Solar and Pellet Combisystem for Apartment Buildings: Heat Losses

Applied Energy xxx (2012) xxx–xxx

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Applied Energy

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Solar and pellet combisystem for apartment buildings: Heat losses and efﬁciency improvements of the pellet boiler ⇑ Aivars Zˇandeckis , Lelde Timma, Dagnija Blumberga, Claudio Rochas, Marika Roša¯

Institute of Environment and Energy Systems, Riga Technical University, Riga, Latvia highlights

" The improvements for the performance of the pellet boiler. " Ratio of the supplied combustion air in polynomial order affects CO emission and efﬁciency. " The location of the intake point for combustion air inﬂuences the thermal losses. article info abstract

Article history: This paper is based on the analysis of the solar combisystem installed in a 4-storey apartment building in Received 9 December 2011 Sigulda, Latvia. The combisystem consists of: 42 m2 of flat plate solar collectors, a 2.35 m3 accumulation Received in revised form 16 March 2012 tank and a 100 kW pellet boiler as the auxiliary heater. This paper focuses on the optimisation of the ther- Accepted 25 March 2012 mal performance of the pellet boiler. Available online xxxx During laboratory tests on the 25 kW pellet boiler, the influence of the supply ratio of the combustion air, the chemical and heat losses in the flue gases on the performance of the boiler was established. The Keywords: results demonstrate the optimum thermal performance, CO emissions, chemical and heat losses, which Combustion air supply are related to the amount of free oxygen (O ) in the flue gases. Pellet boiler 2 Solar and pellet combisystem The results of the laboratory tests were then applied to optimise the operation of the 100 kW pellet boi- Thermal and chemical heat losses ler. The flue gas measurements for the 100 kW boiler were performed in order to identify the O2 concentration, CO emissions and flue gas temperature. The results showed that what is required to optimise performance is to reduce the amount of air supplied to the boiler’s combustion chamber. Changes were made to the boiler’s control algorithms to achieve the desired result. Additionally, boiler performance was improved by changing the location of the air intake point. Decreasing temperatures in the heat accumulation tank were achieved by making modifications to the boiler’s control algorithms. Ó 2012 Elsevier Ltd. All rights reserved.

1. Introduction preconditions required for the development of pellet combustion technologies [2]. Households in Nordic countries account for a large share of the A substitution of natural gas by biomass fuels is economically consumption of heat energy. In Latvia 70% of the total heat energy justified in work done by Chau et al. [3]. Based on the paper pre- produced in the country was used for household purposes in 2010 sented by Bram et al. [4] use of woody biomass in the long term [1]. The increase in price for fossil fuels has forced households to can become the major energy resources. Studies by Thür et al. [5] consider alternative heating systems. and Persson [6] have shown that primary energy savings can be If non fossil energy sources for heat production in Latvia are achieved by introducing solar thermal technologies for heat sup- compared, then wood logs and wood chips for stoves and boilers ply. When combining solar thermal technologies and pellet boilers, today have the greatest share of the renewable energy market. At a reduction of pellet consumption can thereby be achieved. the same time, wood pellet fired boilers have also become popular Possibilities for the integration of pellet stoves and solar heating [1]. As Latvia had the 5th largest pellet production capacity in systems for single detached homes are discussed by Persson et al. the European Union in 2008, Latvia would seem to satisfy all the [7]. Weiss [8] and the SOLARGE project report [9] present examples of solar combisystems integrated into multi-family buildings. ⇑ Corresponding author. Tel.: +371 22334510; fax: +371 67089908. Work done by Lundh et al. [10] examines the influence of heat E-mail address: [email protected] (A. Zˇandeckis). stores dimensions on fractional energy saving in a medium-sized

Please cite this article in press as: Zˇandeckis A et al. Solar and pellet combisystem for apartment buildings: Heat losses and efﬁciency improvements of the pellet boiler. Appl Energy (2012), http://dx.doi.org/10.1016/j.apenergy.2012.03.049 2 A. Zˇandeckis et al. / Applied Energy xxx (2012) xxx–xxx

Nomenclature

B mass of the test fuel (kg h1) g efﬁciency (%) C carbon content of test fuel (as ﬁred basis) (% of mass) Q heat output, kW

CO carbon monoxide content in the dry ﬂue gases (% of Qa thermal heat losses in the ﬂue gases, referred to the unit volume) of mass of the test fuel (kJ kg1)

CO2 carbon dioxide content in the dry flue gases (% of QB heat input (kW) volume) Qb chemical heat losses in the flue gases, referred to the 1 1 1 Cp specific heat of water (kJ kg K ) unit of mass of the test fuel (kJ kg ) Cr carbon content of the residue, referred to the quantity of qa proportion of losses through specific heat in the flue test fuel fired (% of mass) gases Qa, referred to the calorific value in the test fuel Cpmd specific heat of dry flue gases in standard conditions, (as fired basis) (%) depending on temperature and composition of the gases qb proportion of losses through latent heat in the flue gases 1 3 (kJ K m ) Qb, referred to the calorific value in the test fuel (as fired CpmH2O specific heat of water vapour in flue gases in standard basis) (%) 1 3 conditions, depending on temperature (kJ K m ) ta flue gas temperature (°C) H hydrogen content of the test fuel (as fired basis) (% of pa draught in the chimney (Pa) mass) tb.in boiler input temperature (°C) 1 Hu lower calorific value of the fuel (as fired basis) (kJ kg ) tb.out boiler output temperature (°C) 1 Mw water flow rate (kg h ) tr room temperature (°C) O2 oxygen content of the dry flue gases (% of volume) W water content of the test fuel (as fired basis) (% of mass)

solar combisystems for residential buildings. Use of solar combi- savings of 20%. A study on the influence of pellet type on boiler system for Net Zero Energy House is discussed by Leckner and efficiency was conducted by González et al. [30]. Zmeureanu [11]. The sizing of the solar combisystems, both for Eskilsson et al. [31] analyse the relationship between emission space heating (SH) and domestic hot water (DHW) preparation, rates and the critical parameters for NOx minimisation in pellet at different loads has been studied by Lund [12]. Work done by burners. Hang et al. [13] ascertains that solar combisystems, used for hot In this study, laboratory tests on a 25 kW pellet boiler were per- water heating in residential buildings, can compete with conven- formed. The influence of the supply ratio of the combustion air and tional heat supply systems. Experimental research by Rochas [14] the chemical and heat losses in the flue gases on the performance on the optimisation of the two parameters – heat storage and pel- of the boiler was identified. The results of these laboratory tests let boiler constructive parameters – was also done. were then applied to optimise the operation of the 100 kW pellet In addition to the primary energy savings gained by means of boiler installed for the solar combisystem. Flue gas measurements the solar combisystem, emissions from boilers can be significantly for the 100 kW boiler were performed in order to evaluate the reduced. During summer months, limits can be placed on both the amount of free O2. Modifications were introduced in order to boiler’s working time and the number of start/stop routines. Both increase the efficiency of the 100 kW pellet boiler as well as to emissions and the thermal performance of the auxiliary heater reduce emissions and fuel consumption. should be taken into account to optimise the performance of the solar combisystem. The CO emissions released by solar and pellet 2. Methodology combisystems was studied by Fiedler [15], and Fiedler and Persson [16]. Persson et al. [17] concluded that a solar pellet combisystem 2.1. Theoretical background can reduce pellet consumption by 25% and CO emissions by 44%. There are already regulations in place in Sweden and Germany, Heat losses in the flue gases and the efficiency of the boilers for example, that limits emissions in flue gases from pellet boilers were calculated according to the ISO EN 13240:2001 [32] and EN [18–22]. Stricter limits are identified by eco-labels. One such label 303-5:1998 [33] standards. is ‘‘Svanmark’’ [23]. The limits for emission levels are expected to An indirect efficiency calculation method (ISO EN 13240:2001 decline still further in the coming years. [32]) was used to calculate chemical and thermal heat losses. Studies by Fiedler et al. [24] showed how to size and control Thermal heat losses in the flue gases Q , which refers to the unit commercially-available solar and pellet heating systems. The oper- a of mass of the test fuel, were calculated according to the following ation of the solar combisystem in Greece has been investigated by equation: Chasapis et al. [25]. They concluded that the main reason for the lower performance of the whole solar combisystem was the poorly Q ¼ðt t Þ a ar designed biomass burner. In the papers presented by Verma et al. Cpmd ðC CrÞ CpmH2O 1:244 ð9 H WÞ [26,27] it is concluded that wood pellet boilers have from 2% up to þ ð1Þ 0:536 ðCO þ CO Þ 100 6.4% lower combustion efficiency in the real life condition if com- 2 pared with the standard laboratory conditions. The proportion of losses through the specific heat in the flue gases

The work by Fiedler et al. [28] proposes an optimisation method Qa, refers to the calorific value of the test fuel (as fired basis) Hu, see for the solar and pellet combisystem. The main goal of this is to re- the following equation: duce CO emissions and fuel consumption. Unfortunately, the q ¼ 100 Q =H ð2Þ improvements achieved were not significant. Research on improv- a a u ing boiler controls and potential energy savings in heating systems Chemical heat losses in the flue gases Qb, which refer to the unit of is explained by Liao and Dexter [29]. Their results show that when mass of the test fuel, were calculated according to the following boiler controls are improved it is possible to achieve energy equation:

Please cite this article in press as: Zˇandeckis A et al. Solar and pellet combisystem for apartment buildings: Heat losses and efﬁciency improvements of the pellet boiler. Appl Energy (2012), http://dx.doi.org/10.1016/j.apenergy.2012.03.049 A. Zˇandeckis et al. / Applied Energy xxx (2012) xxx–xxx 3

12644 CO ðC CrÞ The solar combisystem consists of: Q b ¼ ð3Þ 0:536 ðCO2 þ COÞ100 (a) Heat suppliers – a wood pellet boiler with a nominal capac- The proportion of losses through the specific heat in the flue gases ity of 100 kW, and solar collectors with a total aperture area Q , refers to the calorific value of the test fuel (as fired basis) H , see b u of 37.38 m2. the following equation: (b) Heat storage – an accumulation tank with a volume of 2.35 m3. qb ¼ 100 Q b=Hu ð4Þ (c) Heat consumers – SH and DHW systems (preparation and A direct method based on ISO EN 303-5:1998 [33] was used for the recirculation). calculation of boiler efficiency g, see the following equation: (d) A technical unit containing all necessary components for the functioning of the combisystem – pumps, expansion vessels, g ¼ Q=Q ð5Þ B valves, heat exchangers, etc.

Heat input QB is based on the amount of the fuel supplied into the The system was installed in a container with limited space. The boiler per unit of time B and lower caloriﬁc value of fuel Hu, see the following equation: study was carried out by Bolonina et al. [34] in order to determine the optimal volume of the pellet storage unit and the heat accumu-

Q B ¼ B Hu ð6Þ lation tank. The heat accumulation tank is designed to store solar thermal Heat output Q was calculated according to the following equation: energy in three bottom layers of the accumulation tank (1.8 m3 of water). The upper layer of the tank is kept as an aux- Q ¼ C M ðt t Þð7Þ p w b:out b:in iliary volume. Temperature levels are maintained there by the pellet boiler. 2.2. Studied systems Hot water from the accumulation tank is supplied to the heat exchangers HEX1 and HEX2 for SH and DHW. See Fig. 1. The de- A pilot project of the solar and pellet combisystem was fined temperature for SH and DHW preparation is controlled by launched in Sigulda, which is a city in Latvia with a population of three-way valves M1 and M2. The set point temperature for 11,000. DHW is 55 °C. The supplied temperature for the SH system is regulated as a function of the outdoor temperature and a predefined 2.2.1. Description of the solar combisystem heating curve. The return temperature from the SH and DHW The solar combisystem for DHW and SH purposes was installed (>35 °C) is sent to the middle of the accumulation tank. The in a 4-storey multi-family building. Wood pellets were chosen as three-way switching valve M3 directs the return DHW flow to the biomass fuel for the auxiliary boiler. The hydraulic scheme of the bottom of the tank if flow temperature is below the 35 °C set the combisystem and the monitored parameters are shown in point. This technical solution maintains low temperature in the Fig. 1. bottom of the tank.

Fig. 1. The hydraulic scheme for the solar combisystem and the monitored parameters.

Please cite this article in press as: Zˇandeckis A et al. Solar and pellet combisystem for apartment buildings: Heat losses and efﬁciency improvements of the pellet boiler. Appl Energy (2012), http://dx.doi.org/10.1016/j.apenergy.2012.03.049 4 A. Zˇandeckis et al. / Applied Energy xxx (2012) xxx–xxx

Table 1 The start-up phase of the boiler includes automatic ignition The working intervals of the screw-type conveyor for the different power modes. after sufﬁcient amount of pellets have been supplied to the burner. Power mode (kW) Conveyor ON (s) Conveyor OFF (s) If, after start-up, the temperature in the boiler is below the deﬁned 100 15 15 temperature for QMAX, then the boiler switches to the 100 kW re- 65 10 20 gime. When the temperature reaches the QMAX, the boiler switches 35 5 25 to the 65 kW power mode. The boiler works at 65 kW capacity un-

til the temperature reaches the QMIN. At temperatures above QMIN the boiler works at 35 kW until the ‘‘OFF’’ temperature is reached. 2.2.2. Description of the on-site 100 kW boiler When this happens, the boiler works at the thermostat-off regime The boiler used for the solar combisystem has three power until the ‘‘ON’’ temperature is reached again. The set points for the modes. These include 35 kW, 65 kW, 100 kW. The boiler has an temperatures QMAX, QMIN, OFF, ON can be pre-set by the user and overfeed vertical burner with cleaning system, an automatic fuel can therefore be set at different temperature levels in the algo- ignition system, an air blower and a lambda sensor for controlling rithm. The default factory settings are shown in Table 4. the air supply. The pellets are fed into the burner by a screw-type conveyor. 2.2.3. Description of the 25 kW laboratory boiler stand The power mode of the boiler is manipulated by the controller. The laboratory testing stand consists of a 25 kW pellet boiler, The controller modulates the working intervals of the screw-type monitoring equipment, a heat accumulation tank and a heat conveyor. The working intervals of the screw-type conveyor are exchanger for boiler cooling. See Fig. 3. The monitored parameters deﬁned for each power mode. See Table 1. include the temperature and the chemical composition of the ﬂue The principal algorithm for the boiler controller is shown in gases, the thermal performance of the boiler, the amount of Fig. 2. consumed fuel and the chimney draught.

Fig. 2. The algorithm for the boiler controller.

Fig. 3. The principal scheme for the 25 kW laboratory boiler stand.

Please cite this article in press as: Zˇandeckis A et al. Solar and pellet combisystem for apartment buildings: Heat losses and efﬁciency improvements of the pellet boiler. Appl Energy (2012), http://dx.doi.org/10.1016/j.apenergy.2012.03.049 A. Zˇandeckis et al. / Applied Energy xxx (2012) xxx–xxx 5

Table 2 The chemical and physical parameters of the wood pellets.

Parameter Sample 1 (on-site) Sample 2 (lab. test) Sample 3 (lab. test) Method Value Ash content (%) 0.5 1.3 1.2 CEN/TS 14775 [35] Moisture content (%) 7.4 11.8 6.7 CEN/TS 14774-3 [36] Net caloriﬁc value (MJ kg1) 17.71 16.36 17.76 CEN/TS 14918 [37] Gross caloriﬁc value (MJ kg1) 20.60 20.17 20.50 CEN/TS 14918 [37] Durability (%) n.t. 93.9 97.9 CEN/TS 15210-1 [38] Mean diameter (mm) 8.26 6.54 6.43 – Mean length (mm) 14.28 17.96 19.33 – Cd (%) n.t. 44.13 47.60 CEN/TS 15104 [39] Hd (%) n.t. 5.55 5.78 CEN/TS 15104 [39] Sd (%) n.t. 0.001 0.001 CEN/TS 15289 [40] Nd (%) n.t. 0.36 0.37 CEN/TS 15104 [39] Od (%) n.t. 37.00 38.20 CEN/TS 15104 [39] n.t. – not tested.

The 25 kW boiler is produced by the same manufacturer as the Using software STATGRAPHICS Centurion 16.1.15, a polynomial boiler installed for the solar combisystem. The control algorithm regression model for the experimental data was performed, see for the boiler is based on an ON/OFF working routine and is con- Table 3. trolled by the thermostat. The amount of air supplied to the com- Based on the combustion tests in the laboratory, the correlation bustion chamber is controlled manually by changing ON/OFF between CO emissions in the flue gases and boiler efficiency with intervals of the air blower. the O2 concentration in the flue gases was observed, see Fig. 4. All combustion tests were carried out according to the method- Fig. 4 shows the optimal working interval for the boiler as a ology of the EN 303-5:1998 [33]. In total, 13 combustion tests were function of the amount of combustion air supplied. Optimal boiler carried out using two pellet samples and different ratios of combus- efficiency in this case falls within 5.9% O2 concentration in the flue tion air supply. The results of the tests are described in Chapter 3. gases.

CO emissions in the flue gases increase rapidly at low O2 concentrations in the flue gases (in this case <7.8% O ), since at low 2.2.4. Parameters of the wood pellets 2 O concentrations, increased amounts of un-oxidised fuel are pres- The physical and chemical parameters of the pellets used for the 2 ent in the combustion chamber. CO emissions in the flue gases in- tests are described in Table 2. crease rapidly once again at high O concentrations in the flue Sample 1 corresponds the wood pellets used for the operation 2 gases (in this case >7.8% O ). This increase can be explained, first, of the solar combisystem. Pellet samples 2 and 3 are used in the 2 by the increased power of the air and flue gas flow in the burner. combustion tests in the laboratory. The parameters of the pellets Increased power and velocity of the flow release more small parti- are used for the calculations of the thermal and chemical heat cles of the unburned fuel from the burner and carry them out of the losses Eqs. (1) and (3) and heat input Eq. (6). chamber. An increased flow velocity also reduces the retention time available for chemical reactions to occur in the combustion 3. Results and discussion chamber. Secondly, a significant increase in the air flow reduces the temperature of the combustion process and delays complete The results are divided into two parts: Evaluation of the boiler’s oxidation of the combustibles. efficiency dependence on the O2 load and modifications to the con- The same trend in CO emissions and O2 content in the flue gases trol parameters for the boiler’s optimisation. is discussed by Dias et al. [41] and González et al. [30].

3.1. Evaluation of boiler’s efficiency dependence on the O2 load Table 4 The defined set points for the boiler controller. The 25 kW laboratory boiler tests were performed at constant The temperature set point load conditions. The results of the combustion tests were then applied for optimisation of the 100 kW boiler installed in the solar Factory settings Custom (without SH) Custom (with SH) Q 70 60 65 combisystem. All data were related to the amount of O2 in the flue MAX gases. The amount of combustion air supplied to the chamber is QMIN 75 65 75 OFF 85 75 80 the most crucial parameter that can be regulated during the pellet ON 80 70 70 boiler operation.

Table 3 Data analysis of the combustion tests for the 25 kW boiler in the laboratory.

2 b 2 Correlation of the O2 content in the flue gases to The order of the polynomial model P-value for the order fit test in the variance R (%) Adjusted R (%) table (ANOVA)a Boiler efficiency (%) 2nd 0.0044 91.34 89.61 Thermal heat losses (%) 2nd 0.0214 95.04 94.04 Chemical heat losses (%) 3rd 0.0130 98.91 98.54 3 CO in the flue gases, mg/Nm at 10% O2 3rd 0.0461 98.83 98.44

a Since P-value is less than 0.05, the proposed model is adequate for data describing at 95% conﬁdence level. b The coefﬁcient of determination.

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CO (mg/Nm3 at Efficiency, % 10 % O2) 90 4000

3000 85

2000

80 1000

75 0 4 6 8 10 12 14 16 Oxygen content in flue gases, % Boiler efficiency, Sample 1 CO emmisions, Sample 1 Boiler efficiency, Sample 2 CO emmisions, Sample 2

Fig. 4. Boiler efficiency and CO emissions in the flue gases as a function of O2 concentration in the flue gases (Samples 1 and 2 correspond to the wood pellets parameters in Table 2).

8 Losses, % Losses, 4

0 4 6 8 10121416 Oxygen content in flue gases, %

Chemical heat losses, Sample 1 Thermal heat losses, Sample 1 Chemical heat losses, Sample 2 Thermal heat losses, Sample 2

Fig. 5. Thermal and chemical heat losses in the ﬂue gases as a function of O2 concentration in the ﬂue gases (Samples 1 and 2 corresponds to wood pellets parameters in Table 2).

18 100

O2 content in flue gases, % 15 Power mode, kW 80

12 60 9 40 6 Power mode, kW 20

content in the flue gases, % 3 2 O 0 0 15:16:40 15:25:00 15:33:20 15:41:40 15:50:00 15:58:20 16:06:40 Time

Fig. 6. The actual O2 concentration in the ﬂue gases during the full operation cycle of the 100 kW boiler before optimisation.

Boiler efficiency drops quickly with increased O2 content in the Based on the results from the 25 kW laboratory boiler testing, flue gases (in this case >7% O2), since excess combustion air in- the on-site 100 kW boiler parameters can be analysed and opti- creases the total amount of hot flue gases and consequently in- mised. To determine the actual O2 concentration in the flue gases, creases CO emissions. Boiler efficiency is a function of the the on-site measurements were applied to the full boiler operation chemical and thermal heat losses. See Fig. 5. cycle, starting from the fuel ignition process up to the thermostat Because chemical heat losses and CO emissions in the flue gases off mode. The results are shown in Fig. 6. are related parameters, they show similar trends. See Figs. 4 and 5. The actual free O2 concentration in the flue gases depends on Thermal heat losses increase faster than chemical heat losses with the power mode of the boiler. The average O2 concentration during increased O2 content in the flue gases. Therefore, the total amount the test for 100 kW power mode was 8.6%, for 65 kW – 8.8% and for of thermal heat loss is greater. See Fig. 5. 35 kW – 11.8%, see Fig. 6. The O2 concentration for each power

Please cite this article in press as: Zˇandeckis A et al. Solar and pellet combisystem for apartment buildings: Heat losses and efﬁciency improvements of the pellet boiler. Appl Energy (2012), http://dx.doi.org/10.1016/j.apenergy.2012.03.049 A. Zˇandeckis et al. / Applied Energy xxx (2012) xxx–xxx 7

18 100

O2 content in the flue gases, % 15 Power mode, kW 80

12 60 9 40 6 Power mode, kW 20 content in the flue gases, % 3 2 O 0 0 14:26:40 14:35:00 14:43:20 14:51:40 15:00:00 15:08:20 15:16:40 Time

Fig. 7. The actual O2 concentration in the flue gases during the full operation cycle of the boiler after optimisation. mode was calculated as average value from the flue gas measure- the lambda sensor. The PID controller changes the ON/OFF inter- ments at 5-s intervals. val width for the air blower as a function of the output signal The functioning of the 100 kW boiler in real life conditions is from the lambda sensor. See Fig. 8. The 1 V output is related to linked to the frequency of the ON/OFF procedures, changes in the 4% O2. It is factory-set for minimum O2 concentration in the flue SH and DHW loads, fluctuations in the environmental parameters gasses. Below this factory-set minimum, the blower operates and the negative impact of dust deposits on the internal surfaces continuously. At 5 V output, the signal blower operates for 300 of the boiler. Therefore the actual O2 concentration in the flue gases milliseconds (ms) every second. The frequency of the PID control- for the on-site boiler as compared to the laboratory combustion ler calculation cycle is 1 s. Due to the controller specifics, the tests must be higher. A range of 8–10% O2 was set as a target for 300 ms working interval cannot be changed. The operation time the air supply to the 100 kW boiler. of the blower changes linearly between factory-set minimums Modifications described in Section 3.2 have been made for the and maximums. A disadvantage of this system is the lack of op-

100 kW boiler to reduce the amount of free O2 in the ﬂue gases. tions to set the maximum O2 concentration at which the air The results achieved after modiﬁcations are shown in Fig. 7. blower would turn off.

As shown in Fig. 7, the amount of O2 during the full operation Measurements of the O2 concentration in the flue gases pre- cycle of the 100 kW boiler has decreased. For the 35 kW power sented in Fig. 6 shows increased amounts of O2 during the opera- mode, O2 concentration has decreased by 2%, reaching 9.8% O2 in tion in low power (35 kW) mode. This increase in O2 the flue gases. Average O2 concentration in the flue gases for the concentration is a result of the relatively long operating intervals 100 kW and 65 kW power modes did not change significantly. of the air blower at the 35 kW power mode even when amount

of O2 is high. A powerful natural draught from the chimney (from 3.2. Modifications to the control parameters for the boiler’s 45 up to 55 Pa at +6 °C ambient temperature) increases the air optimisation supply even more. Therefore, in the low power mode a natural draught is able to provide air for the combustion process without Modifications to the lambda sensor output signal, the air intake the need of a forced air supply. point location and the defined temperature set points for the boiler The frequency of the PID controller calculation cycle was chan- are made to increase the efficiency of the 100 kW boiler. ged to 3 s. The lambda sensor was re-programmed to give 5 V out-

put above 10% O2 concentration. This means that in a range of 10– 3.2.1. Lambda algorithm modifications 21% O2 concentration in the flue gases the air blower operates for The control algorithm of the air supply system is based on the 300 ms every 3 s. The achieved reduction of O2 concentration in measurements of the actual O2 concentration in the flue gases by the flue gases is shown in Fig. 7.

1 Output of the lambda sensor, V Output of the lambda sensor, Lambda output signal after modifications Lambda output signal before modifications 0 04812162024

O2 concentration in the flue gases, %

Fig. 8. Lambda algorithm based on output sensor signal and O2 concentration in the ﬂue gases.

Please cite this article in press as: Zˇandeckis A et al. Solar and pellet combisystem for apartment buildings: Heat losses and efﬁciency improvements of the pellet boiler. Appl Energy (2012), http://dx.doi.org/10.1016/j.apenergy.2012.03.049 8 A. Zˇandeckis et al. / Applied Energy xxx (2012) xxx–xxx

3.2.2. Air intake point location of Latvia’’ Programme and by the European Regional Development The air intake point was changed in order to increase the tem- Fund of the European Commission within the framework of the perature of the combustion air and to reduce the energy required ‘‘Central Baltic INTERREG IV Programme 2007–2013’’. for pre-heating the air. The 100 kW boiler is installed in a boiler The authors also express a special acknowledgement to the lab- room, which has dimensions of 3.0 3.0 2.5 m. Since there is a oratory assistant Ms. Anna Beloborodko for her work on wood pel- low air infiltration rate in the boiler room, air inside the boiler let testing, to the laboratory assistant Mr. Vladimirs Kirsanovs on room is stratified. his work on combustion tests and Mr. Colin Hefferon for his help When the air intake point is moved up from a height of 0.5– with proofreading. 2.0 m, the temperature at the air intake point increases by 14° (from 23.7 °C up to 37.7 °C). The flue gas measurements before and after References the air intake modification show a 2% decrease in heat losses. [1] Energy statistics 2010. Central Statistical Bureau of Latvia. Riga, Latvia; 2011. 3.2.3. The defined temperature set points for the boiler controller [2] Pellet market data 2008. Survey data collection 2009. Pellet@las; 2009. Since the combisystem is installed in a multi-family building, it [3] 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 is essential to adjust the system according to the season. The set 2009;86:364–71. point temperature of the auxiliary heater should be lower when [4] Bram S, Ruyck De J, Lavric D. Using biomass: a system perturbation analysis. SH is not required. The algorithm for the on-site 100 kW boiler Appl Energy 2009;86:194–201. [5] Thür A, Furbo S, Shah LJ. Energy savings for solar heating systems. 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Please cite this article in press as: Zˇandeckis A et al. Solar and pellet combisystem for apartment buildings: Heat losses and efﬁciency improvements of the pellet boiler. Appl Energy (2012), http://dx.doi.org/10.1016/j.apenergy.2012.03.049 A. Zˇandeckis et al. / Applied Energy xxx (2012) xxx–xxx 9

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