SYSTEM PERFORMANCE OF A STORAGE INTEGRATED PELLET BOILER Stefan Aigenbauer1, Michael Hartl2, Ivan Malenkovic2; Andreas Simetzberger3, Vijay Kumar Verma1, Christoph Schmidl1 1 Bioenergy2020+ GmbH, Gewerbepark Haag 3, 3250 Wieselburg-Land, Austria www.bioenergy2020.eu,
[email protected], +43 (0)7416 / 522 38-47 2 AIT Austrian Institute of Technology, Giefinggasse 2, 1210 Vienna, Austria 3 SOLARFOCUS GmbH, Werkstraße 1, 4451 St.Ulrich/Steyr, Austria
ABSTRACT: A pellet burner directly integrated into the solar storage provides heat and domestic hot water for small residential applications in an environment-friendly way. The objective of this work was to evaluate the system performance of a storage integrated pellet boiler in laboratory under transient test conditions. Furthermore, the type test results according to ÖNORM EN 303-5 [1] of the last decade were compared with monitoring data of systems with separated boiler and heat storage. The laboratory tests allowed finding relevant parameters and losses, which influence the system performance. A developed computer simulation model shows the potential to optimize the performance of the investigated boiler. Keywords: boiler, energy balance, modelling, monitoring, performance, small scale application
1
AIM
The aim of this work was to evaluate the system performance of a storage integrated pellet boiler in laboratory under transient conditions and to compare type test results according to ÖNORM EN 303-5 with monitoring data of systems with separated boiler and heat storage. As a further step the evaluated data are used for developing a computer simulation model.
2
EXPERIMENTAL APPROACH AND METHOD
By measuring pellet consumption, the consumed heat for room heating and domestic hot water and the losses, it was possible to make a complete energy balance and calculate the system performance. The tested pellet burner in figure 1 is directly integrated into the solar storage. Test object is the boiler and storage tank. Figure 1 shows a sectional view of the investigated storage integrated boiler. The down firing combustion technology boiler was equipped with a 500 l solar storage, pellet tank and heat exchanger pipes.
Q stored heat
Q power supply
Tflow, room expansion vessel
Treturn, room m(t) room
Q heat, room
Qair balance, m(t) fuel
Quseful ,heat m(t2 ) fuel * NCV,wb
* 100[%]
Equation 1: System performance
Qheat,DHW
Qfuel
η SP =
Qlosses Tflow, DHW
chimney
The power range of the investigated pellet boiler varies from 2,9 to 14,9 kW and the maximum flow temperature is 85°C. The boiler was type tested [4] with a thermal efficiency of 93,1 % at nominal load and 89,4 % at minimum load. Furthermore in figure 1 is a sketch about the measured values and the system boundary with the input and outputs. The system performance is defined as efficiency depending on a defined investigation time and defined system boundary. The system performance (ηsp) was calculated on the basis of the fuel energy input (Qfuel) and the boiler heat output (Quseful heat). Fuel energy input was calculated with the fuel consumption and the net calorific value (NCV) based on the wet fuel (wb). For energy balances, investigation time (t) started with the start of the pellet boiler (t0), passes the stop of the boiler operation, (t1), until the end of the discharging phase, when the boiler starts again (t2). At the end of the system balance time load difference of the storage compared to the beginning was considered in the calculation of system performance.
Treturn, DHW m(t) DHW
Q useful heat = Q heat, room + Q heat, DHW + Q stored heat Q losses = Σ (Q exhaust + Q buffer losses) Q fuel = Q losses + Q heat, room + Q heat, DHW + Q stored heat Figure 1: Investigated storage integrated boiler; source: Solarfocus GmbH
The storage integrated pellet boiler was tested in the laboratory under transient operating conditions with selected load profiles for room heating (transition period and winter) and domestic hot water (DHW). According to VDI 4655 guideline [2] and DIN 4702 part 8 [3], four reference days with different heat demands were defined. The investigated boiler has to react to this heat demand curves. According to the VDI 4655, reference days are called: Transition period clear, transition period cloudy, winter clear and winter cloudy. For the energy balance and system performance calculation only defined sections from this reference days were chosen (table I to table IV). In case of hydraulic systems with storage tanks, boilers usually operate at nominal load. The investigated boiler operates also at nominal load (14,9 kW) until the measured boiler temperature reaches a set difference temperature to a wanted boiler temperature. At this point the boiler modulates with a P-type controller from
nominal load until minimum load and then the boiler shuts-off.
Table IV: Defined section of reference day: Winter clear Boiler operation parameters
Table I: Defined section of reference day: Transition period cloudy Boiler operation parameters
Transition period cloudy #1 #2 #3
Investigation time
hh:mm
04:05
04:00
04:31
boiler operation time
hh:mm
01:30
01:31
01:25
average heat load (demand)
kW
average flow temperature
°C
81,7
82,1
80,6
average return temperature
°C
41,3
41,4
40,6
3,56
3,56
2,96
Table II: Defined section of reference day: Transition period clear Boiler operation parameters
Transition period clear #4 #5 #6
Investigation time
hh:mm
05:00
06:00
04:16
boiler operation time
hh:mm
01:26
01:42
01:26
average heat load (demand)
kW
2,58
2,62
3,02
average flow temperature
°C
82,2
80,8
81,7
average return temperature
°C
41,5
42,4
42,5
Table III: Defined section of reference day: Winter cloudy Boiler operation parameters
Winter cloudy #7 #8 #9
Investigation time
hh:mm
02:50
03:05
04:45
boiler operation time
hh:mm
01:41
01:49
03:26
average heat load (demand)
kW
6,58
6,31
7,32
average flow temperature
°C
average return temperature
°C
80,3 36,4
80,0 36,2
#10
Winter clear #11 #12
Investigation time
hh:mm
04:27
02:54
03:02
boiler operation time
hh:mm
03:21
01:51
02:03
average heat load (demand)
kW
7,26
6,64
7,22
average flow temperature
°C
81,8
79,8
79,0
average return temperature
°C
37,0
37,3
37,5
3
BACKGROUND
Modern pellet boilers achieve efficiencies over 90%, depending on the nominal load and in steady state operation under defined test conditions e.g. ÖNORM EN 303-5. These efficiencies are not characteristic for the annual efficiency in field conditions. The annual efficiency of heat supply systems also included the losses from start up, stand by, load change, shut down phases, storage losses, non-optimal temperature levels and part load operation. The difference between the annual efficiency of pellet boilers and the results of the test method ÖNORM EN 303-5 are listed in table V. These values are not comparable with the results from the current investigations since a different method as compared to the annual efficiency method [7] was used. Table V: Difference between the annual system performance and standard test method annual efficiency [%] Method; (n = number of boilers) Annual efficiency Schwarz [7] method; n=2
average
min
75,7
73,7
77,6
max
Schraube [5]
monitoring; n=6
73,0
70
80
Kunde [6]
monitoring; n=6
74,1
69,9
80,4
80,9 test efficiency [%] avermin max age
36,1 Test method EN 303-5 pellet boiler at full load 1999 – 2009 source: FJ BLT Wieselburg; compiled: Bioenergy2020 + Type test; constant operation; n=199
91,6
80,9
96,2
40
4.1 System performance at different heat loads Energy balance of the boiler from test 8 is presented in figure 2. The cumulative energy as a function of test duration is presented in terms of several parameters e.g. nominal load, stored heat, cumulated fuel input, useful heat and the losses of the storage integrated pellet boiler. The pellet screw starts at t0 and fills the combustion chamber with initial pellets. After the pellet ignition at ta, the boiler power changes from 50 % to 80 % and finally (tb) to 95 % (nominal load). When the temperature in the middle of the storage is close to the wanted boiler temperature (tc), the power is modulated down until (td) 35 % (minimum load). In the shut down phase (td to t1) an exhaust fan removes gases from the combustion chamber. The pellet boiler changes in the stand-by mode from t1 to t2.
35
100%
t0
t2 80%
30 25
60%
20 40%
15 10
20% 5 0 00:00
01:00
02:00
03:00
Time Q fuel [kWh]
Q useful heat [kWh]
Q losses [kWh]
nominal load [%]
Q stored heat [kWh]
Figure 4: Energy balance from test #3; Transition period cloudy
95
t2 80%
25
60%
t1
ta
40%
15
td 10 20%
0 00:00
01:00
02:00
03:00
0% 05:00
04:00
Transition period, cloudy
90
Winter clear
85 Winter cloudy
80 monitoring
Monitoring n=6; transition period
0
Q fuel [kWh]
Q useful heat [kWh]
Q losses [kWh]
nominal load [%]
40
100%
25
80%
60%
20 40%
15 10
20% 5
03:00
04:00
0% 05:00
Time Q fuel [kWh]
Q useful heat [kWh]
Q losses [kWh]
nominal load [%]
8
Table VI: Relevant parameters of three laboratory tests
Nominal load [%]
t2
30
02:00
4 6 Heat load [kW]
Figure 5: System performance under different transient operation conditions; results of the type test; monitoring data of separated systems [5]
t0
01:00
2
Q stored heat [kWh]
Figure 2: Energy balance from test #8; winter cloudy
0 00:00
Monitoring n=6; winter
70
Time
Cumulated energy [kWh]
laboratory test
75
5
35
System performance [%]
t0
30
20
Transition period, clear
100%
tc
Nominal load [%]
Cumulated energy [kWh]
35
tb
0% 05:00
04:00
100 40
Nominal load [%]
RESULTS Cumulated energy [kWh]
4
Q stored heat [kWh]
Figure 3: Energy balance from test #9; winter cloudy Figure 5 shows the system performances of the laboratory tests and monitoring data of conventional boiler-storage systems [5] in winter and transition period. It shows that the tested system has a potentially higher performance for the considered operating conditions than the reported systems. Furthermore, system performance increases with a higher average heat load (demand). The difference on an average is around 8 %.
Operation mode
#8; winter cloudy nominal load
#9; winter cloudy modulation
#3; transition cloudy nominal load
storage load and discharging time (t0 to t2)
hh:mm
03:05
04:45
04:31
boiler operation time (t0 to t1)
hh:mm
01:49
03:26
01:25
average heat load (demand) (t0 to t2)
kW
6,31
7,32
2,96
useful heat and storage load (t2)
kWh
19,47
34,75
13,35
fuel input (t2)
kWh
20,88
37,89
15,98
losses (t2)
kWh
1,41
3,14
2,63
storage losses (t2)
kWh
0,19
0,30
0,30
other losses (t2),
kWh
1,22
2,84
2,33
average exhaust temperature (t0 t1)
°C
112
111
111
average residual oxygen (t0 - t1)
%
9,14
8,54
10,34
system performance (t0 - t2)
%
93,2
91,7
83,5
•
According to the results in table VI, storage losses for the system performance are insignificant as compared to the overall losses. If there are the same maximum storage loads (temperature level) the relative storage losses are always more or less 2 % to 3 %. These results do not correlate with the monitoring measured storage losses (of boiler separated systems), which vary between 8 % and 18 % of the energy input [5]. The reason for the different system performance between low and high average heat load (demand) is the difference between the average residual oxygen during the boiler operation. If the heat demand is higher, for example on a cold cloudy winter day, the boiler operates longer in the optimal mode with low residual oxygen. The non-optimal modes with higher average residual oxygen (start and shut off phase) are shorter compared to the overall operation time. This results and the high performance of investigated boiler in minimum load (89,4 % type test measured) [4], shows the potential for a optimized modulation operation of the investigated storage integrated boiler. The goal is to avoid short operating cycles, which have a negative influence on the boiler performance. 4.2 Simulation of different thermal capacity The acquired data are being used for developing a simulation model. The simulations were performed in the software TRNSYS. The applied boiler model was developed and validated by Michel Haller [8]. One of the first investigations was to vary the thermal capacitance of the boiler to analyse the cycle behaviour of the boiler – storage system. The thermal capacity of the market available boiler from Solarfocus was determined with parameter identification with the experimental test results and is 200 kJ/K. This is equivalent to a mass of the boiler without water and without thermal storage associated components of 205 kg. The simulation was performed with four different masses of the boiler according to figure 6. The system efficiency was calculated with regard to equation 1.
Figure 6: Cycle efficiency and duty cycle for different masses of the boiler mass without water and without thermal storage associated components. One test cycle for the simulation is defined as following (according to figure 7): • • •
Initial storage temperature is homogeneously 20°C Boiler start-up to reach a maximum storage temperature of 75°C Discharge of the thermal energy storage with constant mass flow rate of 800 kg/h and 20°C return temperature, immediately after boiler shut-down
Shut-down of discharging after reaching steady state initial conditions
Figure 7: Storage temperatures and power for the test cycle with a boiler mass of 205 kg according to the market available pellet boiler As one can observe in figure 6 the cycle efficiency tends to increase with increasing boiler mass. This is caused by the fact that the duty cycle increases with the thermal capacity as well. Since the start-up losses are an absolute value the cycle efficiency increases the longer the boiler is switched on. At the same time this results in a higher amount of energy stored and then discharged as useful energy. This effect, however, is caused due to the higher thermal capacity of the boiler. The higher the thermal capacity of the boiler is, the more energy will be stored in the material of the boiler instead of the water in the storage until the set point of the storage temperature is reached. With some time delay the energy stored in the boiler material will be transferred to the water and leads to an overshoot of the set point of around 1 to 2°C. This can be improved by optimizing the controls which is one of our next steps. However, draft losses may compensate for the increasing cycle efficiency with increasing thermal capacity of the boiler [6]. Since the boiler model applied for the cycle test does not separately account for the draft losses as a function of thermal capacity, but as a mixed heat loss value to the ambient, it cannot be proved yet.
5
OUTLOOK
The laboratory test and also a monitoring study of the investigated boiler with solar collectors are still ongoing. The influence of detected parameters like thermal capacity, temperature levels, duration and numbers of start-stop cycles and so on, will be investigated. With the results of this measurements and the computer model, it
will be possible to optimize the control concept for the concerned device for different operation characteristics. The research project will be finished in spring 2014.
6
ACKNOWLEDGEMENT
This project, called KOMBINE, is funded by means of the “Klima- und Energiefonds” in the framework of the research program “Neue Energien 2020” managed by FFG - Austrian Research Promotion Agency. Furthermore I also want to thank the project coordinator Austrian Institute of Technology and project partner Solarfocus GmbH and Bioenergy2020+ GmbH.
7
REFERENCES
[1] ÖNORM EN 303-5:1999; Heizkessel für feste Brennstoffe, hand- und automatisch beschickte Feuerungen, Nenn-Wärmeleistung bis 300 kW; July 1999 [2] VDI 4655; Reference load profiles of single-family and multi-family houses for the use of CHP systems; Verein Deutscher Ingenieure; May 2008 [3] DIN 4702 part 8; Central heating boiler; Determination of the standard efficiency and the standard emissivity; Deutsches Institut für Normung; March 1990 [4] TÜV Austria 2010; Typenprüfung der Pelletskesseltype octoplus 15 gemäß ÖNORM EN 303-5:1999; St.Ulrich/Steyr; Juni 2010 [5] Schraube, C.; Jung, T.; Wilmotte, J.-Y.; Mabilat, C. und Castagno, F.: Long-term monitoring of small pellet boiler based heating systems in domestic applications. Proceedings of the 18th European Biomass Conference and Exhibition, Lyon 2010 [6] Kunde, R.; Volz, F.; Gaderer, M. und Spliethoff, H.: Felduntersuchungen an Holzpellet Zentralheizkesseln. Beurteilung realer Schadstoffemissionen und Jahresnutzgrade. BWKEnergie- Fachmagazin 61 (1-2); 2009 [7] Schwarz, M; Heckmann, M; Lasselsberger, L; Haslinger, W; Determination of annual efficiency and emission factors of small-scale biomass boiler; Central European Biomass Conference 2011 [8] Haller, M. Y.: Combined solar and pellet heating systems - Improvement of energy efficiency by advanced heat storage techniques, hydraulics, and control Institute of Thermal Engineering - Graz University of Technology, 2010
