Performance Evaluation of Two 25 kW Residential Wood Pellet Boiler

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Performance Evaluation of Two 25 kW Residential Wood Pellet Boiler Heating Systems Kui Wang, Mauro Masiol, Devraj Thimmaiah, Yuanyuan Zhang, and Philip K. Hopke Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b01868 • Publication Date (Web): 16 Oct 2017 Downloaded from http://pubs.acs.org on October 17, 2017

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Energy & Fuels

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Performance Evaluation of Two 25 kW Residential

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Wood Pellet Boiler Heating Systems

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Kui Wang,1.2 Mauro Masiol,1 Devraj Thimmaiah, 1 Yuanyuan Zhang, 1 Philip K. Hopke1,2*

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1. Center for Air Resources Engineering and Science

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2. Department of Chemical and Biomolecular Engineering

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Clarkson University

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Potsdam, NY 13699 USA

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ABSTRACT

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A significant increase in the use of wood pellets for residential space heating has occurred over

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the past decade. The performance of two modern residential wood pellet boilers (designated PB

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and WPB) were evaluated including boiler thermal efficiency, thermal energy storage (TES) tank

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discharge efficiency, and system efficiency. A correlation applicable to both systems between the

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boiler thermal efficiency (ηth, in %) and the boiler output load (χ, in %) was found in the form of

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ηth = 52.69 ln χ – 137.7 with R2 = 0.79 (for 25 < χ < 75). This equation provides an easy,

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accurate estimation of the boiler thermal efficiency in field operations. The boiler thermal

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efficiency decreased with time and this decline was determined using a Mann-Kendall trend

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analysis with Sen’s slope. This decrease was primarily the result of fouling in the heat

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exchanger and thus, this analysis identifies the need for manual cleaning of the heat exchanger

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tubes to restore maximal system performance. The evaluation of the TES tank performance

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found that the TES tank discharge efficiency was correlated with a dimensionless function of

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tank inlet Reynolds number (Red) and temperature differences in the tank and inlet and outlet

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pipes. Overall system efficiency showed a seasonal average of 62.8%, 62.0%, and 75.8% for

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three heating seasons of the PB system. These results provide a comprehensive performance

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evaluation of these wood pellet boiler heating systems in the field over an extended period of

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operation.

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Keywords: wood pellet boiler; thermal efficiency; Sen’s slope; TES tank performance

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Nomenclature Btotal: energy to the tank from boiler, kJ

∆Td:

cp,w: water specific heat, kJ/(kg·K) Echa: energy to building during tank charge, kJ Edis:

energy to building discharge, kJ

during

tank

HHV: pellet fuel higher heating value, MJ/kg

TES tank inlet/outlet temperature difference, °C

∆Td,ave: average TES tank inlet/outlet temperature difference during discharge, °C ∆Ts: TES tank top and bottom temperature difference, °C

Mw: pellet fuel moisture content (w.b.)

∆Ts,ave: average TES tank top and bottom temperature difference during discharge, °C

N: boiler nominal load, kW

ε: boiler stoker feed fraction

Q: boiler loop flow rate, m3/h

ηth: boiler thermal efficiency, %

Q*: building loop flow rate, m3/h

ηsys: system efficiency, %

Red: inlet pipe Reynolds number

κ100: boiler stoker feed rate at 100%, kg/h

tc: time of a complete burn cycle, h

ξ: TES tank flow factor

td: boiler off time or tank discharge time, h

ρw: water density, kg/m3

T1: boiler output water temperature, °C

φ: TES tank overall efficiency (cycle basis), %

T1*: TES tank output water temperature, °C T2: boiler return water temperature, °C T2*: TES tank return water temperature, °C

φdis: TES tank discharge efficiency, % χ: boiler output load, %

∆t: boiler flow meter recording interval, s


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Energy & Fuels

1. Introduction Because of the increasing interest in the use of renewable energy, biomass has been

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increasingly utilized over the past decade. According to IEA, bioenergy provides 10% of world

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primary energy supply currently1 and biofuels can provide up to 27% of world transportation fuel

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by 2050.2 Generally, biomass refers to any material that is derived directly or indirectly from

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plants.3 Biomass, especially wood and wood pellets, is becoming more widely used as an

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alternative energy source for domestic space heating due to its relatively high energy density and

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uniform quality compared to cordwood or chips.4 In the European Union, wood pellet use is

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growing rapidly due to government policies to meet renewable energy targets.3,4,5

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Wood pellets offer both economic and climate benefits. Wood can be harvested sustainably

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and provide a substantial local energy supply as well as offering extra employment to collect and

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process the wood. Chau et al.6 provided a detailed techno-economic evaluation of a small scale

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wood pellet heating system. Wood pellet boiler technology has also improved over the past

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decades particularly driven by European interest in utilizing a local, renewable fuel. Modern

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wood pellet boilers have high efficiency and low pollutant emissions. Fiedler7 reviewed the state

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of the art of small-scale pellet combustion units for domestic use and the individual EU country

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regulations where more stringent regulations are likely to appear in the future. Öhman et al.8

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studied the slagging tendencies in residential wood pellet boilers related to ash sintering

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temperatures. They found that wood pellets have a low sintering tendency (sintering

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temperatures higher than 1100 °C) mainly due to the low alkali and Si content in the fuel.9 Hence,

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modern wood pellet boilers utilize moving grates or sliding grate ash removal systems.7 Better

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combustion chamber and flow geometry can be designed and optimized by applying

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computational fluid dynamics (CFD) simulations.10,11 In addition, studies have shown that when

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firing with non-woody biomass pellets, better boiler control such as fuel feeding, air intake, and

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ash management is required.12

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A key index for assessing the performance of wood pellet boiler is its thermal efficiency.

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There are many factors affecting the thermal efficiency of wood pellet boilers. Chandrasekaran et

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al. characterized the efficiency of two wood pellet boilers13 and wood stoves burning multiple

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types of pellets14. Efficiencies ranged from 70% to 91% for different boilers running with wood

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pellets and from 75% to 85% when burning with non-woody pellets. Verma et al.15,16 found that

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the performance of a domestic pellet boiler is a function of the boiler operational loads. Carlon


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et al.17 reported a non-linear relationship between thermal efficiency and a modified load factor.

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Several factors lead to the efficiency-load relationships. The most important factor is the excess

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air ratio that is usually higher under low load conditions. In addition, low load conditions cannot

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generate sufficient turbulence for complete combustion, which leads to low combustion

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efficiency and temperatures.16 Heat transfer is also reduced during low load conditions. Thus, it

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is better for the boiler to operate at high or full load condition so that maximum thermal

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efficiency can be achieved.

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Depending on the building load and the ambient temperature, the building may not require all

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of the heat produced by the boiler when it operates at full load. The use of thermal energy

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storage (TES) enables the wood pellet boiler to operate for longer periods at higher loads and,

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consequently, reduces the number of boiler ignitions and avoids short-cycling during warmer

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weather. TES in the form of large insulated water tanks was initially utilized in solar water

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heating systems because the stratification inside the tank can help to extract more useful heat

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from the tank.18 Many experimental and numerical studies have been performed to study the

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performance of TES tank in terms of temperature stratification, efficiency, and operational

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behavior.19-25

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The present work systematically evaluated the performance of two modern wood pellet

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heating systems in terms of: boiler thermal efficiency at various loads, TES tank discharge

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efficiency and overall system efficiency based on long-term monitoring from 2015 to 2017.

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Currently, there are no such systematic evaluations of in-use wood pellet heating systems for

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such an extended period.

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2. Material and Methods

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2.1 The Two Systems

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In this project, two nominally identical 25 kW wood pellet boilers were installed in two

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homes in northern New York State with different configurations (as shown in Figure 1). In each

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boiler, wood pellets are fed by a stoker screw auger into the combustion chamber from the top of

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the burner pot. A hot air gun ignites the pellets. Primary air enters the combustion chamber from

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the bottom of the burner pot. Hot combustion gas passes through a 3-stage fire tube heat

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exchanger to transfer heat into jacket water and exhausts through the chimney. The induced draft

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fan at the end of the flue gas pathway controls the primary air flow through a Lambda sensor (O2


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Energy & Fuels

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control) (calibrated with an external combustion analyzer) and a negative pressure sensor (draft

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control). Within each fire tube, there is a twisted turbulator to improve heat transfer as well as to

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clean the inside surfaces of the fire tubes by moving the turbulators up and down. The residual

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ash is removed from the bottom ash collector. The boiler is controlled by software that records

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its operational data.

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2.2 Pellet Fuel Characterization

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The pellets used in this study are PFI (Pellet Fuels Institute26) certified premium pellets.

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Samples were collected during bulk delivery to the pellet bins. The pellets were characterized

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for heating value, moisture, composition, and ash content. Detailed analytical methods and

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results are listed in Table 1. The pellets have a mean calorific content of 19.12 MJ/kg (or 8221

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Btu/lb) higher heating value (HHV), 3.88% moisture, 0.58% ash, and 46.64% carbon.

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Table 1. Summary of analytical methods and results Parameter

Method

Result

HHV

ASTM E 711-87 (2004)

19.12 MJ/kg (a.r.)

Moisture

ASTM E 871-82 (2006)

3.88% (w.b.)

Ash

ASTM D 1102-84 (2007)

0.58% (d.b.)

Carbon

ASTM E 777-87 (2004)

46.64% (d.b.)

Hydrogen

ASTM E 777-87 (2004)

6.4% (d.b.)

Nitrogen

ASTM E 778-87 (1996)

0.14% (d.b.)

Oxygen

By difference

46.23% (d.b.)

Chlorine

ASTME 776-87 (2009)

0.00388% (d.b.)

Sulfur

ASTM E 775-87 (2004)

0.00689% (d.b.)

107 108


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2.3 Experimental Methods Figure 1 shows the two system configurations as operated during the 2016-17 heating season.

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These systems had each undergone modification over time and detailed histories of each system

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are presented in the Supporting Information. The PB (Parishville boiler) system was installed in

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a modified shipping container near a family house in Parishville NY, USA (44.63 N, 74.83 W,

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271 m a.s.l.) replacing their existing outdoor wood boiler (OWB). Hot water from the boiler was

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stored in two identical TES tanks (A.O. Smith Co., volume of each = 0.45 m3) and then delivered

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to the house for space heating through an underground circulation system. The tanks provided

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36.0 liter per kW (2.8 gallons per kBtu/h) storage, above manufacturer recommendations of 25.7

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liter per kW (2 gallons per kBtu/h). Return water from the building flowed to the bottom of the

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tanks. This house has a total heating area of 406.5 m2 with an estimated peak load of 27 kW

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(92000 Btu/h). The tanks store hot water from the boiler at temperature ranging from 55°C

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(131°F) to 75°C (167°F) that is used for a radiant floor heating system with return water

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temperatures ranging from 35°C (95°F) to 45°C (113°F), depending on outdoor temperature and

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the thermostat settings.

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The other nominal 25 kW boiler was installed in the basement of a family house located in

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West Potsdam, NY, USA (44.69 N, 75.09 W, 122 m a.s.l.). An existing oil boiler was integrated

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into the system as an auxiliary boiler. This system is denoted as WPB (West Potsdam boiler)

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system. A single 210-gallon TES tank (0.79 m3) was installed in the system. In this system, the

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TES tank was installed in a 2-pipe configuration such that both the boiler and the heat demand

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are on the same side of the tank. In addition, WPB system had a 0.1 m3 domestic hot water

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(DHW) tank that provided domestic hot water. The additional oil boiler and the DHW tank

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made both the system control and the boiler behavior different from PB system.

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For both systems, flow and temperature were monitored in two loops: boiler to tank loop and

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tank to building loop to calculate heat output from the boiler and the TES tank, respectively. A

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certified ultrasonic flow meter (EF-10 series, Spire Metering Technology) and two temperature

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probes were installed in each loop. The boiler was controlled by two temperature sensors in the

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TES tank (bottom ON and top OFF).

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To determine the energy input rate, the boiler stoker feed auger was calibrated at several feed

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rates. The pellets were collected in a weighed bucket and after a fixed time, the bucket was

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weighed. From these results, the maximum feed rates (κ100) were determined to be 12.98 kg/h


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Energy & Fuels

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and 12.17 kg/h for PB and WPB, respectively. The constant speed augers are modulated by

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operating them only a part of the time (feed fraction, ε). The actual feed fractions are recorded at

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1 second intervals by each boiler. Using the fuel characterization data, the heat input into the

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boiler can be calculated as the product of the mass flow rate and the calorific content of the

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pellets. It was found that the augers were quite reproducible over time such that multiple

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calibrations produced essentially identical relationships between time of auger operation and

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mass flow rates of fuel delivered into the boiler.

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The operation of these boiler-thermal storage systems was characterized in terms of “system

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cycles” that began with the start-up of the boiler, steady-state combustion, boiler shut down, and

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finally depletion of heat from the thermal storage system to the building until the boiler restarted

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because the control temperature at the top of the thermal storage tank had been reduced to the

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setpoint value. Within these system cycles, there are “boiler cycles.” A complete boiler cycle

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includes three phases: start-up, steady state combustion, and burnout/shutdown.

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For PB system, measurements were made over three heating seasons denoted as PB-1, PB-2,

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and PB-3. PB-1 included data from February 1, 2015 to April 11, 2015 representing 493 cycles;

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PB-2 included data from December 1, 2015 to April 1, 2016 with a total of 392 cycles; and PB-3

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included data from January 5, 2017 to April 5, 2017 with a total of 372 cycles. During PB-1 and

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PB-2, the thermal storage tanks were continuously connected to the circulation loop that

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conducted heat to the building (See Supplemental Figure SI-3). Water was continuously

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circulated through this loop to avoid freezing in the underground line. Before PB-3, a 3-way

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valve was installed (Figure 1, top) such that the circulation loop was separated from the thermal

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storage tanks when there was no demand for heat from the house. Local weather data obtained

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from Weather Underground27 were used in this study.

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For WPB system, major changes had been made over the prior heating seasons to address

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operational problems that had been identified (see supplemental information). Thus, only WPB-

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3 data were used for the performance analysis. WPB-3 ran from December 28, 2016 to April 12,

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2017 and included a total of 644 cycles.

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2.4 Efficiency Calculations Because of the slower start-up and shut down associated with solid fuel combustion, the boiler thermal efficiency was calculated based on each boiler cycle. Overall efficiency was


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calculated based on system cycles. For each boiler cycle, the boiler thermal efficiency (ηth) was

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calculated as the ratio of heat output to heat input over each cycle time (tc) =

∑ , ( – ) ∙ Δ ∑(HHV ∙ ∙ )

× 100%

(1)

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where ∆t is ultrasonic flowmeter data sampling interval (10 s). The boiler stoker feed fraction ε

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was recorded every 1 s.

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System efficiency is defined based on system cycles. It is the total energy delivered into the

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building during a system cycle divided by the total energy input into the boiler for each boiler

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cycle, as #$# =

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∑&' ∗ , ( ∗ – ∗ ) ∙ Δ ∑(HHV ∙ ∙ )

× 100%

where td is the time when boiler is off until next boiler cycle.


(2)

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Energy & Fuels

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Figure 1. Schematic of the PB (top) and WPB (bottom) systems with data acquisition devices (S-

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1: flue gas O2 control signal, S-2: boiler pump control signal; S-3: Boiler ON signal; S-4: Boiler

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OFF signal)

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Energy & Fuels

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3. Results and Discussion

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3.1 Boiler Thermal Efficiency

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During a boiler cycle, the instantaneous efficiency of the boiler changes. Low efficiency

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occurs during start-up due to inefficient combustion during the ignition and stabilization stages

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when there are also high CO and particle emissions.28 As the combustion stabilizes, the boiler

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thermal efficiency reaches its highest value during its steady state operation. Low efficiencies

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again occur as the boiler shuts down and the residual fuel burns out. Thus, boiler thermal

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efficiency over each cycle is an appropriate metric for boiler performance.

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Figure 2 shows the regression between boiler thermal efficiency and boiler output load

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percentage for both boiler systems. For the PB system, due to boiler software updates and

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frequent setting change of TES tank ON/OFF temperatures during PB-2 and PB-3, a relatively

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low r2 = 0.68 was determined. However, when these parameters remained fixed in PB-1, the r2

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rose to 0.86. For the WPB system, the operation of DHW significantly affected the boiler cycling

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characteristics, as explained in the supporting information. Because the DHW tank represented a

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low thermal mass, very little energy was dissipated into the domestic hot water. When the boiler

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was ON because of demand from the thermal storage tank and the DHW was also demanding

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heat, the DHW tank acts as an independent TES tank. However, the low heat dissipation in the

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DHW tank resulted in very high boiler return water temperatures that approached the safety limit

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set temperature of the boiler. Therefore, the boiler reduced its output quickly sometimes causing

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boiler short cycling. The problem was reduced during WPB-3 by defining wider temperature set-

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points for DHW tank but controls alone could not eliminate the problem.

206 207

A general regression equation based on four datasets for the two systems was determined by merging the data and fitting an equation as follows: ηth = 52.69 ln χ – 137.7

(for 25< χ i), n is the number of total data points, and sgn(xj - xi ) is the

272

sign function defined as

273

+1, sgn234 − 3 6 = : 0, −1,

(7)

Mann30 and Kendall31 have shown that when n ≥ 18, S is approximately a normal distribution

274 275

34 − 3 > 0 34 − 3 = 0 34 − 3 < 0

with expectation of 0 and variance as >

(-)

?(? − 1)(2? + 5) − ∑B 8 ( − 1)(2 + 5) = 18


(8)

Energy & Fuels

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where m is the number of tied groups and ti denotes the number of ties of extent i. A tied group is

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defined as a set of samples with the same value. The standard normal test (or Z-test) statistic ZS

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is computed as

279

-−1 , if - > 0 I GJ> (-) DE = 0, if - = 0 H -+1 G , if - < 0 FJ> (-)

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usually used to estimate the slope of the trend (also called Sen’s slope analysis). Figure 4 shows

281

the analysis results for PB and WPB systems.

282

(9)

ZS > 0 means increasing trends while ZS < 0 means decreasing trends. A Theil-Sen’s line is

A decreasing trend in boiler thermal efficiency can be observed in Figure 4(a) when there

283

was no manual cleaning performed. High boiler thermal efficiency of around 85% was initially

284

observed after the boiler installation. An average rate of -0.022 % per boiler cycle was

285

determined for PB-1. Therefore, if the boiler must be cleaned manually when the thermal

286

efficiency has dropped by 10%, PB has to be cleaned after 455 boiler cycles. Given an average

287

of 7 boiler cycles per day, the boiler needs to be cleaned after ~65 days of continuous operation.

288

Figure 4(b) shows a relatively constant trend of boiler thermal efficiency for PB-2, which

289

suggests that the boiler can only maintain around 70% thermal efficiency based on its internal

290

cleaning mechanism. After two manual cleanings were performed during PB-3, the boiler

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thermal efficiency significantly improved compared to PB-2. For WPB-3, an average boiler

292

thermal efficiency rate of -0.038% per boiler cycle was determined, which means WPB must be

293

manually cleaned after 263 boiler cycles or ~38 days of continuous operation given the same

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evaluating criterion as PB. WPB-3 had the higher rate of decline in the boiler thermal efficiency.

295

One factor for this boiler could be the high moisture content in the basement. This old basement

296

has little ventilation and standing water was often observed on the basement floor on rainy days.

297

When the boiler was manually cleaned at the beginning of WPB-3, the cleaning turbulators had

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to be replaced due to substantial fouling and corrosion. In a high moisture environment, it is

299

much easier to have condensation in the boiler, which facilitates boiler fouling and corrosion

300


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Energy & Fuels

(a). PB-1

(b). PB-2

(c). PB-3

(d). WPB-3

301 302

Figure 4. Sen’s slope of boiler thermal efficiency for PB and WPB systems (c.i. – confidence

303

interval)


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304 305

process. Thus, operation of a dehumidifier in these circumstances may improve the boiler performance.

306 307

3.3 Thermal Storage Tank Performance

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The TES tank top and bottom temperatures are measured by the ON and OFF temperature

309

sensors inserted into sensor wells in the tank as shown in Figure 1. The TES tank top and bottom

310

temperature difference (∆Ts) can be used to assess tank stratification since a well stratified tank

311

has a larger ∆Ts. Figure 5 shows tank behavior during a complete system cycle in PB-1.

312 313

Figure 5. TES tank temperature distribution during one complete boiler burn cycle in PB-3

314 315

The vertical dashed lines divide the system cycle into charge and discharge periods. During

316

the TES tank charging period, the boiler heats the tank to the set temperature. Then the boiler

317

shuts down, and the tank discharges heat to the building. The amount of energy discharged is

318

determined by the stratification inside the tank since a well stratified tank has higher exergy than


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Energy & Fuels

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a completely mixed tank.32 The TES tank can continuously supply energy to the building without

320

interruption depending on the building demand.

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A well stratified tank can discharge more energy than a completely mixed tank, as shown in earlier studies.19-24 The overall tank efficiency (φ) is defined as φ = (Echa + Edis)/Btotal × 100%

323

(10)

Simultaneously, tank discharge efficiency (φdis) is defined as φdis = Edis/(Btotal – Echa) × 100%

(11)

324

where Echa and Edis are energy delivered to the building during charge and discharge periods,

325

respectively; Btotal is the total energy delivered to the tank during a complete boiler cycle.

326

Tank discharge efficiency, φdis, is a good indicator of the stratification inside the tank when

327

the boiler is off. Studies have shown that two major factors affect the stratification: tank

328

geometry (inlet/outlet positions,20 tank aspect ratio,19 baffle plates,23 etc.) and operational

329

conditions (temperature difference,33 flow rates,33 operation modes,21,22 etc.). The present

330

evaluation focuses on these operational conditions because the tanks had a fixed geometry during

331

the entire monitoring period and the TES tank was operated with continuous flow on the building

332

side for PB system during the first two heating seasons.

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Figure 6(a) shows TES tank top and bottom average temperature difference (∆Ts,ave) over

334

each discharge period as a function of corresponding discharge time td (or boiler off time) for

335

PB-1 data. The correlation equation is shown below (R2 = 0.83) ∆Ts,ave = 66.75(td)-1 – 150.0(td)-2 + 119.3(td)-3 – 5.965


(12)

Energy & Fuels

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336 337

Figure 6. (a): TES tank top and bottom average temperature difference as a function of discharge

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time for PB-1; (b): Tank discharge efficiency correlated with dimensionless flow factor for PB-1

339 340

Longer discharge times indicate less heat demand from the building (i.e. warmer weather)

341

that result in smaller temperature differences in the tank since the TES tank supply and return

342

water temperatures will be closer in temperature. Then, no clear thermocline may develop in the

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tank.34 Consequently, when the building is demanding heat, a larger ∆Ts,ave will occur due to the

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larger temperature difference between the TES tank supply and return water temperatures. Figure

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6(a) shows that stratification of a TES tank in the heating system is highly dependent on the

346

building heat load. This result suggests that maintaining stratification inside the tank at low

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building heat loads can be attained by decreasing the flow rate to create a larger return water

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temperature difference from the tank top temperature. Figure 6(b) shows TES tank discharge efficiency φdis is correlated with a dimensionless

349 350

number ξ by the following equation (r2 = 0.73) ln φdis = 8.3 – 175.3ξ

(13)

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where φdis is in %, and ξ is expressed in pipe Reynolds number (Red) and temperature differences

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as ξ = (∆Ts,ave /∆Td,ave)0.35(Red)-0.41

(14)

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where ∆Ts,ave is the average TES tank top and bottom temperature difference during discharge

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period and ∆Td,ave is the average TES tank supply and return temperature difference during the

355

same discharge period. Figure 6(b) indicates that an increase in Red and decrease in ∆Ts,ave


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/∆Td,ave will lead to an exponential increase in tank discharge efficiency. For a specific TES tank,

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increasing the Red means an increased flow rate, which corresponds with previous studies that

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the TES tank discharge efficiency (or extraction efficiency) increases with increasing flow

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rate.19,24,33 Thus, inlet flow diffusers and baffles23,24,35 are used to minimize flow mixing and

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maintain a high level of stratification. The ratio of ∆Ts,ave /∆Td,ave is an indicator of the amount of

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heat delivered to the building. The maximum value of ∆Ts,ave /∆Td,ave is 1 when there is no heat

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delivered to the building or the return water temperature is equal to the temperature at the bottom

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of TES tank. Small ratios suggest large TES tank supply and return temperature differences,

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which further indicate large amounts of heat consumed by the building.

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For PB-2 and PB-3, correlations between ∆Ts,ave and td, φdis and ξ have much lower R2 values

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compared to PB-1 data as shown in Figure 6. This result is caused by the constant TES tank

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ON/OFF setting maintained over the whole heating season for PB-1, but a variety of TES tank

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ON/OFF settings were explored during PB-2 and PB-3 that in turn reduced the correlation. For

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WPB-3, the TES tank stratification (∆Ts) was constantly disturbed by the DHW tank operation.

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Therefore, the correlation was worse than for PB-2 and PB-3. In addition, the 2-pipe

371

configuration prohibits the determination of the exact amount of energy delivered into the TES

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tank since part of the energy produced while the boiler is on can be directed to the DHW tank.

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Therefore, no correlation for TES tank discharge efficiency can be established. An additional

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flowmeter on the DHW tank loop would have been required to permit this calculation.

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376 377

3.4 System Efficiency System efficiency evaluates the efficiency of the whole system based on the heat delivered to

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the building and the heat input into the boiler. The average seasonal system efficiency values

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were 62.8%, 62.0%, and 75.8% for PB-1, PB-2, and PB-3, respectively. Low system efficiencies

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during PB-1 and PB-2 was mostly due to the low thermal efficiencies and low TES tank

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discharge efficiencies when there was no 3-way valve installed to prevent the constant

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circulation of the building circulator as shown in Figure 1 (as discussed in supplemental

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information). After effective boiler cleaning and system modification, the system efficiency in

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PB-3 significantly increased. System efficiencies for WPB could not calculated because the

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current measurement system did not permit the determination of the amounts of heat input

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transferred into both the TES tank and the building based on the 2-pipe configuration system.

387 388

4. Conclusions and Suggestions

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4.1 Conclusions

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The objective of this study was to evaluate the performance of two 25 kW residential wood

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pellet boilers in terms of boiler thermal efficiency, thermal energy storage (TES) tank discharge

392

efficiency, and system efficiency based on long term field site monitoring results. A general

393

correlation between boiler thermal efficiency (ηth, in %) and boiler output load percentage (χ,

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in %) was determined to be ηth = 52.69 ln χ – 137.7 with r2 = 0.79 (for 25 < χ < 75) for both

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systems. Evaluation of thermal efficiency bins showed that most of time, the PB boiler was

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operating above 70% efficiency with seasonal average thermal efficiencies of 73.6%, 70.8%, and

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79.0% for PB-1, PB-2, and PB-3, respectively. However, the WPB boiler was operating below

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70% thermal efficiency most of the time caused by the DHW induced load modulation and

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severe boiler fouling. A Sen’s slope analysis on the boiler thermal efficiency can determine the

400

boiler thermal efficiency decreasing rate and provide information on when additional cleaning is

401

needed. The results showed that the PB boiler had a thermal efficiency drop of 0.022% per boiler

402

cycle and could maintain around 70% boiler thermal efficiency without manual cleaning. The

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WPB boiler had a thermal efficiency drop of 0.038% per boiler cycle and must be cleaned every

404

38 days (given an average of 7 boiler cycles per day). TES tank performance showed a

405

correlation of the tank discharge efficiency with a dimensionless number that is a function of

406

inlet pipe Reynolds number and temperature differences in both the tank and pipe inlet/out. The


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system efficiency evaluation revealed that effective boiler cleaning and system modification can

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enhance overall system performance. Overall system efficiency showed seasonal averages of

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62.8%, 62.0%, and 75.8% for PB-1, PB-2, and PB-3, respectively. Future work will be focused

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on continuing monitoring and performing dynamic process simulation of the system for further

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optimization. The boiler emissions will be discussed separately in a future manuscript.

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4.2 Suggestions for Improvement (1). High moisture content in the combustion air can accelerate wood pellet boiler fouling, as

415

shown in WPB installation. Therefore, it is important to ensure a low moisture content

416

environment for the wood pellet boiler such as adding a dehumidifier to high moisture content

417

environment.

418

(2). The 2-pipe configuration does not necessarily perform better than the traditional 4-pipe

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configuration. It is important to recognize the effect of the thermal mass of the different heat

420

demanding units connected to the system such that the boiler short-cycling and load modulation

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shown in WPB system can be avoided. An alternative 3-pipe configuration might solve the

422

problem.

423

(3). Frequent manual cleaning of the heat exchanger is essential to maintaining high overall

424

system performance for these wood pellet heating systems. The decline in the thermal efficiency

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can serve as an indicator of the need for additional cleaning.

426 427

Acknowledgements

428

This work was supported by the New York State Energy Research and Development Authority

429

(NYSERDA) under agreements #32977 and 63007. We want to express our thanks to the

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homeowners of the boiler systems for their willingness to work with us to assess the

431

effectiveness of this technology. John Siegenthaler of Appropriate Designs and Khaled Yousef

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of Pyramid Energy Engineering Services, PLLC are acknowledged for their advice on system

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design and modifications. We also want to acknowledge the cooperation of the boiler

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manufacturer in helping to address issues as they were identified.

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References (1) International Energy Agency (IEA), available at https://www.iea.org/topics/renewables/subtopics/bioenergy/. (2) International Energy Agency (IEA), available at https://www.iea.org/newsroomandevents/pressreleases/2011/april/biofuels-can-provide-up-to27of-world-transportation-fuel-by-2050-iea-report-.html. (3) Demirbas, A. Biomass resource facilities and biomass conversion processing for fuels and chemicals. Energy Convers Manage 2001, 42, 1357-78. (4) Goh, C.S.; Junginger, M.; Cocchi M.; Marchal, D.; Thrän, D.; Hennig, C.; et al. Wood pellet market and trade: a global perspective. Biofuels Bioprod Biorefin 2013, 7, 24-42. (5) Parikka, M. Global biomass fuel resources. Biomass Bioenergy 2004, 27, 613-20. (6) Chau, J.; Sowlati, T.; Sokhansanj, S.; Preto, F.; Melin, S.; Bi, X. Techno-economic analysis of wood biomass boilers for the greenhouse industry. Appl Energy 2009, 86, 364-71. (7) Fiedler, F. The state of the art of small-scale pellet-based heating systems and relevant regulations in Sweden, Austria and Germany. Renew Sustain Energy Rev 2004, 8, 201-21. (8) Öhman, M.; Boman, C.; Hedman, H.; Nordin, A.; Boström, D. Slagging tendencies of wood pellet ash during combustion in residential pellet burners. Biomass Bioenergy 2004, 27, 585-96. (9) Vega-Nieva, D.J.; Torres, L.; Tabares, J.; Morán J. Measuring and Predicting the Slagging of Woody and Herbaceous Mediterranean Biomass Fuels on a Domestic Pellet Boiler. Energy Fuels 2016, 30, 1085-95. (10) Porteiro, J.; Collazo, J.; Patiño, D.; Granada, E.; Gonzalez, J.C.M.; Míguez, J.L. Numerical modeling of a biomass pellet domestic boiler. Energy Fuels 2009, 23, 1067-75. (11) Chaney, J.; Liu, H.; Li, J. An overview of CFD modelling of small-scale fixed-bed biomass pellet boilers with preliminary results from a simplified approach. Energy Convers Manage 2012, 63, 149-56. (12) Carvalho, L.; Wopienka, E.; Pointner, C.; Lundgren, J.; Verma, V.K.; Haslinger, W.; et al. Performance of a pellet boiler fired with agricultural fuels. Appl Energy 2013, 104, 286-96. (13) Chandrasekaran, S.R.; Laing, J.R.; Holsen, T.M.; Raja, S.; Hopke, P.K. Emission characterization and efficiency measurements of high-efficiency wood boilers. Energy Fuels 2011, 25, 5015-21. (14) Chandrasekaran, S.R.; Hopke, P.K.; Newtown, M.; Hurlbut, A. Residential-scale biomass boiler emissions and efficiency characterization for several fuels. Energy Fuels 2013, 27, 4840-9. (15) Verma, V.K.; Bram, S.; Gauthier, G.; Ruyck, J.D. Performance of a domestic pellet boiler as a function of operational loads: Part-2. Biomass Bioenergy 2011, 35, 272-9. (16) Verma, V.K.; Bram, S.; Delattin, F.; Ruyck, J.D. Real life performance of domestic pellet boiler technologies as a function of operational loads: A case study of Belgium. Appl Energy 2013, 101, 357-62. (17) Carlon, E.; Schwarz, M.; Golicza, L.; Verma, V.K.; Prada, A.; Baratieri, M.; et al. Efficiency and operational behaviour of small-scale pellet boilers installed in residential buildings. Appl Energy 2015, 155, 854-65. (18) Hollands, K.G.T.; Lightstone, M.F. A review of low-flow, stratified-tank solar water heating systems. Sol Energy 1989, 43, 97-105. (19) Lavan, Z.; Thompson, J. Experimental study of thermally stratified hot water storage tanks. Sol Energy 1977, 19, 519-24.


Page 23 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

(20) Shah, L.J.; Furbo, S. Entrance effects in solar storage tanks. Sol Energy 2003, 75, 337-48. (21) Fernandez-Seara, J.; Uhia, F.J.; Sieres, J. Experimental analysis of a domestic electric hot water storage tank. Part I: Static mode of operation. Appl Therm Eng 2007, 27,129-36. (22) Fernandez-Seara, J.; Uhia, F.J.; Sieres, J. Experimental analysis of a domestic electric hot water storage tank. Part II: dynamic mode of operation. Appl Therm Eng 2007, 27,137-44. (23) Levers, S.; Lin, W. Numerical simulation of three-dimensional flow dynamics in a hot water storage tank. Appl Energy 2009, 86, 2604-14. (24) Han, Y.M.; Wang, R.Z.; Dai, Y.J. Thermal stratification within the water tank. Renew Sustain Energy Rev 2009, 13, 1014-26. (25) Rosen, M.A.; Dincer, I. Exergy methods for assessing and comparing thermal storage systems. Int J Energy Res 2003, 27, 415-30. (26) Pellet Fuels Institute, available at http://www.pelletheat.org/joining-the-standards-program. (27) Weather History & Data Archive, available at https://www.wunderground.com. (28) Win, K.; Persson, T. Emissions from residential wood pellet boilers and stove characterized into start-up, steady operation, and stop emissions. Energy Fuels 2014, 28, 2496-505. (29) Sippula, O.; Hytönen, K.; Tissari, J.; Raunemaa, T; Jokiniemi, J. Effect of wood fuel on the emissions from a top-feed pellet stove. Energy Fuels 2007, 21, 1151-60. (30) Mann, H.B. Nonparametric tests against trend. Econometrica, 1945, 13, 245–259. (31) Kendall, M.G. Rank Correlation Methods. 1975, Griffin, London, UK. (32) Rosen, M. A. The exergy of stratified thermal energy storages. Sol Energy, 2001, 71, 173185. (33) Hariharan, K.; Murthy, M.V.K. Temperature stratification in hot-water storage tanks. Energy 1991, 16, 977-82. (34) Abdoly, M.A.; Rapp, D. Theoretical and experimental studies of stratified thermocline storage of hot water. Energy Convers Manage 1982, 22, 275-85. (35) Altuntop, N.; Arslan, M.; Ozceyhan, V.; Kanoglu, M. Effect of obstacles on thermal stratification in hot water storage tanks. Appl Therm Eng 2005, 25, 2285-98.


Performance Evaluation of Two 25 kW Residential Wood Pellet Boiler

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