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Appliance interaction with a ZEB is an important design consideration. Appliances interact in different manners with a ZEB. Energy usage by appliances in a ZEB will often outweigh the climate’s energy impact on the building’s shell. Appliance interactions may be favorable or unfavorable depending on the appliance and the building’s geographic location. Different technologies within an appliance group, such as heat-pump water heaters and electric-resistance water heaters also impact a ZEB differently.
The appliance industry has relentlessly improved its products for several decades, and today’s modern appliances offer consumers excellent quality, coupled with high energy efficiency. In order to effectively design ZEBs, however, it is important to understand how appliances affect the building’s comfort-conditioning system and the ability to reach net zero energy levels.
So, how do appliances impact a highly sealed, energy-efficient house? Well, it depends on the type of appliance and where the house is located. A refrigerator with a nominal 400 kW/hr energy tag has a net energy impact of 300 kW/hr if located in Illinois due to beneficial winter heating outweighing detrimental summer heating. In Phoenix, the refrigerator impact is 470 kW/hr, while in Fairbanks, Alaska, the net impact is 200 kW/hr.
Learning from ZEBsWhile appliances impact conventional homes in a similar manner, the impact is less significant because the house energy load is shell (climate) dominated. To further assess energy impacts on zero-energy homes, three geographic locations were selected and super-efficient homes were built: Urbana, Ill., Phoenix, Ariz., and Fairbanks, Alaska.
In Urbana, Ill., a house was constructed called the Equinox House-a 2,100 square-foot ranch home. This home has an energy requirement that is 25% of the energy for conventional construction due to super-insulation, super-sealing and proper window design. Human-activity-related energy uses such as lights, appliances and metabolism in a super-insulated, super-sealed house, are approximately twice the energy of the house-shell requirements for central Illinois.
The predicted annual energy requirement for the Equinox House with four people is 11,000 kW/hr. The same house without any human-related loads requires 3,700 kW/hr of energy in order to maintain a comfortable room temperature. Supplying fresh air at ASHRAE 62.2 recommended levels (15 to 20 cfm per person) for four occupants, increases the energy load to 6,000 kW/hr. The remaining 5,000 kW/hr energy requirements are for major appliances-including water heating-and other electrical loads, including television, computers, lights and hairdryers.
Illinois is a location that represents regions with significant summer air conditioning and winter heating loads. Phoenix and Fairbanks, Alaska, are used as extremes for cooling-dominated and heating-dominated climates. The ZEB design for Phoenix and Fairbanks are similar to the ZEB in Urbana, Ill., except that the Phoenix house design has more window overhang protection and the Fairbanks home does not have window overhang protection.
A ZEB in Phoenix with four people has a base-energy load of 8,000 kW/hr while a Fairbanks ZEB has an annual energy load of 16,000 kW/hr, compared to the Illinois-located ZEB with 11,000 kW/hr.
The house energy findings that follow are based on an all-electric house. House-conditioning-system efficiencies for air conditioning and heating are assumed to have reasonable seasonal energy efficiency ratio/coefficient of performance (SEER/COP) values.
The actual home-conditioning performance factors will affect the magnitude of the appliance effects, but not the trends that are the primary items of interest in this article. The assumed appliance usage levels are likewise meant to be representative examples. As many studies have shown, two identical homes with identical appliances and the same number of people can have widely varying energy requirements based on the residents’ living habits.
Refrigerators and FreezersFrom the viewpoint of a house, refrigerators are electric resistance heaters. Depending on the climate and time of the year, heat from a refrigerator’s operation may or may not be beneficial. A typical refrigerator has an annual energy usage of 300 to 500 kW/hr, which is related to its size (volume), style (top-mount, side-by-side), component efficiency, usage patterns and features (through-the-door ice and water).
For a refrigerator with a “yellow tag” annual energy load of 400 kW/hr, the ZEB located in Urbana, Ill., has an annual energy impact of 313 kW/hr. That is, for central Illinois, the refrigerator’s annual energy addition of 400 kW/hr to the interior of the house benefits its winter heating requirement more than its penalty for the additional summer cooling needed. So, the real impact of the refrigerator on the house is 22% less than its nominal energy listing.
For a ZEB located in Phoenix, the 400 kW/hr refrigerator results in 485 kW/hr of house energy load due to the extra air conditioning required to maintain comfort. The ZEB located in Fairbanks, Alaska, has an energy load of 284 kW/hr due to the 400 kW/hr of refrigerator operation because its heating effect is almost always beneficial.
Dishwashers, Laundry and CookingDishwashers, clothes washers and cooking appliances (microwave ovens, cook tops and ovens) are grouped together because they all add energy to a residence in a similar manner.
Some of a dishwasher’s operational energy remains in the house as added heat and moisture, while a portion of the energy runs down the drain with the water. The assumed annual energy load for a dishwasher is 300 kW/hr for a house based on a typical dishwasher with Energy Star performance and assumed usage, with the understanding that this may be discounted by the fraction of energy that travels with the drain water.
In this article the moisture issue will not be addressed, although the simulated model does include moisture effects for the house. Moisture conditioning (dehumidification and humidification) is generally less than 10% of the house energy requirements and is dominated by human moisture generation, such as respiration, perspiration, showers and cooking, and fresh air ventilation. It is important to note that the instantaneous moisture load of a ZEB may dominate its comfort-conditioning load unlike conventional homes that are climate or shell dominated.
Clothes washers have an assumed annual energy requirement of 300 kW/hr. This energy load is assumed based on a typical household of 2.6 people, using approximately 200 kW/hr annually with 400 clothes washing loads. For the family of four, a proportional scaling results in 300 kW/hr. The energy does not include energy for hot water, which will be treated separately.
Cooking energy has been decreasing over the past three decades and is less than half of the level it was in the 1970s. Based on a recent U.S. Department of Energy report, cooking with electricity has an annual load of 250 kW/hr for cook top/oven usage and 150 kW/hr for microwave oven usage for an annual total of 400 kW/hr.
Combining dishwasher, clothes washer and cooking energies, we have 1,000 kW/hr of annual energy usage. In a nearly proportional manner to the refrigerator load, this additional energy can be beneficial or detrimental. On average, this level of appliance usage is equivalent to a continuous power of 114 W inside the house. For Phoenix, additional air conditioning requires 245 kW/hr in addition to the 1,000 kW/hr load, while Fairbanks realizes 302 kW/hr less than the appliance load.
One should note that these appliance loads may be sufficient enough for certain locations and ZEB designs that, on average, a month that may have required heating becomes a time period that requires cooling. In conventional homes, appliance effects are too small relative to climatic load on the home to have this same impact.
Clothes DryersTwo interesting classes of clothes dryers are the conventional, vented dryers in which essentially all clothes dryer energy is exhausted from the house; and the ventless, heat-pump dryer in which all of the dryer energy stays in the house. A third category is the ventless, “condensing dryer,” which is a less efficient version of the ventless, heat-pump dryer.
Conventional, vented dryers heat the dryer air to a level that vaporizes water from clothing. No Energy Star rating specifications are currently available for clothes dryers. From our laboratory studies, an average dryer energy load of 2.5 kW/hr is assumed. Advances in clothes washers in which high-spin speeds reduce clothing water retention along with moisture sensors that help conserve energy by avoiding over drying, are reducing clothing-appliance energy requirements.
For a family of four, 615 drying cycles are assumed-the same as used for clothes washing. A dryer cycling time of 0.5 hours per load is assumed with 5 kW of power, resulting in 1,500 kW/hr per year. Because the clothes dryer energy is exhausted from the house, this energy does not impact house comfort directly.
Our clothes dryer ventilation studies indicate an average, vented-dryer air flow of 0.07 kg/s (120 cfm) during a drying cycle. For a family of four, with the above assumptions, the dryer operates 300 hours per year, or about 3% of the time. The fresh air ventilation for a super-sealed home is 0.008 kg/s per person (15 to 20cfm per person). On average, the dryer exhaust adds 0.00062 kg/s of uncontrolled ventilation per person to the house ventilation level, which affects the comfort- conditioning system’s load.
For the Urbana ZEB, 310 kW/hr of home-conditioning energy occurs for additional summer cooling and winter heating of the house. For Phoenix, only 44 kW/hr of additional load is realized. The Fairbanks ZEB has an additional comfort-conditioning load of 518 kW/hr above the 1,500 kW/hr dryer energy.
Ventless dryers with heat pumps essentially remove water from clothing by temporarily vaporizing the water. Heat from the condenser coils vaporizes water in the clothing, which is then re-condensed in the evaporator-similar to a dehumidifier. The energy used for the drying stays within the house, and no additional ventilation load is placed on the house.
Ventless, heat-pump clothes dryers are appearing on the market with half the drying cycle energy required by resistive heating dryers. Based on the vented dryer assumptions in this article, a ventless, heat-pump dryer requires 750 kW/hr to operate over the course of a year, with the energy staying inside the house in the form of additional heat.
For the Urbana ZEB, the house energy due to clothes drying is 578 kW/hr compared to the 1,810 kW/hr required by the vented dryer. For Phoenix, the ventless, heat-pump dryer imposes an annual energy load of 986 kW/hr in comparison to a vented dryer’s 1,544 kW/hr. In Fairbanks, annual energy for a ventless dryer realizes 526 kW/hr compared to 2,018 kW/hr.
Water HeatingWater heating is perhaps the most interesting of all these appliances, because of its energy significance, as well as new heat-pump technology moving into the market. For our family of four, we assume that 280 kg of hot water (approximately 80 gallons) are required per day for showering, clothes washing and other hot water needs.
Electric-resistance water heating has an annual energy load of 5,292 kW/hr per year based on the above assumptions. No allowance for storage tank losses is assumed for this article. Such losses may be heat gains to the surrounding space that may or may not impact the home’s energy depending on the location of the water heater.
Although some fraction of the energy in the hot water does transfer into the comfort space through both direct heating of the home and through water vaporization from showers and other uses, this amount will be neglected for this discussion. All energy used for heating the water is assumed to go down the drain with the water.
Newer heat-pump water heaters entering the market, significantly reduce water heating energy requirements while also impacting the comfort conditioning of the ZEB. Here, the heat-pump water heater is assumed to be located in the comfort space, although this may not be true in all cases. For a heat-pump coefficient of performance (COP) of 2, half of the energy transferred to the water comes from the interior of the comfort space and half from electrical for the heat pump’s power requirement. Assuming a COP of 2, the heat-pump water heater requires 2,646 kW/hr of annual energy for water heating, compared to the electric water heater with an annual energy load of 5,292 kW/hr.
Because the heat-pump water heater is cooling the house throughout the year, it has a negative impact on cold-climate homes, as evidenced by the annual water-heating load of 3,234 kW/hr for Urbana and 3,418 kW/hr for Fairbanks. Phoenix nearly always benefits from the cooling, with only 1,995 kW/hr of net house energy due to the water heater.
Although it would seem counterproductive to cool a house in the winter for northern locations, a heat-pump water heater, in combination with a home’s heating system, should be viewed as a “two-stage” refrigeration system in which one system is able to move energy from cold outdoor ambient to interior room ambient temperature conditions.
The heat-pump water heater is then able to “lift” room ambient temperature energy to the desired water temperature level. Also, the energy required for heating one’s water with a heat pump that has an average COP of 2 is nearly equivalent to the energy from a person’s metabolism within the house. That is, the heat from a person over the course of a day is similar to the energy needed to heat one’s daily shower water. The heat pump can be thought of as a means to redirect the metabolism energy to a useful purpose before exhausting it down the drain.
Design engineers can benefit from understanding the impact of appliances on ZEB energy requirements. In general, efficiency gains by all appliances are desired. When analyzing the energy flows and interactions for a house, the energy requirements of each appliance must be assessed in order to define how it affects the house and the ability to maintain comfort in a home. This interaction is very important when looking at the life-cycle cost for both the individual appliances, as well as the building as a whole.
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