Technical Test File

Comparative wood stove tests to determine fuel savings with the use of a Thermoelectric fan to maintain thermal comfort

THEORY

An experimental study is presented that provides credible evidence that the use of a thermo-electric fan on a wood stove reduces the amount of fuel needed to maintain a prescribed level of thermal comfort. Test procedures and protocols based on existing standards and standards have been integrated into a testing facility that allows detailed examination of the dynamic heat transfer characteristics associated with wood stove operation in a controlled environment. The experimental results are validated using numerical simulations which further substantiate that the test results show that the use of a thermo-electric fan during the operation of the wood stove provides an average fuel saving of 14% for a range of testing conditions studied while maintaining user comfort level for extended periods.

INTRODUCTION

Rising fuel costs have increased the demand for efficient, solid fuel appliances, as well as associated consumer products that promise to strengthen and improve the thermal performance of these appliances. These improvements and enhancements are ultimately to reduce fuel consumption and operating costs. Most wood stove manufacturers suggest that fuel consumption can be improved, by fitting a blower or blower to redistribute heat to the rear of the stove thus improving convective heat transfer conditions. This will result in more even heating of the room which feels more comfortable to the occupants.

A review of existing standards and methodologies determined that there are no commonly accepted standard tests for performing comparative testing of solid fuel appliances and associated consumer products, under "real world" conditions. Existing methods are primarily engine legislative and aim to determine combustion rates and/or stack emissions. The development of comparative test procedures for wood stoves in "real life" is very complex, because the operation of wood stoves is dynamic and steady state conditions are rarely achieved and the measured values ​​are constantly changing. Therefore, data analysis to determine comparative results becomes very complicated due to data variability.

When developing a standardized test method to properly address this situation; existing standards, including "ASHRAE Standard 55-2004, Thermal Environmental Conditions for Human Occupancy" [1] "EPA Method 28 Certification and Auditing of Wood Stoves" [2], and "ISO 7730:2005 ,Ergonomics of the thermal environment” [3 ] were used as guidelines.

TEST PROCEDURE

The main objective of this test is to determine the amount of fuel that could be saved by maintaining a comfortable thermal environment for occupants through comparative tests with and without a thermo-electric fan. Most wood stove manufacturers and consumers anticipate that a fan-assisted wood stove leads to better thermal performance based on the fact that forced heat transfer by convection

Comparative wood stove tests to determine fuel savings with the use of a Thermoelectric fan to maintain thermal comfort, induced by a fan, redistributes hot air that is trapped around the wood stove, as well as stagnant hot air at the top of the room. However, few studies have been done in a controlled study to support this belief. To study the improvement of thermal comfort, the ASHRAE 55-2004 guidelines [1] was referenced in the design of a controlled test facility in which sensors of air temperature, humidity and humidity speed were installed to assess the level of thermal comfort. The test facility, thermal environment, test method and procedures are described as follows.

Test facility

The test facility used in this study consists of a 9.75 meter long by 6.4 meter wide by 2.4 meter high chambers, built to local building code, within a structure existing. There is an air gap of 30.5 centimeters on three sides of the test facility, between the existing structure and the exterior of the test facility walls. The walls and ceiling are insulated to a level of R12 and R20, respectively. The outside air is conditioned by an air ventilation system, which is equipped with a heating device and capable of circulating the conditioned air outside and maintaining a specified temperature between the wall of the installation test and the existing structure. The test facility is heavily instrumented with sensors for temperature detection, weight measurement and air flow velocity at locations recommended by the ASHRAE standard [1]. A computerized data acquisition system is used to log and record the data. Figure 1 shows the layout of the test setup.

Figure 1 Test setup

As shown in Figure 1, a wood stove (EPA Drolet Pyropak [4]) is placed on the west wall of the test facility. A layer was placed 3 meters in front of the stove; where the occupant operating temperature (T o)

Is obtained.

By ASHRAE Standard 55-2004 [1], T o can be calculated with sufficient approximation as the mean value of air and mean radiation temperature, where the relative air velocity is low (<0 .2 m/s), or when the difference between the average radiation and air temperature is small (<4 °C) [1]. air temperature at the occupant's position is measured at four different heights; 0.1m, 0.6m, 1.1m and 1.7m. These locations are at the ankle, knee and sitting/standing head positions of the occupant. The average air temperature is the average of the temperature readings at these locations. The average radiation temperature is defined as the temperature of a uniform, black

2

Comparative wood stove testing to determine fuel savings with the use of a thermoelectric fan to maintain thermal comfort enclosure that exchanges the same amount of thermal radiation to the occupant as the actual enclosure [1]. To obtain radiant temperatures, a black box is placed at the occupant's head level above the sofa with T-type thermocouples attached to all its faces. The mean radiation temperature is the average of these thermocouple readings. The operating temperature is then calculated from:

T o = ( T a + T r)	(1)
2

where T a and T r are the air and radiation temperatures, respectively. In addition, the air temperature in other parts of the test facility was measured at 1.1 m and 2.0 m from all walls of the test facility and the four spaces of the test facility. air between the test facility and the existing structure. Surface temperatures at midpoint locations inside and outside all walls as well as the ceiling and roof were recorded. The wood stove top, back and sides and chimney surface temperatures were also measured and recorded. The static pressure of the combustion gases in the chimney was also measured.

Test Facility heat loss calculation

As an index of thermal comfort, an ambient temperature of 22.5 °C was taken for occupant of a sedentary or near sedentary metabolic rate and an average insulation of clothing.

The heat loss of the test facility was calculated based on the final air temperature and availability deviation of 0 °C and 22.5 °C, respectively. The power required to raise the starting chamber temperature 0°C to 22.5°C in a given time can be calculated from equation 2.

where m and C p are mass and specific heat at constant air pressure, t is the time interval and AT is the difference between the gap temperature and the final room temperature. Substituting the appropriate terms, the total energy required is estimated at 1152 W.

To calculate the heat loss for the test installation, the thermal resistance of the walls, the two windows and the ceiling must be evaluated.

Table 1 shows the calculated values ​​of these thermal resistances with their associated regions.

Table 1. Thermal resistance of test facility limits

	Boundaries	Thermal resistance (m2 ° C/W)	Area (m 2)
	Walls	2,406	78.8
	the Windows	0.176	4.5
	Ceiling	3,952	62.5

The heat loss by transmission across the boundaries of the test facility can be calculated according to equation (3).

=			(3)
	Tr RTAQ

3

Comparative wood stove tests to determine fuel savings with the use of a Thermoelectric fan to maintain thermal comfort

where A, R, and AT, are the region, thermal resistance, and difference between ambient and gap temperatures, respectively. Substituting values ​​for walls, windows, ceiling and ATin equation 3 yields:

Q Tr = 78.8 X 22.5					(4)
2,406 + 4.5 X 220,176.5 + 62.5 X 3.95222. = 1662W
Heat loss through the floor slab is per perimeter and is evaluated using equation 5:
		(5)
	ground	Ô' = TPUQ

where U”, P, and AT are edge coefficient, floor perimeter, and difference between ambient and mass temperatures, respectively. Heat loss through the floor is rated at 1799 W, assuming an edge coefficient of 2.47 W/m°C for the floor with minimum insulation and a bulk temperature of 0°C. Heat loss total transmission will be the sum of heat losses through the limits of the installation is:

Q Tr = 1662 = 3461 1799 W

(6)

Total heat loss equals transmission heat loss plus infiltration heat loss according to equation 7.

where, Q i is the infiltration heat loss which is caused by the exchange of air between the test installation and outside. Assuming one air exchange per hour, the infiltration heat loss would equal 1152 W. Substitution for the appropriate terms in Equation 7 would produce the total heat loss in the test installation.

Q T = Q Tr + Q I = 3461 + 1152 = 4613 W

(8)

Numerical analysis of the test facility

To calculate and evaluate the impact of using a thermo-electric fan on a wood stove on the thermal environment of the test facility, a fluid dynamics analysis was carried out. FloEFD Pro 9TM,

a commercial CFD package by Mentor Graphics [5] was used to simulate the real case scenarios of the test facility with and without fans. The CFD images in Figure 2 clearly show that hot air is more evenly distributed when a thermo-electric fan is used. In Figure 2a, when no fan is used, the hot air rises directly to the top, where it spreads and stays at the top of the room. In Figure 2b however, when a fan used hot air is pushed lower into the room by forced convection and mixes and distributes the hot air evenly.

Figure 2 CFD image of the test setup with a) without a fan and b) with a fan

 

Comparative wood stove tests to determine fuel savings with the use of a Thermoelectric fan to maintain thermal comfort

Test procedure

A new testing protocol was developed as the objective of this investigation was unique. There have been a number of studies done on wood stoves, but they are designed to evaluate wood stove burn rates and stove efficiency or stack emissions. However, there was no previous work intended to evaluate the fuel savings when a specified thermal comfort for the occupant was maintained, using a thermoelectric fan. It is important to note that the fuel savings focus of this study is not directly related to the efficiency of wood stoves. The net space heat lost to the ambient outside is the same if a fan is used under the same environmental conditions. Using a fan does not change the heat output of the stove, but it does circulate and distribute the hot air throughout the room. In other words, a thermo-electric fan draws stagnant hot air from the top and back of a stove and forces it into the middle of the room. The enhanced forced heat transfer by convection causes occupants to experience a more comfortable room temperature. It results; Less fuel refills over prolonged use of the wood stove.

It was important to design a testing procedure that mirrored real-life scenarios as closely as possible. The goal of this investigation was to create a testing method that could capture the actual behavior of occupants attempting to use the stove to achieve and maintain thermal comfort. With this consideration, the tests were designed long where the test technician tried to maintain TO at 22.5°C. To do this, the wood stove started with a fixed amount of kindling and the pretest ignition fire, with additional fuel charges during the transition from the initial test facility temperature to the prescribed ambient temperature, after which the test would begin. Wood fuel will only be added when the fuel remaining in the stove has reached a minimum mass of 1 kg. The test technician was allowed to control the temperature by adjusting the air damper setting. In doing so, the fuel air would change resulting in a change in combustion rate thus affecting the heat output of the stove. The stove was placed on an electronic scale and the mass of existing or consumed wood could be calculated in any case of time. All data were acquired electronically by a computer data acquisition system. At the end of the test, the raw data was processed to determine the total wood consumption over the duration of the test and when divided by time, the burn rate in kg/h were estimated. The combustion rates were further normalized relative to AT, to account for the difference in temperature gradients. A detailed description of the test procedure analysis and data can be obtained from Caframo Ecofan Fuel Utilization Test Procedure Document [6]

For the testing protocol, EPA Method 28 for Certification and Verification of Wood-Burning Appliances [2] was used as a general guideline. There were gaps; one of which was the fuel type. The fuel used in this study was the airless untreated bark of two-year-old furniture grade dry white ash, compared to Douglas fir used in EPA Method 28 [2].

At the start of each test, 19 mm x 19 mm x 380 mm white ash (1.0 kg) is consumed for ignition, followed by 50 mm x 50 mm x 225 mm white ash (2.25 kg) of pre -ignition, and 50 mm x 100 mm x 380 mm of white ash (3.25 kg) for fuel charges. All burn rates were in EPA Method 28 Category 2 (0.8 to 1.25 kg/hr).

The moisture content of the wood fuel was measured by a moisture meter before placing it in the stove hearth. The wood fuel used in this study had a moisture content ranging from 18 to 23%. The moisture mass was deducted from the total mass to obtain the dry mass of wood.

An initial fuel consumption test procedure was developed and several test iterations were competed before the procedure was finalized. The evolution of the test protocol is briefly explained in the following section.

Comparative wood stove tests to determine fuel savings with the use of a Thermoelectric fan to maintain thermal comfort

Protocol and procedure Evolution of the wood stove Use of test fuel

Phase I: As outlined by EPA Method 28, the goal of this phase is to standardize the pretest

lighting procedures. Pretest ignition is necessary to ensure that a uniform charcoalization of the test fuel bed is achieved prior to test fuel loading. The pretest ignition consisted of loading crumpled newspaper,

1.0 kg of kindling and a pre-test fuel load of 2.25 kg. The pretest ignition is allowed to burn until the weight of the fuel is consumed at approximately 20 - 25% of the weight of the test fuel load. Stove operations and ambient temperature characterization was monitored and documented. At the end of this phase the ignition procedure was updated pretest and finalized. Phase II:

the test facility was brought to an initial state of 10°C and the ambient temperature outside relative

humidity. Once the pretest ignition was completed, the wood stove was charged with a single test load (3.25 kg) at a pre-run combustion air setting. Ambient temperature and stove operation were monitored and documented. Several tests were performed at different combustion air flow rates and resulting burn rates. It was determined that the desired comfort level of 22.5°C could not be achieved with a single fuel charge from an initial room condition of 10°C. Test procedure was revised to integrate multiple fuel loads. Phase III:

This phase was similar to Phase II, however, multiple fuel loads of 3.25 kg were added to

set time intervals and a preset combustion air setting. Ambient temperature and stove operation were monitored and documented. Several tests were performed at different combustion air flow rates and resulting burn rates. It was determined that the desired comfort level of 22.5°C could be achieved with multiple fuel loads, however, as the test progressed the fuel loads were not fully consumed during the prescribe time intervals and The wood stove became more filled with coals in various stages of combustion. After two consecutive fuel loads, additional fuel loads cannot be added and combustion BECOMES stifled with lower combustion air parameters. Phase IV:

Test multiple fuel loads of 3.25 kg were used, with a preset combustion air setting. However, additional fuel loads were added when the only total weight of fuel in the wood stove was

1.2kg. This ensured that there was enough room for the next load of fuel to be added to the wood stove and that there was an even burn. Test runs were typically 7 to 10 hours in duration, and the desired thermal comfort temperature was achieved. However, it was determined that the resulting ambient temperature varied significantly due to the cyclical nature of the combustion rate, the size of the fuel load (3.25 kg), and environmental influences. An analysis of the collected data determined that due to the significant variation in temperatures the data cannot be used for fan/any fan comparisons. The analysis also determined that the majority of test time and fuel consumed were used to bring the test facility from the initial conditions of 10°C to 22.5°C. As a result the actual test times at the desired comfort temperature were too short. Phase V:

Test procedure has been revised to reduce fuel load (50mm x 100mm x 380 white ash

Weighing approximately 1.25kg), with additional fuel loads only added when the total weight of fuel in the wood stove was 1.2kg and the combustion air settings were pre-set. The initial conditions of the test facility were varied from 10°C to 21°C, to minimize the transition time between the initial temperature conditions of desired thermal comfort. Environmental conditions for February and March indicated unusually higher daytime temperatures than normal. The dynamic environmental conditions significantly influenced the heat loss of the test facility and ultimately gave rise to a significant variation in the ambient temperature of the test facility. An analysis of the collected data determined that due to the significant variation in daytime temperatures, the data cannot be used for valid fan/no fan comparison.

Comparative wood stove tests to determine fuel savings with the use of a Thermoelectric fan to maintain thermal comfort

Phase VI: The test was started in the evening and conducted at night. During the night

Environmental conditions tend to be more stable and outdoor temperatures are lower and more consistent. Test procedures consisted of reduced fuel loads, with additional fuel being added when the total weight of fuel in the wood stove was 1.0 to 1.2 kg. In this phase, the test technician was allowed to adjust the combustion air setting, as necessary to maintain a stabilized thermal comfort temperature. To compensate for abnormally high seasonal temperatures and ensure constant heat loss rates from the test facility; The thermal comfort temperature was defined as 22.5°C above the temperature of the stabilized air space.

RESULTS AND DISCUSSIONS

In January and February 2010, a large number of tests were carried out. However, all test series could be taken into account in the data analysis. Some of the previous tests were designed to characterize the wood stove and test protocol. Test data that could be compiled and processed for an interpretation of thermal comfort and fuel consumption is listed in the table

Comparative wood stove tests to determine fuel savings with the use of a Thermoelectric fan to maintain thermal comfort

But a greater source of variability in the data is how the wood stove was used during the experiments. Since the main criterion was to maintain the comfort temperature around 22.5°C, the operator may have had to change the air damper setting a number of times in a given test series. Obviously, changing the air damper setting would change the air to fuel and in turn change the fuel burn rate and efficiency accordingly. Changing the air damper setting is a means of controlling and maintaining the temperature and reflects the real-life scenario of wood stove operation. In the absence of an automatic control system, the occupant changes the air setting to achieve desirable thermal comfort. It is evident from the data that the behavior of occupants in maintaining a thermally comfortable environment appears different when using a fan on the wood stove. Using a fan ensures a higher and more uniform comfort temperature, so the occupant would tend to operate the stove at a lower air damper setting. At lower settings, more complete combustion takes place and burn efficiency increases. This results in fewer refills of wood fuel over prolonged use of the wood stove.

FUEL SAVING

Wood is not the only advantage of using a fan with a wood stove. A fan would create a thermal environment that is more pleasant and comfortable for the occupant. ASHRAE Standard [1] has a number of scales for measuring local temperature discomfort, one which is vertical air temperature difference. According to ASHRAE standards [1], thermal stratification which results in the air temperature at the head being warmer than the ankle can cause thermal discomfort [1]. In assessing the vertical temperature difference, the temperature at the ankle level could not be used because it was observed that the temperature sensing assembly at that level had touched the ground at some point and using these values ​​would have distorted the results. Otherwise, temperature at knee level was used instead. Figure 3 compares the vertical temperature difference for the fan/no fan condition in the different test pairs. As is evident, the vertical temperature difference is lower than a fan in all tests performed. The difference is between 0.2°C to 0.9°C with an average of 0.5°C. It should be noted that the difference would have been even greater had the temperature at the ankle been used in the assessment of the vertical temperature difference.

This investigation establishes strong indications that the use of a fan on a wood stove saves fuel and improves the thermal comfort of occupants. However, to provide a more precise quantitative measure of fuel economy and thermal comfort, additional studies are needed. The authors intend to design and conduct improved experiments in the fall of 2010 to adequately quantify the benefits of using a fan when used with a wood stove. The new experiments will be conducted in a testing facility with enhanced controlled environment that allows sub-0°C settings.

CONCLUSION

The test results presented in the previous section of this report strongly suggest improved fuel economy and occupant thermal comfort when a thermo-electric fan is used with wood stoves.

In all tests, a consistent and considerable percentage in fuel savings is reported when the electric thermo fan is used with the wood stove. Fuel economy is 6% to 28% with an average of 14.1%. The large variability seen in fuel economy between experiments was expected. However, the test pairs are comparable because they were conducted in the same time frame and similar environmental conditions. There is also a strong trend and indication that the use of a thermo-electric fan improves the thermal comfort of the environment. In each test, the vertical temperature difference between the occupants' head and knee was less when a fan was used. The difference is 0.2°C to 0.9°C with an average of 0.5°C. The difference would have been even more if the difference had been assessed between the occupants' head and ankle.

NOMENCLATURE

A = area

C p = The specific heat of air at constant pressure

m = Air mass of the test facility

P = Perimeter

Q = Energy

R = Thermal resistance

t = time T a = Air temperature T o = Temperature of

occupant functioning

T r = Radiant Temperature

AT = temperature gradient between final and initial test facility

U » Edge coefficient