The unit has a coefficient of performance (COP) of 3.7 (measured using a 36[degrees]F (2.2[degrees]C) water tank as the surrogate soil) and a cooling energy efficiency rating (EER) of 16.6 at 77[degrees]F (25[degrees]C) tank temperature, which is the average temperature of the soil in July near the ground heat exchanger. The average earth temperature next to the ground coil in the heating seasons is around 47[degrees]F (8.3[degrees]C) in this location and a slightly milder winter than average. Figure 6 shows the average monthly 5 ft (1.5 m) deep ground temperatures at four distances from the copper tubing for each month from January to December 2004. These temperature sensors are located at the same depth as the pipe, out from a point 50 ft (15 m) from the compressor outlet. The temperature closest to the tubing is the warmest in the cooling season, but the difference between this and the far-field temperature, a temperature more than 15 ft (5 m) away from the pipelines, never exceeds 5[degrees]F (3[degrees]C). This indicates that the ground is accepting the house's heat rejection in the summer with little soil temperature impact. A diagram of the entire geothermal direct exchange system and desuperheater is shown in Figure 7.

[FIGURE 6 OMITTED]

[FIGURE 7 OMITTED]

A validated computer model has been developed for this direct exchange geothermal heat pump system to facilitate the use of current ZEH technologies and optimize them for other climatic zones in US. This model consists of a heat pump model with horizontal ground pipes and a detailed building model that reflects heating and cooling loads in the TRNSYS simulation environment (SEL 2006). The developed model has been calibrated and validated with the measurement data. This model predicts annual energy consumption and the performance of the heat pump in cooling and heating seasons. The simulation model calculates annual energy consumption within 10% of the field measurements, as shown in Figure 8.

Ventilation. The mechanical ventilation system has an ASHRAE Standard 62.2 compliant (ASHRAE 2004) average supply rate of 40 cfm (1.13 [m.sup.3]/min) that is pulled through a 6 in. (0.15 m) fresh air vent into the return duct of the space heat pump indoor fan unit. House air is simultaneously exhausted through the bathroom fan unit. If the compressor is not needed for at least 20 successive minutes, the damper on the ventilation air supply duct opens, allowing fresh air to be filtered and sent to the interior space. Once the damper is open, the air distribution fan runs at 50% capacity for 10 minutes and then shuts off for another 20 minutes before running again. If the compressor is running, the mechanical damper is open long enough to allow the prescribed amount of fresh air to come in; the air is immediately conditioned. This is particularly desirable in the summer, in the mixed humid climate, to remove the moisture prior to entering the space.

Because the near zero-energy houses are built with a superbly airtight envelope (< 1 ACH at 50 Pa), they can be designed to bring in a prescribed amount of fresh air per day to satisfy the requirement defined by ASHRAE Standard 62.2-2004, Ventilation and Acceptable Indoor Air Quality in Low-rise Residential Buildings (ASHRAE 2004). While bringing in adequate fresh air significantly improves the air quality in the home, the relative humidity levels tend to be higher than desirable in the heavy cooling months of June through August in the mixed humid climate. Therefore, more energy is required to control the relative humidity levels at a comfortable condition. Figure 9 displays the average monthly interior temperatures and relative humidities for ZEH3. The temperature stays around 72[degrees]F (22[degrees]C) in the summer and the relative humidity averages just above 60%. ZEH2's average temperature is around 75[degrees]F (24[degrees]C), and its relative humidity levels average 35% in the heating season. Both ZEH2 and ZEH3 have programmable thermostats that attempt to control dehumidification. The thermostats in ZEH2 and in ZEH3 were both programmed for precision control of RH set at 55%. If the RH is higher than that, they continue to cool at a slow indoor fan speed until the air temperature drops 3[degrees]F (1.7[degrees]C) below the thermostat setpoint. At the beginning of the cooling season, both thermostats were programmed 24/7at 75[degrees]F (24[degrees]C) and the desired RH of 55%.

Figure 9 also shows the average monthly temperatures and relative humidities in the crawlspace. The summertime RH in the crawlspace is higher than ideal, although it is very common to find 100% RH in vented crawlspaces during the summer in the east Tennessee climate.

[FIGURE 8 OMITTED]

[FIGURE 9 OMITTED]

Photovoltaic System. The PV system used on the house has a 1.98 kWp capacity comprising twelve 165 W multi-crystalline silicon modules. The entire system is 12.68% efficient. The PV system is attached to the 26.6[degrees] tilted roof (6/12 pitch) and faces directly south. With only 12 panels, the PV resembles an attractive skylight (Figure 10). Three to four times more PV modules could fit on the south-facing roof. The solar has the potential to provide about 24% of the total energy load. A key finding here is that there is room on the roof to expand the PV to attain zero-energy status.

All PV cells contain a semiconductor material, usually silicon, that absorbs the light energy from the sun. This energy allows electrons to break loose and flow freely. Electric fields in the PV cell force the electrons to flow in a certain direction, and the current that results can be drawn by placing metal contacts on the top and bottom of the cell. The direct current (DC) from the sun's light is then converted to usable alternating current (AC) by a grid-tied solar inverter located on the north side of the house. This AC current then goes to the electrical panel box on the house before being used in the house or sent to the utility grid. There are two electric utility meters on the north side of the house as well. One of these keeps track of the total amount of AC electricity the PV is producing. The other meter is a net-meter and has the capability of going both forward and backward, depending on whether the house is producing more energy than it is using or vice versa. The sum of these two meter readings yields the total amount of energy the house consumes.

[FIGURE 10 OMITTED]

Instrumentation

There are a total of 49 sensors capturing the continuous thermal and energy performance of ZEH3. The sensors include thermocouples to measure the temperatures of HVAC equipment and different interior and exterior spaces, duty cycle valves to monitor equipment usage, and electric power transducers to keep track of the amount of energy being used throughout the house. Unique to ZEH3 are the thermocouples that measure the temperature of the ground 1, 6, 12, and 24 in. (25, 150, 300, and 600 mm) away from the copper pipeline 50 ft (15 m) out along an outgoing loop, as mentioned previously. There is also a thermocouple on a weld of one of the copper loops 100 ft (30 m) out, where the pipe goes from 3/8 to 1/4 in. (10 to 6 mm) diameter, and one at a far distance from the pipelines (far field). The measurements from all 49 sensors are sent to the data acquisition system, located in the crawlspace, every 15 minutes. The data acquisition system then transmits the previous day's collected data to a computer in the ORNL laboratory at 2 a.m. over a phone line. This house was monitored for more than two years of occupancy.

Energy Usage and Cost

An unexpected occurrence happened in ZEH3 during the summer of 2004. The lady of the house became the daily caretaker of four additional children, making the total occupancy during the daytime six. Figure 11 shows the energy loads on ZEH2 and ZEH3 during the summer months of 2004. ZEH3 was using 30% more energy, with most of the increased load coming from "other" plug loads, such as lighting, washer and dryer usage, and other appliances.

[FIGURE 11 OMITTED]

ZEH3 has a very airtight envelope. The calculated air changes per hour (ACH) were 0.03 at 4 Pa for the envelope alone. This measurement was obtained by performing a blower door test upon completion of construction. With this test, one can carefully measure the amount of air leakage through the house walls, windows, electrical outlets, etc. The whole house ACH at 4 Pa was 0.06. Ten conventional older houses in this same area were blower door tested with results ranging from 0.24 to 1.15 ACH at 4 Pa. The average new house built by the same HfH affiliate that has constructed all five of the near zero-energy houses with the more traditional 2 x 4 wood frame rather than SIPs has a 0.20 ACH at 4 Pa. Prior to our involvement, the older wood-frame houses were measured to have an average natural ACH of around 0.26. Such an airtight building allows for adequate ventilation to supply superior indoor air quality to the residents. Therefore, the zero-energy houses bring in more fresh air per day in the spring, summer, and fall but considerably less air in the winter than conventional construction, which generally has more air exchanges than necessary to attain good indoor air quality, resulting in excess consumption of heating energy.

Throughout the heating season, ZEH3 performed very well as compared to ZEH1 and ZEH2. The geothermal direct exchange system worked extremely efficiently to condition the house at a low energy cost. The HERS is 91.5, and the house is approximately 60% more energy efficient than traditional HfH homes. The total raw energy use of the house for an entire year was 11,000 kWh, while a traditional older home of its size uses around 26,970 kWh for a full year. It should be noted that 67% of the raw energy used by ZEH3, 60% in ZEH1, 44% in ZEH2, and 54% in ZEH4 is for lights, appliances, and other plug loads. The internal loads in ZEH3 are about 8 kWh per day higher than those in ZEH2.

ZEH3 occupants consumed a total of 11014 kWh for one complete year from March 1, 2004, through February 28, 2005. During this period, the homeowners paid $0.068/kWh for their electricity. This house sells green power back to the local utility. The contractual arrangement is that the utility will pay the homeowner $0.15/kWh for all the solar power produced by the PV for 10 years whether the homeowner uses it or not. The utility turns around and sells this "green power" at a about $0.10/kWh to other customers on the grid. During this same annual period, the solar system generated 2241 kWh. Twenty-nine percent of the solar was collected at a time when it was not needed in the house. Table 3 shows the energy usage broken down into heating, cooling, hot water, and other.

The net daily cost for off-site energy to run this all-electric house was $1.13. The other loads in this house, totaling 7388 for an entire year, were much higher than ZEH1 (5907 kWh/yr), ZEH2 (4314 kWh/yr), and the suggested internal loads from the Building America benchmark house (6512 kWh/yr). In part this is explained by the house being occupied during the day on most days, with occasional day-care services, and a significant load attributed to Christmas decorations from November to January (the homeowner reported 100 strings of lights). To be able to more directly compare ZEH3 with the other houses and the Building America benchmark, the "other loads" for ZEH3 are reduced for comparison purposes. The average of ZEH1, ZEH2, and two Building America benchmark house "other" loads is 5604 kWh/yr, or $1.04 /day. This would reduce the other energy loads by 1784 kWh and the cost to the homeowner for off-site energy, shown in Table 3, by $0.34/day, resulting in an average daily cost for all off-site energy of $0.79/day. This compares to ZEH1 using the same $0.068 kWh at $1.01/day (Christian 2005) and ZEH2 at $0.88/day (Christian et al. 2006).

The HVAC cost for ZEH3 with the geothermal heat pump averaged only $0.44/day. The HVAC cost for ZEH1 with an SEER-13 single-speed compressor using the same $0.068/kWh electricity came to $0.51/day. The final adjusted HVAC daily cost for ZEH2 came to $0.85/day.

With an adjusted "other" load for ZEH3 of 5604 kWh/yr, this all-electric house's fraction of solar energy collected on site amounts to 24% of the total electric demand of 9230 kWh/yr. This is an improvement from ZEH 1, which attained 20% of total electric demand supplied by solar energy. ZEH2 attained 23% of its total energy needs from the solar PV system. ZEH4 reached 27%, with a slightly larger PV system, 2.2 kWp compared to 2.0 kWp.

Construction Costs

ZEH3's total cost was about 60% higher than the cost of a benchmark house of similar size and typical HfH design. The extra cost resulted mainly from the PV system, but also from the geothermal system, desuperheater, high-efficiency water heater, and the high-quality airtight envelope components of the house. Table 4 compares the costs of all the ZEH homes built thus far as well as the base house.

Public Views

All of the zero-energy houses built thus far have received significant recognition by the surrounding community and even other areas of the US and abroad. At the very least, people seem intrigued by the idea that their utility company will pay them a premium for the energy they produce with a PV system.

By issuing a questionnaire to the Tellico Village Kiwanis group of HfH volunteers, most of whom worked on building ZEH2, ZEH3, and ZEH5, we learned that the project is of great interest to many people. Most of those surveyed were not in the market for a new home, but Figure 12 summarizes their willingness to pay more for the construction of a zero-energy home than for a conventional home.

It was also discovered that 77% of those surveyed talked with family members or friends about the zero-energy home project. Eighty-four percent would recommend such a zero-energy housing design to a friend who is in the market for a new home. These results show that the zero-energy housing project is providing an effective avenue to publicize the advantages and importance of energy-efficient residences.

Lessons Learned

One problem that has been identified by the homeowners of ZEH3 is that the roof makes a loud popping noise once every morning and evening on hot sunny days. Around 8 a.m. and 6 p.m., starting on the east side of the south-facing roof and continuing to the west side of the south-facing metal roof, there are approximately 10 popping sounds in a row. The speculation is that this noise is due to the movement of the metal roof when it heats up and cools down. The metal roof bows upward slightly when it gets warm and expands and then pops back to normal when it cools down and contracts. The loudest pops are heard in the evening when the roof pops back down into place from the slightly bowed shape. This same observation was noted by the homeowners of ZEH2, who have the same roofing structure but with a lower solar reflecting paint, which would suggest that the problem should be less severe on ZEH3. One way to fix this problem is by fastening the roofing directly to the roof deck and using standoffs to hold the PV modules on the roof. However, this more common method requires penetrating the roof membrane and flashing, creating a higher risk of a roof leak if not installed correctly. Nevertheless, this is the approach utilized in ZEH4.

Another problem occurred with the geothermal exchange system in the house. This system is expected to provide considerable energy savings year-round, but only significant energy savings in the heating season were observed. In the cooling season, the desuperheater unit was supposed to use the waste heat from the compressor to heat water for nearly all the hot water supply needed. However, this expectation was not observed and it wasn't until June when it was discovered that the inlet and outlet water lines from the desuperheater exchanger and the hot water heater were not plumbed correctly and needed to be reversed. After the plumbing was corrected, the desuperheater still did not perform as well as expected. The desuperheater provides less than 10% of the hot water for ZEH3. Unless temperatures can be adjusted and more intelligent controls allowing predictions of when hot water will be needed are available, it will be hard to pay back the $510 cost of the desuperheater unit in this type of near zero-energy application.

[FIGURE 12 OMITTED]

ZEH3 has produced 24% of the total energy with the PV system, compared to 23% in ZEH2 and 20% in ZEH1. The net cost per day for off-site energy also has been progressing downward. ZEH3 had an average net daily cost for off-site power of $0.79/day, compared to $0.88 for ZEH2 and $1.01 for ZEH1. Using the measured solar PV generation data on this house, which delivered 2241 kWh/year, and assuming that the electric utility was willing to pay $0.13/kWh more for the green power and peak load reduction benefits of houses such as ZEH3, the homeowner would have experienced zero-energy net cost for grid power. In other words, for houses that are very efficient and have a modest amount of solar PV, a second-tier solar buyback arrangement could be created that would provide a very attractive marketing feature to zero-energy houses. The $0.28 kWh solar buyback for all AC solar power generated could only be offered for an all-electric house that meets the 50% heating, cooling, and hot water savings requirements of the US Energy Policy Act of 2005 (IRS 2006).

By monitoring the zero-energy houses, we are able to observe fairly quickly when any equipment is not working correctly. The observations that led to determining that the solar inverter on ZEH2 needed to be replaced, that ZEH2's heat pump needed to be recharged, and that the ZEH3 desuperheater plumbing was incorrect would most likely have gone unnoticed by the typical homeowner. This equipment was still functioning, though not to its greatest efficiency. Therefore, it is important that systems be improved to reduce the risk of improper installation and operation and suboptimal performance.

As the project continued, considerations were given to past performance and cost of the different components of each previous near zero-energy house prototype. The long-term goal after the lessons learned on ZEH3 are clearly that additional research on improving efficiency and first cost reductions are needed to make annual life-cycle analysis that leads to mortgage payments plus energy costs of these near zero-energy houses equal to today's typical new housing.

SUMMARY

This paper reports on the continuous 15-minute thermal performance measurements taken for more than one year of the third in a series of five test houses that are approaching affordable zero energy in Lenoir City, Tennessee. This all-electric house uses SIPS, direct exchange geothermal, 2 kWp solar PV, and extensive energy-efficient details to produce a small house for under $100/[ft.sup.2] of construction cost. This house was heated and ventilated according to ASHRAE Standard 62.2 (ASHRAE 2004) in the winter of 2003-2004 for less than $40. ZEH3, with a 1060 [ft.sup.2] (100 [m.sup.2]) floor plan, produced 24% of the total energy with the PV system, compared to 23% in ZEH2 and 20% in ZEH1. There is room on the south-facing 6/12 pitch roof for four times more PV modules.

A key finding is that the PV could be expanded to attain zero energy status. The net cost per day for all off-site energy also has been progressing downward. ZEH3 had an average net daily cost for off-site power of $0.79/day, compared to $0.88 for ZEH2 and $1.01 for ZEH1. Using the measured solar PV generation data on this house, which delivered 2241 kWh/year, and assuming that the electric utility was willing to pay $0.28/kWh (currently the residential kWh rate is $0.068 and the solar buyback rate is $0.15 kWh) for the green power and peak load reduction benefits of houses such as ZEH3, the homeowner would have experienced zero-energy net cost for all of their off-site energy needs.

REFERENCES

AEC. 2006. Rem/Rate. http://www.eere.energy.gov/buildings/tools_directory/software.cfm/ID=50/page-name=alpha_list. Architectural Energy Corporation, Boulder, Colorado.

ASHRAE. 2004. ANSI/ASHRAE Standard 62.2, Ventilation and Acceptable Indoor Air Quality in Low-Rise Residential Buildings. Atlanta: American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc.

Christian, J. 2005. Small building applications: Ultra-low energy residences. ASHRAE Journal 47(1):20-26.

Christian, J.E., and J. Kosny. 1996. Thermal performance and wall ratings. ASHRAE Journal 38(3):55-65.

Christian, J.E., and J. Kosny. 1995. Towards a national opaque wall rating label. ASHRAE/DOE Thermal Performance of the Exterior Envelopes of Buildings Conference VI, Clearwater Beach, FL, December 4-8.

Christian, J., P. Pate, P. Childs, and J. Atchley. 2006. Small house with construction cost of $100K, Total energy cost of $0.88 a Day. ASHRAE Transactions 112(1):269-80.

DOE. 2005. DOE Building America Benchmark Model. US Department of Energy. http://www.eere.energy.gov/buildings/building_america/docs/benchmark_2005.doc.

Hendron, R., R. Anderson, R. Judkoff, C. Christensen, M. Eastment, P. Norton, P. Reeves, and E. Hancock. 2004. Building america performance analysis procedures, rev. 1. NREL/TP-550-35567. National Renewable Eneregy Laboratory, Golden, CO.

IRS. 2006 Treasury and IRS guidance on energy credit to home builders, IR-2006, February 21. www.irs.gov/newsroom/article/0,,id=154658,00.html. Internal Revenue Service.

SEL. 2006. TRYSYS. http://www.eere.energy.gov/buildings/tools_directory/software.cfm/ID=58/pagename_submenu=renewable_energy/pagename_menu=whole_building_analysis/pagename=subjects. Solar Energy Laboratory, University of Wisconsin, Madison.

UCF. 2005. Energy Gauge. http://energygauge.com/. University of Central Florida, Cocoa, FL.

Vineyard, E.A., R.A. Linkous, and E. Baskin. 2003. Measured performance of conventional and high-velocity distribution systems in attic and space locations. ASHRAE Transactions 109(2):45-51.

Jeffrey E. Christian

Associate Member ASHRAE

Lauren Richards

Phil Childs

Jerry Atchley

Hyeun Moon

Jeffrey E. Christian, Phil Childs, Jerry Atchley, and Hyeun Moon are with Oak Ridge National Laboratory, Oak Ridge, Tennessee. Lauren Richards is in the Mechanical Engineering Department, Cornell University, Ithaca, New York.

Table 1. ZEHs and Base House Building Envelope Features

House                         Base House
Stories                       1
Floor, [ft.sup.2]([m.sup.2])  1056 (100)
Occupancy                     June 2000

Foundation                    Vented crawlspace
First Floor                   R-19 fiberglass batts
                              R-17.9 x h x [ft.sup.2] x [degrees]F/Btu
                              (3.2 [m.sup.2] x [degrees]C/W
Walls                         2 x 4 in. (51 x 102 mm) frame with
                              R-11 x h x [ft.sup.2] x [degrees]F/Btu
                              (1.9 [m.sup.2] x [degrees]C/W
                              fiberglass batts, OSB sheathing,
                              R-10.6 x h x [ft.sup.2] x [degrees]F/Btu
                              (1.9 [m.sup.2] x [degrees]C/W
Windows                       7 windows, U-factor 0.54
Doors                         2 doors: one solid insulated, one half
                              view
Roof                          Attic floor blown fiberglass R-
                              28.4 x h x [ft.sup.2] x [degrees]F/Btu
                              (5 [m.sup.2] x [degrees]C/W)
Roofing                       Gray asphalt shingles

House                         ZEH 1
Stories                       1
Floor, [ft.sup.2]([m.sup.2])  1056 (100)
Occupancy                     Nov 2001

Foundation                    Unvented crawlspace
First Floor                   6.5 in. (165 mm) SIPS 1#EPS
                              R-20 x h x [ft.sup.2] x [degrees]F/Btu
                              (3.5 [m.sup.2] x [degrees]C/W) structural
                              splines
Walls                         4.5 in. (114 mm) SIPS 1#EPS
                              R-15 x h x [ft.sup.2] x [degrees]F/Btu
                              (2.6 [m.sup.2] x [degrees]C/W)
                              surface splines, house wrap, vinyl
Windows                       9 windows, 0.34 U-factor, 0.33 SHGC,
                              sill seal pans
Doors                         2 doors: one solid insulated, one half
                              view
Roof                          8 in. (203 mm) SIPS 1#EPS
                              R-28 x h x [ft.sup.2] x [degrees]F/Btu
                              (4.9 [m.sup.2] x [degrees]C/W) surface
                              splines
Roofing                       Hidden raised metal seam

House                         ZEH 2
Stories                       1
Floor, [ft.sup.2]([m.sup.2])  1060 (100)
Occupancy                     Dec 2003

Foundation                    Mechanically vented crawlspace with
                              insulated walls 2 in. (51 mm)
                              polyisocyanurate boards,
                              R-12 x h x [ft.sup.2] x [degrees]F/Btu
                              (2.11 [m.sup.2] x [degrees]C/W)
First Floor                   R-19 x h x [ft.sup.2] x [degrees]F/Btu
                              (3.3 [m.sup.2] x [degrees]C/W) fiber glass
                              batts, 0.75 in. (19 mm) XPS boards
                              installed on bottom side of 9.5 in.
                              (241 mm) I-joist
                              R-24 x h x [ft.sup.2] x [degrees]F/Btu
                              (4.2 [m.sup.2] x [degrees]C/W)
Walls                         4.5 in. (114 mm) SIPS 2#EPS
                              R-15.5 x h x [ft.sup.2] x [degrees]F/Btu
                              (2.7 [m.sup.2] x [degrees]C/W)
                              structural splines, house wrap, vinyl
Windows                       8 windows, 0.34 U-factor, 0.33 SHGC,
                              sill seal pans
Doors                         2 doors: one solid insulated, one half
                              view
Roof                          6.5 in. (165 mm) SIPS 2#EPS
                              R-23 x h x [ft.sup.2] x [degrees]F/Btu
                              (4 [m.sup.2] x [degrees]C/W) structural
                              splines
Roofing                       15 in. (0.38 m) green standing 24GA steel
                              seam, 0.17 reflectivity

House                         ZEH 3
Stories                       1
Floor, [ft.sup.2]([m.sup.2])  1060 (100)
Occupancy                     Dec 2003

Foundation                    Unvented crawlspace with insulated walls 2
                              in. polyisocyanurate boards
                              R-12 x h x [ft.sup.2] x [degrees]F/Btu
                              (2.11 [m.sup.2] x [degrees]C/W)
First Floor                   R-19 x h x [ft.sup.2] x [degrees]F/Btu
                              (3.3 [m.sup.2] x [degrees]C/W) fiber glass
                              batts, 0.75 in. (19 mm) XPS boards
                              installed on bottom side of 9.5 in.
                              (241 mm) I-joist
                              R-24 x h x [ft.sup.2] x [degrees]F/Btu
                              (4.2 [m.sup.2] x [degrees]C/W)
Walls                         6.5 in (165 mm) SIPS 1#EPS
                              R-21 x h x [ft.sup.2] x [degrees]F/Btu
                              (3.7 [m.sup.2] x [degrees]C/W), structural
                              splines, house wrap, vinyl
Windows                       8 windows, 0.34 U-factor, 0.33 SHGC,
                              sill seal pans
Doors                         2 doors: one solid insulated, one half
                              view
Roof                          10 in (0.25 m) SIPS 1#EPS
                              R-35 x h x [ft.sup.2] x [degrees]F/Btu
                              (6.2 [m.sup.2] x [degrees]C/W),
                              surface splines
Roofing                       15 in. (0.38 m) green standing 24GA steel
                              seam, 0.23 reflectivity

House                         ZEH 4
Stories                       2
Floor, [ft.sup.2]([m.sup.2])  1200 (111.5)
Occupancy                     July 2004

Foundation                    Walk-out basement with insulated precast
                              (nominal steady-state R-value
                              of R-16 x h x [ft.sup.2] x [degrees]F/Btu
                              [2.8 [m.sup.2] x [degrees]C/W])
First Floor                   Concrete slab
Walls                         2nd floor 4.5 in. (114 mm) SIPS polyiso.,
                              pentane blown
                              R-26 x h x [ft.sup.2] x [degrees]F/Btu
                              (4.6 [m.sup.2] x [degrees]C/W), surface
                              splines
Windows                       10 windows, 0.34 U-factor, 0.33 SHGC,
                              sill seal pans
Doors                         3 doors: one solid insulated, one full
                              view, one half view
Roof                          8 in (0.2 m) SIPS, polyiso., pentane
                              blown, surface splines
                              R-48 x h x [ft.sup.2] x [degrees]F/Btu
                              (8.5 [m.sup.2] x [degrees]C/W)
Roofing                       Light gray metal simulated tile,
                              0.032 aluminum

Table 2. ZEHs and Base House Mechanical Features

House          Base House     ZEH 1                 ZEH 2

Solar System   None           48-43 W amorphous     12-165 W multi-
                              silicon PV modules,   crystal silicon PV
                              2.06 kWp              modules, 12.68% eff,
                                                    1.98 kWp
Heating and    Unitary 2 ton  1-1/2 ton air-to-air  Two-speed
Cooling        HP, SEER 12    HP, SEER 13.7,        compressor, 2 ton
                              two-speed ECM         air-to-air HP, SEER
                              indoor fan            14, HSPF 7.8, CFM
                                                    cooling 700 (20
                                                    [m.sup.3]/minute),
                                                    variable-speed
                                                    ECM indoor fan
Mechanical     None           Supply to return      Supply to return
Ventilation                   side of coil          side of coil,
                                                    C[O.sub.2] sensor,
                                                    bath fan exhaust
Duct Location  Crawl space    Inside conditioned    Inside conditioned
                              space                 space
Water Heater   Electric       Integrated HPWH       Integrated HPWH,
                              linked to unvented    linked to crawl,
                              crawl                 which has motorized
                                                    damper

House          Base House     ZEH 3                   ZEH 4

Solar System   None           12-165 W multi-crystal  20-110 W
                              silicon PV modules,     polycrystalline
                              12.68% eff, 1.98 kWp    2.2 kWp
Heating and    Unitary 2 ton  2 ton direct exchange   2 ton air-to-air
Cooling        HP, SEER 12    geothermal, R-417a,     HP, SEER 17,
                              variable-speed ECM      variable-speed
                              indoor fan              compressor, ECM
                                                      indoor and
                                                      outdoor fan
Mechanical     None           Supply to return side   Supply to return
Ventilation                   of coil, bath fan       side of coil,
                              exhaust                 bath fan exhaust
Duct Location  Crawl space    Inside conditioned      Inside
                              space                   conditioned space
Water Heater   Electric       Desuperheat for hot     HPWH vented to
                              water, EF 0.94          half bath, which
                                                      is exhausted for
                                                      ventilation

Table 3. ZEH3 Monthly Energy Usage

                                                          Total
              Space Heat  Space Cool  Hot Water  Other    Electric
Month         (kWh)       (kWh)       (kWh)      (kWh)    (kWh)

March         69                       108        486       663
April          0            77         108        489       674
May            0           319          90        560       969
June           0           346          76        511       933
July           0           394          76        569      1039
August         0           352          76        603      1031
September      0           290          79        483       852
October       57             0          99        560       716
November      50             0         104        738       892
December     132             0         148       1174      1454
January      176             0         144        620       940
February      85             0         171        595       851
Total        569          1778        1279       7388     11014
Annual cost  $39          $121         $87       $502      $749
Daily cost    $0.11         $0.33       $0.24      $1.38     $2.05

             Solar AC   Solar to
             Generated  the Grid
Month        (kWh)      (kWh)

March          231      116
April          226      100
May            221       48
June           213       56
July           232       48
August         222       41
September      201       55
October        154       49
November       135       37
December       142       28
January        131       40
February       133       41
Total         2241      659
Annual cost  -$336
Daily cost     -$0.92

Table 4. Construction Cost ($) of All Near Zero-Energy Houses and Base
House, Including Cost of Lot and Infrastructure

                        Base House  ZEH1     ZEH2     ZEH3

House                   59,295       78,914   83,953   87,889
Lot and infrastructure  14,500       14,500   14,500   14,500
Photovoltaic system          0       22,388   16,000   16,000
Total cost              73,795      115,802  115,953  122,329