Abstract
The annual energy savings (AES) associated with the application of the wireless mobile thermostat for an average two-story residential house in the United States are projected to be approximately in the range of 4.5%-15.7% for heating and 3.1%-8.8% for cooling, respectively, depending on geographic location. The simple payback for the new control system serving space heating and space cooling loads is estimated to be approximately 1.9-3.5 years.
TYPICAL CLIMATE CONTROL ARRANGEMENT IN RESIDENTIAL BUILDINGS
The so-called forced air system with central furnace is the most frequently used system in the United States. The forced air system is utilized for space heating in 56.1 million households out of 101.5 million residential houses (or in 55.3% houses), including apartment complexes. The forced air system is quite extensively used for air conditioning (space cooling) and heating, utilizing the same ductwork to distribute heating and cooling throughout a house. Approximately 47.5 million houses out of 73.7 million single-family houses (or 64.5% houses) equipped with central air conditioning system also utilize a central furnace.[1]
About 30.1 million households (or 40.8%) among 73.7 million single-family houses with space heating and cooling are either two-story (26.9 million or 36.5%) or three-story (3.2 million or 4.3%) houses. The rest of the houses (43.6 million) are single-story houses (about 59.2%). Of these 73.7 million households, only approximately 400,000 households (about 0.55%) utilize two zones, with each zone (most frequently subdivided by floor) served by a dedicated furnace, which has its own stationary thermostat to control heating and cooling.[2]
A vast majority of residential houses with forced air systems have only one thermostat to control both heating and cooling modes of operation.
PREVIOUS AND CURRENT INVESTIGATIONS
The previous conducted investigation established that a significant temperature differential exists between the first and second floors when control of space heating and cooling is implemented from the central stationary thermostat located in the living room on the first floor. The air temperature is also subject to significant variation within each floor.[4] A temperature differential between various locations in the rooms is a function of many factors, such as the distance to the heating/cooling discharge air outlets and the windows, as well as occupancy level, heat gains from appliances, wind direction, orientation toward the sun, etc.
The application of a mobile thermostat allows for control of air temperature on demand at any time in any location of the building. This leads to better indoor climate control and reduces annual thermal and electrical energy consumption.1-4’51 The potential AES for an average household in New England were analyzed earlier by Burd and Burd.[6] This paper evaluates the potential AES that could be realized via utilization of the new control system, which features a mobile thermostat in residential buildings at different geographic locations in the United States. Although the application of a mobile thermostat would reduce energy consumption in residential buildings due to multiple factors (including horizontal temperature variation within each floor), in our feasibility evaluation for the developed system, we conservatively considered energy savings that are caused by the vertical temperature variation only. These savings are associated with the temperature differential due to forced and natural convections[3] between floors in multistory buildings (i.e., two- and three-story houses).
INVESTIGATION OF TEMPERATURE DIFFERENTIAL BETWEEN SECOND AND FIRST FLOOR IN A RESIDENTIAL TOWNHOUSE
This investigation was conducted in a typical residential townhouse built in 1986 and located in the state of Connecticut (New England region of the United States). The two-floor townhouse has approximately 92.9 m2 (1000 ft2) of total area. The height of each floor is 2.44 m (8 ft). The townhouse has a living room, a kitchen, and a half bathroom located on the first floor and two bedrooms and a full bathroom on the second floor (the smaller bedroom is used as a study room). In addition, the townhouse has a full unfinished basement used as storage. The basement is also heated and cooled via two diffusers supplying the warm and cold air; the forced-air furnace and associated air distribution ductwork are located in the basement. The townhouse is situated in the middle of a four-unit residential building and has easterly- and westerly-oriented outside exposures.
The monitored residential townhouse was built after the new ASHRAE ventilation norms were adapted and, thus, could be rated as a “tight” house. The infiltration rates in ACH (air change per hour) for residential buildings in the United States were adapted from the data published by McQuiston. The ACH indicates the ratio of the hourly infiltration air volume to the volume of the house. These rates vary from 0.51 ACH (“tight” houses) to 1.3 ACH (“loose” houses). Natural gas and electricity are used for space heating and space cooling, respectively. The furnace has an evaporator (cooling coil); the cooling compressor and condenser units of the cooling system are located outside of the house. The air treated in the furnace (heated or cooled) is delivered throughout the house via ductwork, supply diffusers, and registers, and then it is returned back to the furnace via return grills and ductwork. Two people live in the townhouse. The house has one stationary, nonprogrammable thermostat located in the living room, which controls both heating and cooling.
The temperature differential between the second and the first floors (TDSFF) depends on both forced and natural convection. For the purpose of this analysis, in Fig. 1, we considered a separate impact on TDSFF from the forced and natural air convection.
Natural convection is present in one way or the other during the entire heating season. However, its impact is more significant when the forced air space heating system is turned off. Because of that, natural convection impact is more pronounced during relatively warm outdoor air temperatures with fewer hours of space heating operation.
As opposed to natural convection, the impact of forced air convection is more noticeable during lower outdoor air temperatures with frequent space heating system operation. Therefore, the TDSFF, due to the combined effect of the forced air and natural convection, will be somewhat higher during lower outdoor air temperatures, as compared to the high outdoor air temperatures (see Fig. 1).
The combined impact of natural and forced air convection on TDSFF for the entire heating season is represented by line 3 in Fig. 1. The data in line 3 represent the summation of the data in line 1 and line 2.
According to Fig. 1, the average TDSFF during heating season, with an outdoor air temperature of 4.7°C (40.5°F), will be close to about 2.2°C (36°F).
The stationary thermostat set point in the townhouse (for the monitored data used in Fig. 2) was maintained at about 18°C (64.4°F).
WIRELESS MOBILE THERMOSTAT CONTROL SYSTEM
The accuracy of maintaining a desired air temperature in the building greatly depends on the utilized control system.
The closer the control device is to the area where the targeted temperature value is critical to maintain, the more accurate the control becomes. Obviously, the best and most precise control could be achieved by combining various stages of control and, eventually, by utilizing a final (ultimate) control stage when the control device is located at the place where a desired air temperature value has to be maintained.
Fig. 1 Variations in outdoor air temperature and temperature differential between second and first floors (TDSFF). 1. Forced air convection impact; 2. Natural air convection impact; 3. Combined impact of forced air and natural air convection.
Fig. 2 Mobile thermostat operation during nighttime (heating mode). 1. Study room; 2. Bedroom; 3. Living room (dinner table); 4. Thermostat temperature set point.
An innovative control system that utilizes a wireless mobile thermostat has been developed.[7] The mobile thermostat is a battery-operated device, which facilitates the implementation of the final stage of control to accurately balance the heat loss and heat supply in order to maintain the required air temperature in a particular area of the house. A prototype of the wireless mobile thermostat was manufactured to conduct testing of the thermostat’s performance.
The prototype consists of two parts: a receiver and a transmitter. The prototype was installed in the investigated townhouse. The mobile thermostat’s transmitter (remote temperature sensing element) was moved to different areas throughout the house (such as the bedroom, living room, study, etc.) to control air temperature as necessary.
The transmitter of the wireless mobile thermostat is capable of sending a remote radio signal to the receiver which, in turn, activates or deactivates the furnace’s heating or cooling systems to maintain a set point temperature at the thermostat’s transmitter location. This remote signal is proportional to the temperature differential between the wireless mobile thermostat’s set point temperature and the air temperature at the thermostat’s location. This proportionality is realized via the length of time during which the signal is sent from the thermostat to the receiver. The higher the initial deviation of the air temperature at the thermostat’s location from the set point, the longer the time period during which the signal is transmitted from the transmitter to the receiver. This, in turn, would increase the operating time for the heating/ cooling unit to heat/cool the area. A reverse procedure would ocurr if the initial temperature at the thermostat’s location was closer to the thermostat’s set point. Another distinctive feature of the wireless mobile thermostat is its low inertia and ability to react quickly to any temperature changes at the thermostat’s location. A detailed presentation of the wireless mobile thermostat system’s major elements is given in Burd and Burd.[4]
WIRELESS MOBILE THERMOSTAT OPERATION DURING HEATING MODE
The results of the wireless mobile thermostat’s operational tests are shown in Fig. 2.
The upper part of the graph in Fig. 2 shows that at nighttime, the outdoor air temperature was near — 6.5°C (20.3°F). The maximum value of the furnace’s discharge air temperature measured in the basement was near 30°C (86°F). The furnace cycles on and off.
The mobile thermostat was located on the nightstand in the bedroom, and its temperature set point was 18°C (64.4°F). The lower part of the graph shows that the temperature in the bedroom was maintained close to the thermostat’s set point [maximum deviation did not exceed +1 °C (1.8°F) and - 0.5°C (0.9°F)]. The mobile thermostat overrode the stationary thermostat control and turned the furnace on and off, as necessary (as shown in the upper portion of the graph), to maintain the required temperature at the mobile thermostat’s transmitter location.
While the temperature in the bedroom (as well as in the study) closely followed the wireless mobile thermostat temperature set point, the temperature in the living room was maintained at a substantially lower magnitude.
The air temperature difference between the bedroom and the study room was not quite noticeable. On the other hand, the air temperature in the living room was significantly lower—by 1 °C (1.8°F) to 4°C (7.2°F)—than the temperature at the mobile thermostat location. This demonstrates the ability of the wireless mobile thermostat to save energy for space heating by maintaining the desired temperature in the occupied rooms upstairs on demand, while keeping a lower temperature on the first floor when it is not occupied.
PROJECTED ANNUAL ENERGY SAVINGS IN RESIDENTIAL BUILDINGS
The projected AES due to the application of the wireless mobile thermostat are shown in Table 1. The energy savings were calculated for a number of the selected U.S. cities, which represent a wide variety of climatic conditions.
The design space heating outdoor air temperature conditions for these cities vary from — 29.4 ° C (- 20.9°F) for Bismark (North Dakota) to 7.8°C (46°F) for Miami (Florida). Table 1 indicates that the average (per heating season) outdoor air temperature for the selected cities vary from —1.1°C (30.1 °F) for Bismarck (North Dakota) to 14°C (57.2°F) for Miami (Florida). The annual space heating run time for these cities (when the outdoor temperature is lower than the space heating balance temperature) varies from 894 h for Miami (Florida) to 7815 for Seattle (Washington).
The design space’s cooling outdoor air temperature conditions differ from 43.3°C (109.9°F) for Phoenix (Arizona) to 29.4°C (84.9°F) for Seattle (Washington). Table 1 also demonstrates that the average outdoor air temperature over the cooling season for the selected cities varies from 26.1°C (79°F) for Portland (Maine) to 31.3°C (88.3°F) for Phoenix (Arizona). The annual space cooling run time for these cities (when the outdoor air temperature is higher than the space cooling balance temperature) varies from 237 h for Seattle (Washington) to 5390 h for Miami (Florida).
The cumulative annual operating time for space heating and cooling ranges from the maximum of 8052 h (91.9% of the length of the entire year) for Seattle (Washington) to the minimum of 6141 h (70.1% of the length of the entire year) for Tampa (Florida).
The application of the mobile thermostat would allow a user to setback the air temperature during the heating period and to setforward the air temperature during a cooling period. We used a simplified engineering method of calculation based on the assumption that the energy consumption for space heating and space cooling can be expressed as a linear function of the temperture differential between the two major parameters, averaged over a heating/cooling season. These parameters are: indoor dry-bulb air temperature set point and oudoor dry-bulb air temperature. This approach assumes that the outdoor dry-bulb air temperature can be used as a defining parameter, impacting energy conservation. This is utilized for the purpose of initial evaluating analysis only. A detailed presentation of the methodology for energy savings calculations is given in Burd and Burd.[6]
Table 1 shows the potential energy savings associated with the utilization of the mobile thermostat. The AES due to lowering or increasing a stationary thermostat’s set point temperature by + 1 °C during the heating or cooling mode of operation were calculated by the formula:
where AT, the magnitude of lowering or increasing a set point temperature with the mobile thermostat, °C (°F); HCS, the current heating or cooling set point temperature maintained by the stationary thermostat, °C (°F); and Tav.0ut.hc, the average outdoor air temperature during the heating or cooling season, respectively, °C (°F).
Table 1 denotes that the potential energy savings associated with the 1 °C (1.8°F) setback in air temperature via the wireless mobile thermostat during heating season would vary from 4.7% (for Bismarck, North Dakota) to 16.7% (for Miami, Florida). The potential energy savings associated with the 1 °C (1.8°F) setforward in air temperature via the wireless mobile thermostat during cooling season would vary from 15.8% (for Phoenix, Arizona) to 45% (for Portland, Maine). The projected relative value of energy savings [for 1 °C (1.8°F) reset in temperature set point] during the cooling mode of operation is higher than for the heating mode of operation because the temperature differential between the thermostat set point and average seasonal outdoor air temperature is lower for cooling as compared to heating.
Table 1 Potential annual energy savings due to mobile thermostat application for space heating and space cooling for selected cities in U.S.A.
| North latitude | Design outdoor air temperature for heating | Average per heating season outdoor air | Potential energy savings for space heating per 10C Annual space heating thermostat system operating temperature | Design outdoor air temperature for | Average per cooling season outdoor air | Annual space cooling system | Potential energy savings for space cooling per 10C thermostat | Annual space heating and space cooling
systems operating time |
Cumulative percentage of space heating and space cooling systems operating time during a year | |||
| No
1 |
City
Bismarck |
State
North Dakota (ND) |
(Degrees)
47 |
(° C) – 29.4 | temperature (0C) -1.1 | time (h) setback (%)
6,896 4.7 |
cooling (0C)
33.9 |
temperature (0C)
27.8 |
operating time (h)
824 |
setforward (%)
25.4 |
(h)
7,720 |
(%) 88.1 |
| 2 | Chicago | Illinois (IL) | 42 | - 21.1 | 4.4 | 6,075 6.4 | 32.8 | 27.6 | 1,193 | 27.3 | 7,268 | 83.0 |
| 3 | Portland | Maine (ME) | 46 | - 5.6 | 4.5 | 7,265 6.5 | 32.2 | 26.1 | 383 | 45.0 | 7,648 | 87.3 |
| 4 | Buffalo | New York (NY) | 43 | -16.7 | 4.5 | 6,704 6.5 | 30.0 | 26.3 | 616 | 40.9 | 7,320 | 83.6 |
| 5 | Hartford | Connecticut (CT) | 42 | -16.7 | 4.7 | 6,455 6.5 | 32.8 | 27.2 | 934 | 30.0 | 7,389 | 84.3 |
| 6 | Baltimore | Maryland (MD) | 39 | -11.7 | 6.2 | 5,717 7.2 | 33.9 | 27.5 | 1,308 | 27.7 | 7,025 | 80.2 |
| 7 | Charleston | South Carolina (SC) | 33 | - 3.9 | 6.8 | 5,703 7.6 | 34.4 | 27.2 | 1,206 | 30.0 | 6,909 | 78.9 |
| 8 | Seattle | Washington (WA) | 47 | - 5.0 | 9.1 | 7,815 9.1 | 29.4 | 26.3 | 237 | 40.9 | 8,052 | 91.9 |
| 9 | Dallas | Texas (TX) | 33 | - 8.3 | 9.7 | 4,079 9.7 | 37.8 | 28.6 | 3,029 | 21.4 | 7,108 | 81.1 |
| 10 | Phoenix | Arizona (AZ) | 34 | 1.1 | 11.6 | 3,305 11.8 | 43.3 | 30.2 | 3,861 | 15.8 | 7,166 | 81.8 |
| 11 | Tampa | Florida (FL) | 28 | 2.2 | 13.1 | 2,262 14.4 | 33.3 | 27.4 | 3,879 | 28.6 | 6,141 | 70.1 |
| 12 | Miami | Florida (FL) | 26 | 7.8 | 14.0 | 894 16.7 | 32.8 | 26.9 | 5,390 | 32.7 | 6,284 | 71.7 |
Assumptions:1. Space heating balance temperature is assumed to be 16.7°C (62°F)—below 62°F heating is required;2. Space cooling balance temperature is assumed to be 23.9°C (75°F)—above 75°F cooling is required;3. Space heating and space cooling design dry-bulb air temperatures are assumed at their 99.6 percentile.
| No | City | State | Annual space heating energy consumption (kWh) | Annual space heating
energy savings per 10C temperature setback (kWh) |
Annual space cooling energy consumption (kWh) | Annual space cooling energy savings per 10C thermostat
temperature setforward (kWh) |
Cumulative annual space heating and space cooling energy consumption (kWh) | Percentage of space heating energy consumption (%) | Percentage of space cooling energy consumption (%) | Annual space heating and cooling savings per 10C temp. setback for heating and setforward for cooling (kWh) | Cumulative annual
relative savings percentage for space heating and space cooling per 10C temp, setback and setforward (%) |
| 1 | Bismarck | North Dakota (ND) | 35,350 | 1,679 | 459 | 116 | 35,809 | 98.7 | 1.3 | 1,795 | 5.0 |
| 2 | Chicago | Illinois (IL) | 23,007 | 1,479 | 617 | 168 | 23,624 | 97.4 | 2.6 | 1,647 | 7.0 |
| 3 | Portland | Maine (ME) | 27,415 | 1,769 | 120 | 54 | 27,536 | 99.6 | 0.4 | 1,823 | 6.6 |
| 4 | Buffalo | New York (NY) | 25,298 | 1,632 | 212 | 87 | 25,511 | 99.2 | 0.8 | 1,719 | 6.7 |
| 5 | Hartford | Connecticut (CT) | 24,010 | 1,572 | 439 | 132 | 24,449 | 98.2 | 1.8 | 1,703 | 7.0 |
| 6 | Baltimore | Maryland (MD) | 19,254 | 1,392 | 666 | 185 | 19,920 | 96.7 | 3.3 | 1,576 | 7.9 |
| 7 | Charleston | South Carolina (SC) | 18,358 | 1,388 | 567 | 170 | 18,926 | 97.0 | 3.0 | 1,559 | 8.2 |
| 8 | Seattle | Washington (WA) | 20,823 | 1,903 | 82 | 33 | 20,905 | 99.6 | 0.4 | 1,936 | 9.3 |
| 9 | Dallas | Texas (TX) | 10,207 | 993 | 1,994 | 427 | 12,201 | 83.7 | 16.3 | 1,420 | 11.6 |
| 10 | Phoenix | Arizona (AZ) | 6,795 | 805 | 3,450 | 545 | 10,244 | 66.3 | 33.7 | 1,349 | 13.2 |
| 11 | Tampa | Florida (FL) | 3,824 | 551 | 1,915 | 547 | 5,740 | 66.6 | 33.4 | 1,098 | 19.1 |
| 12 | Miami | Florida (FL) | 1,306 | 218 | 2,323 | 760 | 3,629 | 36.0 | 64.0 | 978 | 26.9 |
Assumptions:1. Space heating and space cooling energy consumption changes in direct proportion to the difference between indoor and outdoor air temperatures;2. Historical energy consumption data for the monitored two-story townhouse in Hartford, CT was assumed to be a base for the calculations 0.000293 conversion factor from Btu to kWh.
The continuation of Table 1 shows annual heating and cooling energy consumption as well as cumulative annual energy consumption for space heating and cooling for the considered cities. The heating and cooling annual energy consumption for the monitored townhouse in Connecticut was calculated based on the actual electrical and gas meters data. The annual heating energy consumption (HEC) for the houses in the selected cities was calculated by the formula:
where HECmh, annual energy consumption for heating in the monitored house, kWh (MJ); TIN H, the air temperature inside the house during heating season was assumed to be 20°C (68°F); Tav.out.hsc, average (per heating season) outdoor air temperature for the selected cities, °C (°F); Tav.out.mh, average (per heating season) outdoor air temperature for the monitored house, °C (°F); ASHOTSC, annual space heating operating time for the selected cities, h; and ASHOTMH, annual space heating operating time for the monitored house, h.
The annual cooling energy consumption (CEC) for the houses in the selected cities was calculated by the formula:
where TIN C, the air temperature inside the house during cooling season was assumed to be 23.9°C (75°F); CECMH, the annual energy consumption for cooling in the monitored house, kWh (MJ); Tav.out.csc, the average (per cooling season) outdoor air temperature for the selected cities, °C (°F); Tav.out.mh, the average (per cooling season) outdoor air temperature for the monitored house, °C (°F); ASCOTSC, the annual space cooling operating time for the selected cities, h; and ASCOTMH, the annual space cooling operating time for the monitored house, h.
Table 1 (continuation) also indicates the cumulative potential energy savings of a 1°C (1.8°F) temperature setback for space heating and temperature setforward for cooling. For the majority of the selected cities, the HEC far exceeds the cooling energy consumption. This is only not the case for Miami (Florida), where the annual energy consumption for cooling (64%) is higher than for heating (36%).
Table 2 demonstrates that the cumulative annual relative energy savings for heating and cooling with the wireless mobile thermostat vary from 5.0% for Bismark (North Dakota) to 26.9% for Miami (Florida) of the current (baseline) energy consumption for the considered buildings for each °C (1.8°F) of the temperature setback and setforward.
Table 2 illustrates the potential annual energy cost savings per 1 °C (1.8°F) in temperature setback and setforward for space heating and space cooling, respectively, for the geographic locations considered in Table 1. The cumulative annual space heating and space cooling energy cost savings range from approximately $102 for Portland (Maine) to $447 for Phoenix (Arizona).
Table 3 shows potential annual energy cost savings due to the wireless mobile thermostat application, considering the results of temperature monitoring and the occupancy schedule in the representative house.[6] Based on the results of temperature monitoring discussed earlier, we assumed that the utilization of the wireless mobile thermostat would allow reduction of the air temperature in the house by 2.2°C (3.96°F) during the heating season, while the upstairs areas are occupied. Based on the results of temperature monitoring,[5'6] we also assumed that the use of the wireless mobile thermostat would allow the user to increase the air temperature of the house by 0.5°C (0.9°F) during the cooling season when it is occupied and occupants are on the second floor. The occupancy schedule assumes that the daily, average, per-week occupied, and nonoccupied time is 14.4 and 9.6 h, respectively. The occupancy schedule also assumes that the occupants spend 30 and 70% of the occupied time in the house downstairs and upstairs, respectively.
Considering the above occupancy schedule, the overall wireless mobile thermostat’s daily setback for space heating and setforward for space cooling is assumed to be approximately 0.9°C (1.62°F) and 0.2°C (0.36°F), respectively. These additional savings are once again projected, compared to the existing stationary thermostat located on the first floor of the two-story house. The cumulative annual space heating and space cooling savings vary from $64 for Miami (Florida) to $117 for Phoenix (Arizona). The simple payback for the wireless mobile thermostat would vary from 1.9 years for Phoenix (Arizona) to 3.5 years for Miami (Florida). For the majority of geographic locations considered in the study, the mobile thermostat would have a simple payback of 2.4-2.7 years.
An application of the wireless mobile thermostat in residential buildings with only space heating or space cooling loads might be less advantageous from an economical point of view, as compared to the houses with both loads.
Table 2 Potential annual energy cost savings due to mobile thermostat application per one °C temperature setback for heating and temperature setforward for cooling
| No | City | State | Annual space energy heating savings (kWh) | Annual space heating energy cost savings ($) | Annual space cooling energy savings (kWh) | Annual space cooling energy cost savings ($) | Cumulative annual space heating and cooling energy cost savings ($) |
| 1 | Bismarck | North Dakota (ND) | 1,679 | 84 | 459 | 54 | 138 |
| 2 | Chicago | Illinois (IL) | 1,479 | 74 | 617 | 73 | 147 |
| 3 | Portland | Maine (ME) | 1,769 | 88 | 120 | 14 | 102 |
| 4 | Buffalo | New York (NY) | 1,632 | 81 | 212 | 25 | 106 |
| 5 | Hartford | Connecticut (CT) | 1,572 | 78 | 439 | 52 | 130 |
| 6 | Baltimore | Maryland (MD) | 1,392 | 69 | 666 | 79 | 148 |
| 7 | Charleston | South Carolina (SC) | 1,388 | 69 | 567 | 67 | 136 |
| 8 | Seattle | Washington (WA) | 1,903 | 95 | 82 | 10 | 105 |
| 9 | Dallas | Texas (TX) | 993 | 50 | 1,994 | 235 | 285 |
| 10 | Phoenix | Arizona (AZ) | 805 | 40 | 3,450 | 407 | 447 |
| 11 | Tampa | Florida (FL) | 551 | 27 | 1,915 | 226 | 253 |
| 12 | Miami | Florida (FL) | 218 | 11 | 2,323 | 274 | 285 |
Notes:1. The above calculations are conducted for 1 °C in temperature setback and setforward for space heating and cooling, respectively;2. The installed cost of the mobile thermostat for the residential application is assumed to be $220 [10-12];3. Cost of natural gas used for space heating was assumed to be $1.46 for 29.3 kWh (100,000 Btu) or $0.0498/kWh;4. Cost of electricity used for space cooling was assumed to be $0.118/kWh.
Table 3 Potential annual energy cost savings due to mobile thermostat application considering occupancy pattern in the representative house
| No | City | State | Potential energy savings for space heating thermostat temperature setback (%) | Annual space heating energy savings (kWh) | Annual space heating energy cost savings ($) | Potential energy savings for space cooling thermostat temperature setforward (%) | Annual space cooling energy savings (kWh) | Annual space cooling energy cost savings ($) | Cumulative annual space heating and cooling energy cost savings ($) | Simple payback period (yrs) |
| 1 | Bismarck | North Dakota (ND) | 4.5 | 1,586 | 79 | 4.9 | 89 | 11 | 90 | 2.5 |
| 2 | Chicago | Illinois (IL) | 6.1 | 1,397 | 70 | 5.3 | 120 | 14 | 84 | 2.6 |
| 3 | Portland | Maine (ME) | 6.1 | 1,670 | 83 | 8.8 | 23 | 3 | 86 | 2.6 |
| 4 | Buffalo | New York (NY) | 6.1 | 1,541 | 77 | 8.0 | 41 | 5 | 82 | 2.7 |
| 5 | Hartford | Connecticut (CT) | 6.2 | 1,484 | 74 | 5.8 | 85 | 10 | 84 | 2.6 |
| 6 | Baltimore | Maryland (MD) | 6.8 | 1,315 | 66 | 5.4 | 130 | 15 | 81 | 2.7 |
| 7 | Charleston | South Carolina (SC) | 7.1 | 1,311 | 65 | 5.8 | 110 | 13 | 78 | 2.8 |
| 8 | Seattle | Washington (WA) | 8.6 | 1,797 | 90 | 8.0 | 16 | 2 | 91 | 2.4 |
| 9 | Dallas | Texas (TX) | 9.2 | 938 | 47 | 4.2 | 388 | 46 | 93 | 2.4 |
| 10 | Phoenix | Arizona (AZ) | 11.2 | 760 | 38 | 3.1 | 671 | 79 | 117 | 1.9 |
| 11 | Tampa | Florida (FL) | 13.6 | 520 | 26 | 5.6 | 372 | 44 | 70 | 3.1 |
| 12 | Miami | Florida (FL) | 15.7 | 206 | 10 | 6.4 | 452 | 53 | 64 | 3.5 |
Notes:1. Assumed average per week temperature setback for heating 0.94°C, 1.7°F;2. Assumed average per week temperature setforward for cooling 0.19°C, 0.35°F.
Energy savings for the wireless mobile thermostat will be achieved due to minimization of what we called a “comfort satisfaction safety factor” (CSSF)—when the set point temperature at the stationary thermostat is maintained at a higher level during the heating season and a lower level during the cooling season to compensate for any deviations of the temperature at the occupants’ location to ensure their satisfaction with the indoor climate control based on the emperical anticipation. Similarly, the occupant applies the CSSF for the first floor stationary thermostat set point to satisfy the required comfort conditions on the second floor.
The above leads to a logical conclusion that the utilization of the mobile thermostat would always produce energy savings as compared to a baseline energy consumption with a stationary thermostat. Obviously, the CSSF would be different for various users and applications.
The projected savings in residential buildings with a single stationary thermostat serving radiation or convector steam or hot water heating systems with natural air convection may be somewhat lower as compared to the forced air heating systems. This could be due to the reduced temperature differential between the second and first floors for systems with natural air convection. Further investigation would be necessary to verify this assumption.
In addition to individual residential houses, the developed control system could also be used in a variety of heating and cooling applications in commercial buildings, as well. Wireless mobile thermostat could also be instrumental in reducing energy consumption in district energy systems, such as district heating/cooling systems,[8] as well as for various industrial applications—for climate controls in zones served by rooftop units, where manufacturing equipment is frequently moved around the facility,[9] etc.
CONCLUSION
The wireless mobile thermostat—the ultimate stage of climate control—would allow a user to setback and setforward temperatures in the house for space heating and space cooling, respectively. The conducted initial investigation utilizing a simplified model for different geographic locations in the United States showed that the application of a wireless mobile thermostat in the two-story townhouse would allow savings of 4.5%-15.7% of annual space heating energy. Utilization of the wireless mobile thermostat would also allow savings of 3.1 %-8.8% of energy for space cooling. The simple payback for the new control system serving space heating and space cooling loads would vary from approximately 1.9-3.5 years.
The mass application of the wireless mobile thermostat, considering the scale of energy consumption in residential buildings, which consume more than 20% of the total energy use—including residential, commercial, agricultural and transportation sectors—could be an important step in energy resource.[10]
The ever-increasing cost of fuel could become a contributing factor for wireless mobile thermostat utilization. In addition, the application of the wireless mobile thermostat will have a positive environmental impact due to emissions reduction at power plants as well as at residential heat generation systems.
