Prototypical Biomass-Fired Micro-Cogeneration Systems—Energy and Ecological Analysis

Article Prototypical Biomass-Fired Micro-Cogeneration Systems—Energy and Ecological Analysis

Krzysztof Sornek Faculty of Energy and Fuels, Department of Sustainable Energy Development, AGH University of Science and Technology, Mickiewicza Ave. 30, 30-059 Krakow, Poland; [email protected]

Received: 16 June 2020; Accepted: 28 July 2020; Published: 31 July 2020

Abstract: Combined heat and power systems dedicated to micro-scale applications are currently increasing in popularity. The use of such systems is beneficial from the standpoint of increasing the usage of renewable energy, increasing energy efficiency and reducing CO2 emissions into the atmosphere. This paper shows two examples of prototypical micro-cogeneration systems powered by biomass. In the first, smaller one, electricity is generated in thermoelectric generators using heat from the wood-fired stove. The second one is equipped with a 100 kWt batch boiler and operates according to a modified Rankine cycle. The energy and ecological analysis were conducted and discussed, including selected aspects of heat and power generation and gaseous pollutant emission. Measurements were performed using a dedicated control and measurement station with a PLC controller. As was shown, thermoelectric generators operated respectively with the power of 22.5 We in the case of the air-cooled unit and 31.2 We in the case of the water-cooled unit. On the other hand, the maximum power level of ca. 1145 We was obtained in the system with a batch boiler operating according to a modified Rankine cycle. The ecological analysis showed that the average amount of CO emission during the wood combustion in the tested stove was 1916 mg/m3 (in the combustion phase). In the case of straw combustion, it was characterized by lower CO2 emissions compared to coal, but higher CO2 emissions compared to gasoline and natural gas. Based on the obtained results, some outlines for the systems development were given.

Keywords: cogeneration; thermoelectric generator; Rankine cycle; biomass; renewable energy

1. Introduction Nowadays, many researchers are focused on developing and implementing systems dedicated to micro-scale installations powered by renewable energy sources [1]. In the wide field of climate change, it is necessary to seek alternative fuels, which can be more friendly for the environment [2]. Among other options, cogeneration and trigeneration systems powered by biomass-fired devices are considered [3]. In biomass powered micro-scale applications, the most popular fuels are wood and waste from its processing, straw, plants from energy crops, and agricultural products. The most popular micro-cogeneration technologies are solutions based on the use of the organic Rankine cycle (ORC) [4], Stirling engines (SE) [5], and thermoelectric generators (TEG) [6]. There are also known hybrid systems integrating different micro-cogeneration technologies [7]. The worldwide literature consists also of a few examples of micro-scale systems according to the Clausius–Rankine cycle [8]. The selection of proper micro-cogeneration technology depends on the construction and operational parameters of the heat source and the expected level of electricity generation. This requires an original approach for designing and developing individual combined heat and power systems (CHP). As it is known, biomass may be converted to electricity in the process of its gasification and combustion. The most commonly used biomass-fired devices in the residential sector are multi-fuel boilers, pellet-burning boilers, wood-fired gasification boilers, and wood-fired stoves. On the other

Energies 2020, 13, 3909; doi:10.3390/en13153909 www.mdpi.com/journal/energies Energies 2020, 13, 3909 2 of 16 hand, in the cases of, e.g., farms and housing estates, the application of straw-fueled batch boilers is becoming increasingly popular because straw is inexpensive and widely available fuel. The power range of different biomass-fired units is from a few kilowatts to megawatts. This paper is focused on the presentation of practical possibilities to introduce power generation to two different types of biomass-fired devices: the 100 kWt straw-fired batch boiler and the 8–16 kWt wood-fired steel stove. Based on the preliminary analysis, two cogeneration technologies were selected, taking into account construction parameters of the considered batch boiler and stove, and available temperature and power, which may be obtained from these devices. The approach presented is original—the devices we analyzed are not widely studied in the context of combined heat and power generation (especially batch boiler). Moreover, the proposed configuration of the micro-cogeneration system with a batch boiler is characterized by several unique solutions that distinguish the developed system from the classic Rankine cycle. In both analyzed systems, a dedicated control and measurement system and dedicated control algorithm were used. Some examples of research devoted to thermoelectric generators involved stoves and systems operating according to the Rankine and organic Rankine cycles are presented below.

1.1. Thermoelectric Generators Operating with Stoves There were several studies devoted to thermoelectric generators incorporated with domestic stoves. An example of the air-cooled thermoelectric module ﬁtted to the domestic woodstove has been presented in [9]. The maximum matched load power was obtained at a level of 4.2 We. Another example of a complete system with a multifunctional stove, thermoelectric generators, and other equipment was studied in [10]. The maximum power available for the end-users was in that case ca. 7.6 We. On the other hand, in [11] a forced water cooling system was studied (as a heat source an electric heater was used). The maximum observed power was ca. 10 We. Both air and water-cooled thermoelectric modules were tested and compared in [12]. The maximum output voltage of the studied single TE module was ca. 6.1 V (in case of water cooling) compared to ca. 4.2 V and ca. 2.6 V when air cooling systems were used (forced air and natural respectively). In [13] the possibility of applying the market-available TEGs to a stove was presented. The proposed system was used both for heating the water and charging a lead-acid battery. It was producing an average of 600 Wt and 27 We. In [14] it was shown that connecting 600 thermoelectric modules in series may result in power at a level of 1 kWe. Besides the use of TEGs in domestic mCHP systems, many studies include examples of the other TEGs applications: in automotive, aircraft and vessel sectors, microelectronics, micro-scale systems powered by solar energy and the space industry [15].

1.2. Systems Operating According to Rankine and Organic Rankine Cycles An example of a micro-cogeneration system with a single-stage piston engine operating according to the Clausius–Rankine cycle was investigated in [16]. The thermal power of the system was 104 kWt, while the electrical power was 23 kWe. The electrical and total efficiency of this system was respectively ca. 14% and ca. 78%. Three ways of controlling the steam engine operation were tested: engine operation with variable shaft speed at constant shut-offs, engine operation with variable shut-offs at constant shaft speed and engine operation controlled by a steam pressure regulating valve. In [17], the possibilities of using the Clausius–Rankine multi-stage system to improve the efficiency of the combined heat and electricity production process were discussed. On the other hand, the example of the ORC system powered by the biomass boiler was studied in [18]. The electricity output of that system was ca. 2 kWe, and electrical efficiency was ca. 7.5–13.5%. Another example is an experimental ORC system powered by a 50 kWt pellet-fired boiler. In this case, the maximum electrical power was observed at a level of 860 We [19]. Besides the performance analysis of micro-scale CHP systems, detailed analyses of CHP systems components were carried out (e.g., analyses of turbines [20] and heat exchangers constructions [21]). Moreover, some dynamic simulations devoted to biomass and solar CHP systems were conducted [22]. Among other works, thermoeconomic analysis of an organic Energies 2020, 13, 3909 3 of 16

Rankine cycle powered by medium-temperature heat sources [23] and analysis of an integrated system for sewage sludge drying [24] were performed. The remainder of the paper includes the investigations performed on two prototypical systems, which are currently being developed at AGH UST in Krakow (Poland): (i) a wood-fired stove with a thermoelectric generator and (ii) a straw-fired batch boiler a with steam-condensate circuit operating according to modified Rankine cycle.

2. Materials and Methods The investigations were carried out using two dedicated experimental rigs. Control and measurement systems were based on the use of programmable logic controllers (PLC) operating with a set of dedicated sensors, transducers, and actuators. The idea of the studies was to develop and test micro-cogeneration units powered by two different biomass-fired units (8–16 kWt domestic stove and a 100 kWt batch boiler). Due to the large difference in power generated by both devices, two technologies of combined heat and power generation were considered. Consequently, below the descriptions and results are divided into two parts.

2.1. Experimental Rig 1: A Wood-Fired Stove with Thermoelectric Generators Experimental rig number 1 was equipped with a wood-ﬁred stove (working as a heat source); thermoelectric generators; and a measuring, controlling and visualizing system:

The 8–16 kWt steel plate wood-ﬁred stove was used as the heat source; • Two types of thermoelectric generators were taken into consideration: •

A 45 We thermoelectric generator (TEG1) designed for mounting on a flat hot which consists • of six TE modules, an aluminum plate used for collecting heat and an air fan cooling system (nominal hot side temperature 350 ◦C); A 100 We thermoelectric generator (TEG2) designed for mounting on a flat hot surface which • consists of eight TE modules, an aluminum plate used for collecting heat and a liquid cooling block (nominal hot side temperature 270 ◦C). A measurement, control and visualization system based on the WAGO PFC200 controller and • equipped with temperature sensors located inside the furnace, inside the flue gas channel, on the rear wall, on the air inlet to the TEG1 and the water inlet and outlet to the TEG2. There were two types of temperature sensors: thermocouple sensors (measurement range 0–1150 C, accuracy 2.5 C or 0.75%) and resistance sensors (measurement ranges 0–250 C ◦ ± ◦ ± ◦ and 0–400 C, accuracy (0.3 + 0.005 t)). The regulation of the airstream was realized using ◦ ± · a throttle. The monitoring and process data acquisition system was developed in the WAGO e!Cockpit software.

The original construction of the tested stove was modified following guidelines developed based on the results of previously conducted studies. Among other modifications, an internal accumulation layer was removed from the furnace and an electronically controlled air throttle was installed. The individual control algorithm was developed to provide an efficient operation from the standpoint of heat and power generation. The tested thermoelectric generators were placed on the rear wall of the stove (the exact location was determined using infrared thermography measurements). To provide good thermal contact between the stove’s rear wall and the hot sides of the thermoelectric generators, a thermal heat-conducting paste, characterized by thermal conductivity 2.8 W/(mK), was used. The average rear wall temperature during the main phase of the wood combustion process was generally close to 400 ◦C. On the other hand, the minimum value observed in the upper part of the rear wall was ca. 230 ◦C and the maximum temperature observed in the bottom part of the rear wall exceeded 500 ◦C[25]. The view of the stove with mounted thermoelectric generators is shown in Figure1. Energies 2020, 13, 3909 4 of 16 Energies 2020, 13, x FOR PEER REVIEW 4 of 16 Energies 2020, 13, x FOR PEER REVIEW 4 of 16

(a) (b) (a) (b) Figure 1. The view of (a) the stove with air-cooled TEG1 mounted on its rear wall;wall; ((b)) thethe stovestove withwith water-cooledFigurewater-cooled 1. The TEG2TEG2view ofmountedmounted (a) the stove onon itsits with rearrear wall.air-cooledwall. TEG1 mounted on its rear wall; (b) the stove with water-cooled TEG2 mounted on its rear wall. The general scheme ofof thethe ﬁrstfirst experimentalexperimental rigrig isis shownshown inin FigureFigure2 2.. The general scheme of the first experimental rig is shown in Figure 2.

Figure 2. The scheme of the test rig showing the mounted thermoelectric generators (TEG1 and TEG2). Figure 2. TheThe scheme scheme of of the the test test rig rig showing showing the the mounte mountedd thermoelectric thermoelectric generators (TEG1 and TEG2). 2.2. Experimental Rig 2:2: A Straw-FiredStraw-Fired BatchBatch BoilerBoiler withwith aa Steam-CondensateSteam-Condensate Circuit Circuit Operating Operating According According to the2.2.to the ModifiedExperimental Modified Rankine Rankine Rig Cycle2: CycleA Straw- Fired Batch Boiler with a Steam-Condensate Circuit Operating According to the Modified Rankine Cycle The configurationconfiguration of the experimentalexperimental rig number 2 includes threethree main parts: the oil circuit; steamsteam/condensateThe/condensate configuration circuit;circuit; of and theand coolingexperime cooling water ntalwater circuitrig circnumber anduit measuring,and 2 includes measuring, controlling thre econtrolling main and parts: visualizing and the visualizing oil system:circuit; steam/condensatesystem: circuit; and cooling water circuit and measuring, controlling and visualizing In the oil circuit, oil was heated up in a 100 kWt straw-fired batch boiler. The boiler had an oil •system: • Injacket the oil (replacing circuit, oil water was jacket) heated and up ain dedicated, a 100 kW manualt straw-fired fuel feederbatch boiler. providing The aboiler continuous had an fuel oil • t Injacketcombustion the (replacingoil circuit, process wateroil was (see jacket) heated Figure and4 up). a in dedicated, a 100 kW manual straw-fired fuel feederbatch boiler.providing The aboiler continuous had an fuel oil jacket (replacing water jacket) and a dedicated, manual fuel feeder providing a continuous fuel combustionThermal oil process heated (see up Figure in the 3). boiler was transferred via pipeline to two shell and tube heat • combustion process (see Figure 3). exchangers operated respectively as an evaporator and superheater. Generated steam was then superheated and conditioned. After that process, the steam got to the 20-horsepower, 2-cylinder, double-acting steam engine. Partially expanded steam was condensed in another shell and tube heat exchanger (condenser). Water was then pumped to the degasser, and finally to the evaporator. The steam engine (see Figure3) was connected to a power generator. The power was consumed by an electric load equipped with 20 bulbs with a total power of 2 kWe.

Energies 2020, 13, x FOR PEER REVIEW 5 of 16

Energies 2020, 13, 3909 5 of 16

The operation of the developed installation was controlled by a dedicated automation system • with a WAGO PFC200 PLC controller. Among all controlled parameters were inlet air and flue gas stream; and oil, condensate and cooling water flow. Moreover, several measurements were taken, including temperature, pressure and flow of the flue gas, oil, steam, condensate and cooling water( anda) current and voltage generated in the power( generator.b) The following types ofFigure sensors 3. The were view used: of (a) thermocouple the boiler with temperaturea fuel feeder; ( sensorsb) evaporator (measurement and superheater. range 0–1150 ◦C, accuracy 2.5 C or 0.75%), resistance temperature sensors (measurement range 0–250 C, ± ◦ ± ◦ • Thermalaccuracy oil heated(0.3 + 0.005up int)), the pressure boiler was transducers transferred (measurement via pipeline range to two 0–10 shell bar and and tube 0–16 heat bar, ± · 3 exchangersaccuracy operated0.5%), oil flowmeterrespectively (measurement as an evaporator range and 0–45 superheater. m /h, accuracy Generated0.5%), steamsteam flowmeterwas then ± ± superheated(measurement and rangeconditioned. 0–200 kgAfter/h, accuracythat process,0.5%), the steam water got flowmetersto the 20-horsepower, (measurement 2-cylinder, range 3 ± double-acting0–10 m /h, accuracy steam engine.3%), voltage Partially transducer expanded (measurement steam was condensed range 0–400 in V,another accuracy shell0.5 and V) tube and ± ± heatcurrent exchanger transducer (condenser). (measurement Water range was 0–15 then A, pumped accuracy to0.2 the A). degasser, All controlled and andfinally measured to the ± evaporator.parameters The were steam available engine via (see CoDeSys Figure software4) was connected using specially to a power developed generator. visualization. The power was consumed by an electric load equipped with 20 bulbs with a total power of 2 kWe.

EnergiesFigure 2020 4. 3., 13 TheThe, x FOR steam PEER engine REVIEW and power generator used during the presented investigations. 5 of 16

• The operation of the developed installation was controlled by a dedicated automation system with a WAGO PFC200 PLC controller. Among all controlled parameters were inlet air and flue gas stream; and oil, condensate and cooling water flow. Moreover, several measurements were taken, including temperature, pressure and flow of the flue gas, oil, steam, condensate and cooling water and current and voltage generated in the power generator. The following types of sensors were used: thermocouple temperature sensors (measurement range 0–1150 °C, accuracy ± 2.5 °C or ± 0.75%), resistance temperature sensors (measurement range 0–250 °C, accuracy ± (0.3 + 0.005∙t)), pressure transducers (measurement range 0–10 bar and 0–16 bar, accuracy ± 0.5%), oil flowmeter (measurement range 0–45 m3/h, accuracy ± 0.5%), steam flowmeter (measurement range 0–200 kg/h, accuracy ± 0.5%), water flowmeters (measurement range 0–10 m3/h, accuracy ± 3%), voltage transducer (measurement range 0–400 V, accuracy ± 0.5 V) and current transducer (measurement range 0–15 A, accuracy ± 0.2 A). All controlled and measured (a) (b) parameters were available via CoDeSys software using specially developed visualization. FigureFigure 4. The 3. view The ofview (a) of the (a boiler) the boiler with awith fuel a feeder; fuel feeder; (b) evaporator (b) evaporator and superheater. and superheater. The proposed configuration of the micro-cogeneration system includes many novel solutions, including, e.g., the oil jacket and fuel feeder (typically not used in batch-boilers), and shell and tube The• Thermal proposed oil conﬁguration heated up in of the the boiler micro-cogeneration was transferred system via pipeline includes to many two novelshell solutions,and tube heat including, e.g., the oil jacket and fuel feeder (typically not used in batch-boilers), and shell and tube exchangers operated respectively as an evaporator and superheater. Generated steam was then heat exchangerssuperheated operating and conditioned. as an evaporator After and that superheater process, the (insteadsteam got of to the the classic 20-horsepower, steam generator) 2-cylinder, and a steamdouble-acting engine (replacing steam engine. typically Partially used steam expanded turbines). steam The was simpliﬁed condensed scheme in another of the shell oil andand tube steam-condensateheat exchanger circuits (condenser). of the developed Water system was isthen shown pumped in Figure to5 .the degasser, and finally to the evaporator. The steam engine (see Figure 4) was connected to a power generator. The power was consumed by an electric load equipped with 20 bulbs with a total power of 2 kWe.

Figure 4. The steam engine and power generator used during the presented investigations.

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Energiesheat 2020exchangers, 13, 3909 operating as an evaporator and superheater (instead of the classic steam generator)6 of 16 and a steam engine (replacing typically used steam turbines). The simplified scheme of the oil and steam-condensate circuits of the developed system is shown in Figure 5.

Oil circuit

Steam-condensate circuit

FigureFigure 5. 5.The The simpliﬁed simplified schemesschemes ofof thethe oil circuit and steam-condensate steam-condensate circuit. circuit. 3. Results 3. Results AmongAmong the the many many measurement measurement series series conducted, conducted, only oneonly set one of resultsset of wasresults selected was selected and presented and inpresented this paper. in this paper. 3.1. Results Obtained with the Experimental Rig 1 3.1. Results Obtained with the Experimental Rig 1 3.1.1. The Temperature of the Stove’s Rear Wall 3.1.1. The Temperature of the Stove’s Rear Wall Hot surface temperature is one of the main parameters aﬀecting the thermoelectric generators’ Hot surface temperature is one of the main parameters affecting the thermoelectric generators’ operationoperation (it (it is is directly directly connectedconnected with the the heat heat transferred transferred from from the the furnace furnace area area to the to thehot hotside sideof ofthermoelectric thermoelectric generators). generators). The The stove’s stove’s rear rear wall wall temperature temperature varied varied in time in and time was and connected, was connected, e.g., e.g.,with with the the amount amount of ofwood wood input, input, the the way way of of fuel fuel combustion combustion (single (single or or continuous continuous process) process) and and control of the stove’s operation. During this study, two fuel inputs were tested: 4 and 6 kg. The typical Energiescontrol 2020 of, the13, x stove’s FOR PEER operat REVIEWion. During this study, two fuel inputs were tested: 4 and 6 kg. The typical7 of 16 variationsvariations in in the the rear rear wall’s wall’s temperature temperature are are shown shown inin FigureFigure6 6..

Figure 6. Temperature variations on the stove’s rear wall during combustion of 4 and 6 kg of wood. Figure 6. Temperature variations on the stove’s rear wall during combustion of 4 and 6 kg of wood.

During the analyzed measurement series, the maximum observed rear wall temperatures were respectively 305 °C (obtained in minute 24, when 4 kg of wood was burned) and 367 °C (obtained in minute 25, when 6 kg of wood was burned). On the other hand, the average rear wall temperature was significantly lower. Taking into account hot side temperature recommended for TEG1 (350 °C), it was achieved only in case of combustion 6 kg of wood between 18 and 28 min (lines B-C in Figure 6). On the other hand, the temperature level corresponding to the TEG2 requirements (270 °C) was observed between 18 and 28 min during the combustion of 4 kg wood (lines B-C) and between 9 and 37 min during combustion of 6 kg wood (lines A-D). To extend the time, when the rear wall temperature is equal to power thermoelectric generators, a continuous combustion process should be provided.

3.1.2. The Operating Characteristics of the Thermoelectric Generators During the presented investigations, cooling medium temperature was ca. 14–15 °C both for water and air cooling TEGs). The temperature of the cold side of TEGs was significantly higher and equal to respectively ca. 140 °C and ca. 71 °C (it was determined based on the calculations of the Seebeck coefficient) [26,27]. On the other hand, the hot side temperature was directly related to the amount of wood input and the combustion process phase. Comparing the I–V characteristics of the TEG1 and TEG2, the higher short-circuit current and open-circuit voltage were observed in the case of TEG2 (see Figure 7).

Figure 7. Current–voltage (I–V) characteristics of the tested TEGs.

As a result of the current and voltage variations, the power generated by TEGs was significantly lower than their nominal power. The TEG1 operated at 22.5 W at the maximum power point (50% of

Energies 2020, 13, x FOR PEER REVIEW 7 of 16

Energies 2020 13 Figure, 6., 3909Temperature variations on the stove’s rear wall during combustion of 4 and 6 kg of wood.7 of 16

DuringDuring thethe analyzedanalyzed measurementmeasurement series,series, thethe maximummaximum observedobserved rearrear wallwall temperaturestemperatures werewere respectively 305 °C (obtained in minute 24, when 4 kg of wood was burned) and 367 °C (obtained in respectively 305 ◦C (obtained in minute 24, when 4 kg of wood was burned) and 367 ◦C (obtained in minute 25, when 6 kg of wood was burned). On the other hand, the average rear wall temperature minute 25, when 6 kg of wood was burned). On the other hand, the average rear wall temperature was was significantly lower. Taking into account hot side temperature recommended for TEG1 (350 °C), significantly lower. Taking into account hot side temperature recommended for TEG1 (350 ◦C), it was it was achieved only in case of combustion 6 kg of wood between 18 and 28 min (lines B-C in Figure achieved only in case of combustion 6 kg of wood between 18 and 28 min (lines B–C in Figure6). On the 6). On the other hand, the temperature level corresponding to the TEG2 requirements (270 °C) was other hand, the temperature level corresponding to the TEG2 requirements (270 ◦C) was observed observed between 18 and 28 min during the combustion of 4 kg wood (lines B-C) and between 9 and between 18 and 28 min during the combustion of 4 kg wood (lines B–C) and between 9 and 37 min 37 min during combustion of 6 kg wood (lines A-D). To extend the time, when the rear wall during combustion of 6 kg wood (lines A–D). To extend the time, when the rear wall temperature is temperature is equal to power thermoelectric generators, a continuous combustion process should equal to power thermoelectric generators, a continuous combustion process should be provided. be provided. 3.1.2. The Operating Characteristics of the Thermoelectric Generators 3.1.2. The Operating Characteristics of the Thermoelectric Generators During the presented investigations, cooling medium temperature was ca. 14–15 ◦C both for waterDuring and air the cooling presented TEGs). investigations, The temperature cooling of the medium cold side temperature of TEGs was was significantly ca. 14–15 higher°C both and for water and air cooling TEGs). The temperature of the cold side of TEGs was significantly higher and equal to respectively ca. 140 ◦C and ca. 71 ◦C (it was determined based on the calculations of the Seebeckequal to coe respectivelyfficient) [26 ca.,27 140]. On °C the and other ca. hand,71 °C (it the was hot sidedetermined temperature based was on the directly calculations related toof thethe amountSeebeck of coefficient) wood input [26,27]. and the On combustion the other hand, process the phase.hot side Comparing temperature the was I–V directly characteristics related ofto thethe TEG1amount and of TEG2, wood the input higher and short-circuit the combustion current process and open-circuit phase. Comparing voltage the were I–V observed characteristics in the case of the of TEG2TEG1 (see and Figure TEG2,7 ).the higher short-circuit current and open-circuit voltage were observed in the case of TEG2 (see Figure 7).

FigureFigure 7. 7.Current–voltage Current–voltage (I–V)(I–V) characteristicscharacteristicsof of the the tested tested TEGs. TEGs.

AsAs aa resultresult ofof thethe currentcurrent andand voltagevoltage variations,variations, thethe powerpower generatedgenerated byby TEGsTEGs waswas signiﬁcantlysignificantly lowerlower than than their their nominal nominal power. power. The The TEG1 TEG1 operated operated at at 22.5 22.5 W W at at the the maximum maximum power power point point (50% (50% of of its nominal power), while the TEG2 operated at 31.2 W (31.2%). Such a situation was caused mainly by the fact that non-uniform surface temperature reduced the heat ﬂux to the hot side of the TEGs and consequently the obtained power (see Figure8). In both considered thermoelectric generators the net power was substantially lower because of the controller operation (controller consumes ca. 1.5–6 We). Moreover, in the case of TEG2, further reduction in the net power may be observed if power consumed by the water pump is included in the calculations. Energies 2020, 13, x FOR PEER REVIEW 8 of 16 its nominal power), while the TEG2 operated at 31.2 W (31.2%). Such a situation was caused mainly by the fact that non-uniform surface temperature reduced the heat flux to the hot side of the TEGs and consequently the obtained power (see Figure 8). In both considered thermoelectric generators the net power was substantially lower because of the controller operation (controller consumes ca. Energies 2020, 13, 3909 8 of 16 1.5–6 We). Moreover, in the case of TEG2, further reduction in the net power may be observed if power consumed by the water pump is included in the calculations.

Figure 8. Power–voltage (P–V) characteristics of the tested TEGs. Figure 8. Power–voltage (P–V) characteristics of the tested TEGs. The reduction in the power generated in thermoelectric generators could be also analyzed in the The reduction in the power generated in thermoelectric generators could be also analyzed in the example of a comparison of the TEG1’s operating parameters in testing and laboratory conditions example of a comparison of the TEG1’s operating parameters in testing and laboratory conditions (see Figure9). In testing conditions, when the existing stove was used as a heat source, contact between (see Figure 9). In testing conditions, when the existing stove was used as a heat source, contact TEG1’s hot side and the stove’s surface was not ideal, and non-homogeneous temperature distribution between TEG1’s hot side and the stove’s surface was not ideal, and non-homogeneous temperature was occurring on the stove’s rear wall (it was analyzed in [25]). distribution was occurring on the stove’s rear wall (it was analyzed in [25]).

Figure 9. The comparison of TEG number 1 operating parameters in testing and laboratory conditions. Figure 9. The comparison of TEG number 1 operating parameters in testing and laboratory conditions. 3.1.3. Emissions Generated during the Wood-Fired Stove Operation 3.1.3. Emissions Generated during the Wood-Fired Stove Operation Analysis of the environmental aspects of the stove operation was carried out for two types of biomassAnalysis fuel: of spruce the environmental wood and briquette. aspects of Tests the werestove performed operation was for twocarried situations: out for two a stove types with of anbiomass internal fuel: accumulation spruce wood layer and (originalbriquette. configuration Tests were performed of the unit) for and two a situations: stove without a stove an internalwith an accumulationinternal accumulation layer (unit layer modified (original to use configuration thermoelectric of the generators). unit) and Analyzing a stove without the obtained an internal data— theaccumulation lowest CO emissionlayer (unit was modified obtained to for use combustion thermoelectric of the generators). wood in the Analyzing stove without the anobtained accumulation data— layer.the lowest The averageCO emission CO emission was obtained was 1916 for mg comb/m3 inustion the combustion of the wood phase in (i.e.,the instove time, without when fluean accumulation layer. The average CO emission was 1916 mg/m3 in the combustion phase (i.e., in time, gasaccumulation temperature layer. was The higher average than CO 250 emission◦C). This was value 1916 is mg/m muchin more the thancombustion Eco-Design phase requirements, (i.e., in time, whichwhen limitflue gas CO emissionstemperature for roomwas higher heaters than to 1500 250 mg °C/m). 3This. On thevalue other is much hand, themore average than valueEco-Design of CO emissionrequirements, during which other limit series CO was emissi evenons higher, for room achieving heaters a maximum to 1500 mg/m level3.of On 2904 the mg other/m3 hand,when the briquetteaverage value was burningof CO emission in the stove during with other an accumulation series was even layer. higher, The average achieving values a maximum of CO and level other of flue gas components (CO2,O2, SO2, NOx) emission in the combustion phase are shown in Table1. By analyzing ecological parameters of the tested stove operation, it may be concluded that removal of the accumulation layer does not impact the CO emissions negatively. Energies 2020, 13, 3909 9 of 16

Table 1. The average emissions of CO, CO2,O2, NOx and SO2 during wood and briquette combustion.

CO CO O NO SO Parameter 2 2 x 2 mg/m3 % % ppm ppm Combustion of the wood, stove with an accumulation layer 2227 3.2 17.9 62.3 26.3 Combustion of the wood, stove without an accumulation layer 1916 3.8 17.3 53.0 15.4 Combustion of the briquette, stove with an accumulation layer 2904 3.8 17.3 98.5 43.9 Combustion of the briquette, stove without an accumulation layer 2812 4.0 17.0 53.7 23.9

3.1.4. Economical Aspects of the Use of Thermoelectric Generators

During typical operation of the stove, the controller consumes 1.5–6.0 We. Assuming the average daily time of the stove’s operation at a level of 6 h (during the heating season), the controller consumes less than 20 Whe per day. If the water pump is taken into consideration (during operation of TEG2), an additional 40–50 Whe should be added (on the other hand, the water circuit may be an independent system or serve as a part of the hot water system). Thus, it may be assumed that 20 Whe or 70 Whe is required for a self-suﬃcient operation of the stove. Taking into account the maximum power obtained from the considered thermoelectric generators during tests, it will be possible to generate ca. 112.5 Whe and 156 Whe respectively from TEG1 and TEG2 (electricity generated only in combustion phase). As a result, net power at a level of ca. 92.5 Whe (in the case of TEG1) and ca. 86.0–136.0 Whe (in the case of TEG2) may be stored in batteries and used to power domestic appliances. Considering only the economic reasons for using the TEG in the analyzed system, it would be unproﬁtable without external funding. Assuming that the price of the thermoelectric generator is 1000 EUR (in cases of both TEG1 and TEG2), yearly generated electric energy is equal to ca. 27 kWhe (TEG1) and 37.4 kWhe (TEG2), and the value of electrical energy in Poland is equal to 0.20 EUR per kWhe, calculated simply payback time (SPBT) = 185.2 years for TEG1 and 133.7 years for TEG2. This contraindicates the use of the TEGs from a purely economic point of view. On the other hand, the economy of the usage of TEGs is not the only argument. By introducing a power generating system, one becomes independent from the electric grid.

3.2. Results Obtained on the Experimental Rig 2

3.2.1. Oil Temperature during Single and Continuous Combustion Processes At the beginning of this part of the studies, a single and continuous combustion process was analyzed. During the analyzed single combustion process four rectangular straw bales with total weight ca. 40 kg were burned. In the case of the continuous combustion process, the initial input was also four bales, but the next bales were successively added. Each load matched as “input x” represents one bale weighed ca. 8 kg. In total, ca. 110 kg of straw was burned during the analyzed continuous combustion process. The input of the additional bales was realized by a specially designed and developed fuel feeder. The time shift between two inputs was typically 9–10 min (in cases of fuel inputs 1–7), but for the two last inputs it increased to 14–15 min. The variations in the oil temperature at the outlet from the boiler during the single and continuous combustion process are shown in Figure 10. The maximum and average oil temperature in the case of a single combustion process were respectively ca. 154 ◦C and ca. 115 ◦C, while in case of continuous combustion process these values were ca. 222 ◦C and 169 ◦C. Temperature ﬂuctuations were slight due to the thermal inertia of the boiler. Time when the observed oil temperature was greater than 200 ◦C was ca. 1 h in case of the analyzed continuous combustion process. EnergiesEnergies 20202020,, 1313,, xx FORFOR PEERPEER REVIEWREVIEW 1010 ofof 1616 ofof continuouscontinuous combustioncombustion processprocess thesethese valuesvalues werewere ca.ca. 222222 °C°C andand 169169 °C.°C. TemperatureTemperature fluctuationsfluctuations Energies 2020, 13, 3909 10 of 16 werewere slightslight duedue toto thethe thermalthermal inertiainertia ofof thethe boboiler.iler. TimeTime whenwhen thethe obseobservedrved oiloil temperaturetemperature waswas greatergreater thanthan 200200 °C°C waswas ca.ca. 11 hh inin casecase ofof thethe analyzedanalyzed continuouscontinuous combustioncombustion process.process.

Figure 10. The variations in the thermal oil temperature at the boiler’s outlet in case of single and FigureFigure 10.10. TheThe variationsvariations inin thethe thermalthermal oiloil temperaturetemperature atat thethe boiler’sboiler’s outletoutlet inin casecase ofof singlesingle andand continuous combustion processes. continuouscontinuous combustioncombustion processes.processes. 3.2.2. Power Distribution in the Oil Circuit 3.2.2.3.2.2. PowerPower DistributionDistribution inin thethe OilOil CircuitCircuit Hot thermal oil ﬂew from the boiler to the superheater; evaporator; and additionally, the oil–water Hot thermal oil flew from the boiler to the superheater; evaporator; and additionally, the oil– heat exchanger,Hot thermal respectively. oil flew from The the heat boiler was to used the tosu evaporateperheater; waterevaporator; and superheating and additionally, steam, the and oil– to water heat exchanger, respectively. The heat was used to evaporate water and superheating steam, heatwater the heat water exchanger, in the water respectively. circuit (oil–water The heat was heat used exchanger to evaporate was switching water and on superheating at the end of steam, each andand toto heatheat thethe waterwater inin thethe waterwater circuitcircuit (oil–wat(oil–waterer heatheat exchangerexchanger waswas switchingswitching onon atat thethe endend ofof measurement series). The average boiler power during the combustion phase was ca. 67 kWt for the eacheach measurementmeasurement series).series). TheThe averageaverage boilerboiler popowerwer duringduring thethe combusticombustionon phasephase waswas ca.ca. 6767 kWkWtt forfor continuous combustion process (comparing to ca. 19 kWt for single combustion process). The variations the continuous combustion process (comparing to ca. 19 kWt for single combustion process). The inthe power continuous transferred combustion from oil process to condensate, (comparing steam to and ca. water19 kW aret for shown single in combustion Figure 11. process). The variationsvariations inin powerpower transferredtransferred frfromom oiloil toto condensate,condensate, steamsteam andand waterwater areare shownshown inin FigureFigure 11.11.

Figure 11. The variations in power transferred from oil to condensate, steam and water during the FigureFigure 11.11. TheThe variationsvariations inin powerpower transferredtransferred fromfrom oioill toto condensate,condensate, steamsteam andand waterwater duringduring thethe continuous combustion process. continuouscontinuous combustioncombustion process.process. Figure 12 shows the percentage dissipation of the power generated in the boiler in the oil circuit. FigureFigure 1212 showsshows thethe percentagepercentage dissipationdissipation ofof thethe powerpower generatedgenerated inin thethe boilerboiler inin thethe oiloil circuit.circuit. It may be noted that ca. 71% of heat was transferred to the condensate and steam, while 20.5% was ItIt maymay bebe notednoted thatthat ca.ca. 71%71% ofof heatheat waswas transferredtransferred toto thethe condensatecondensate andand steam,steam, whilewhile 20.5%20.5% waswas transferred to water (mainly in the post-combustion phase). Ca. 8.5% of the heat was lost. transferredtransferred toto waterwater (mainly(mainly inin thethe post-combustionpost-combustion phase).phase). Ca.Ca. 8.5%8.5% ofof thethe heatheat waswas lost.lost.

Energies 2020, 13, 3909 11 of 16 Energies 2020, 13, x FOR PEER REVIEW 11 of 16 Energies 2020, 13, x FOR PEER REVIEW 11 of 16

Heat losses Heat losses 8,5% 8,5%

Oil-water heat Oil-water heat exchanger Evaporator exchanger Evaporator 20,5% 43,3% 20,5% 43,3%

Superheater Superheater 27,7% 27,7%

Figure 12. The percentage dissipation of the power generated in the boiler. Figure 12. The percentage dissipation of the power generated in the boiler. Figure 12. The percentage dissipation of the power generated in the boiler. 3.2.3. Temperature Variations in the Steam-Condensate Circuit 3.2.3.3.2.3. Temperature Temperature Variations Variations in in the the Steam-Condensate Steam-Condensate Circuit Circuit The most important steam parameters from the standpoint of power generation are pressure, temperatureTheThe mostmost and importantimportant flow. The steamsteam variations parametersparameters in the steam fromfrom temperaturethethe standpointstandpoint at the ofof power inletpower to generationgeneration the evaporator, areare pressure,pressure, between temperaturethetemperature evaporator and and and flow. flow. superheater; The The variations variations at the in in outletthe the steam steam from te te themperaturemperature superheater; at at the the atinletinlet the to to inlet the the evaporator, toevaporator, the steam between between engine; theandthe evaporatorevaporator the outlet from andand superheater; condensersuperheater; are atat shown thethe outletoutlet in Figure fromfrom the the13. superheater; Thesuperheater; condensate atat thethe (steam) inletinlet to flewto thethe in steamsteam counterflow engine;engine; andrelativeand thethe tooutletoutlet the thermal fromfrom condensercondenser oil. The processareare shownshown of evaporation inin FigureFigure 13.13. was TheThe initialized condensatecondensate in (ste minute(steam)am) 16flewflew and inin the counterflowcounterflow reducing valverelativerelative was toto opened thethe thermalthermal in minute oil.oil. TheThe 35. processprocess The steam ofof evaporatevaporat engine operatedionion waswas initializedbetweeninitialized 83 inin and minuteminute 104 min. 1616 andand The thethe fluctuations reducingreducing valveinvalve the was condensatewas opened opened in temperaturein minute minute 35. 35. resultedThe The steam steam from engine engine the factopoperatederated that two-statebetween between 83 control83 and and 104 104 of condensatemin. min. The The fluctuations fluctuations level was inapplied.in thethe condensatecondensate In time, when temperaturetemperature the steam resultedresulted engine fromfrom operated, thethe fafa thectct thatthat average two-statetwo-state temperature controlcontrol at ofof the condensatecondensate steam engine levellevel inlet waswas applied. In time, when the steam engine operated, the average temperature at the steam engine inlet wasapplied. 155 ◦ InC, time, while when the average the steam steam engine temperature operated, at the the average superheater temperature outlet was at the 195 steam◦C. engine inlet waswas 155 155 °C, °C, while while the the average average steam steam temperature temperature at at the the superheater superheater outlet outlet was was 195 195 °C. °C.

Figure 13. Figure 13. TheThe variations variations in steam temperature in the steam-condensate circuit. Figure 13. The variations in steam temperature in the steam-condensate circuit. 3.2.4. Pressure Variations in the Steam-Condensate Circuit 3.2.4.3.2.4. Pressure Pressure Variations Variations in in the the Steam-Condensate Steam-Condensate Circuit Circuit In the initial part of the evaporating process, steam pressure was rising from 0 to 7.5 bar (maximum valueInIn was thethe reached initialinitial inpartpart minute ofof thethe 73). evaporatingevaporating When the steam process,process, engine steamsteam was pressure switchedpressure was on,wassteam risingrising pressure fromfrom 00 dropped toto 7.57.5 barbar to (maximum3.5(maximum bar at the value value outlet was was from reached reached the superheater in in minute minute 73). 73). and When When 3.0 bar the the at steam steam the inlet engine engine to the was was steam switched switched engine. on, on, Between steam steam pressure pressure 73 and 104droppeddropped min, whentoto 3.53.5 the barbar steam atat thethe engine outletoutlet fromwasfrom working, thethe superheatersuperheater the average andand steam3.03.0 barbar pressure atat thethe inletinlet at the toto inletthethe steamsteam to the engine. engineengine. wasBetweenBetween 3.5 bar 73 73 andand and the104 104 dimin, min,ﬀerence when when between the the steam steam steam engine engine pressure was was working, working, at the outlet the the fromaverage average superheater steam steam pressure pressure and at theatat the the inlet inlet inlet to totheto thethe steam engineengine engine waswas was 3.53.5 ca. barbar 1 andand bar. thethe This differencedifference reduction betweenbetween in steam steamsteam pressure pressurepressure was caused atat thethe by outletoutlet a partial fromfrom expansion superheatersuperheater of and at the inlet to the steam engine was ca. 1 bar. This reduction in steam pressure was caused by a steamand at in the the inlet steam to the bus steam (including engine pipes, was reducingca. 1 bar. valveThis reduction and other in steam steam equipment). pressure was The caused variations by a partial expansion of steam in the steam bus (including pipes, reducing valve and other steam inpartial steam expansion pressure and of ﬂowsteam are in shown the steam in Figure bus 14(including. pipes, reducing valve and other steam equipment).equipment). The The variations variations in in steam steam pressure pressure and and flow flow are are shown shown in in Figure Figure 14. 14.

Energies 2020, 13, 3909 12 of 16 EnergiesEnergies 2020 2020, 13, 13, x, xFOR FOR PEER PEER REVIEW REVIEW 1212 of of 16 16

Figure 14. FigureFigure 14. 14. Steam SteamSteam pressure pressurepressure at atat the thethe outlet outletoutlet from fromfrom superheater superheatersuperheater and andand inlet inletinlet to toto the thethe steam steamsteam engine engineengine and andand the thethe steam steamsteam flow in the steam bus. flowflow in in the the steam steam bus. bus. 3.2.5. Power Generation Depending on the Operation of Electric Load 3.2.5.3.2.5. Power Power Generation Generation Depending Depending on on the the Operation Operation of of Electric Electric Load Load The power generated in the power generator resulted from the actual steam pressure and flow The power generated in the power generator resulted from the actual steam pressure and flow andThe from power the settings generated of electric in the power load. Steamgenerator parameters resulted from affected the theactual engine’s steam rotationpressure speed and flow and and from the settings of electric load. Steam parameters affected the engine’s rotation speed and andconsequently from the thesettings generated of electric power. load. Figure Steam 15 shows parameters the variations affected in the generated engine’s power rotation when speed constant and consequentlyconsequently the the generated generated power. power. Figure Figure 15 15 shows shows the the variations variations in in generated generated power power when when constant constant and various electric load settings were applied. The maximum power level of ca. 825 We was observed andand various various electric electric load load settings settings were were applied. applied. The The maximum maximum power power level level of of ca. ca. 825 825 W We ewas was observed observed when a constant electric load was set, while ca. 1145 We was obtained when the various electric load when a constant electric load was set, while ca. 1145 We was obtained when the various electric load whenwas used a constant (this value electric does load not was include set, while power ca. required 1145 W fore was fans, obtained pumps when and other the various electric electric components’ load waswas used used (this (this value value does does not not include include power power requir requireded for for fans, fans, pumps pumps and and other other electric electric components’ components’ operation, which was ca. 5 kWe). Thus, it may be concluded that the use of the maximum power point operation, which was ca. 5 kWe). Thus, it may be concluded that the use of the maximum power point operation,tracker (MPPT) which should was ca. be 5 kW an essentiale). Thus, it part may of be the concluded system. that the use of the maximum power point trackertracker (MPPT) (MPPT) should should be be an an essential essential part part of of the the system. system.

Figure 15. The relationship between generated power, steam pressure and steam ﬂow. FigureFigure 15. 15. The The relationship relationship between between generated generated po power,wer, steam steam pressure pressure and and steam steam flow. flow. 3.2.6. Environmental and Economic Aspects of the Introduction of the Micro-Cogeneration System 3.2.6.with3.2.6. aEnvironmental Environmental Straw-Fired Batch and and Economic Boiler Economic Aspects Aspects of of the the Introduction Introduction of of the the Micro-Cogeneration Micro-Cogeneration System System withwith aThe a Straw-Fired Straw-Fired actual version Batch Batch Boiler Boiler of the prototypical micro-cogeneration system is not optimized yet. By introducingTheThe actualactual versionallversion improvements ofof thethe prototypicalprototypical and increasing micro-cogenerationmicro-cogeneration hot oil temperature, systemsystem it is willis notnot be optimizedoptimized possible to yet.yet. reach ByBy introducingca.introducing 10 kWe, asall all estimatedimprovements improvements through and and the increasing increasing preliminary hot hot oil simulationoil te temperature,mperature, with it TRNSYSit will will be be possible softwarepossible to [to28 reach reach,29]. ca. Inca. the10 10 ﬁrst version of TRNSYS model, the following components were included: Type 5 (heat exchanger— kWkWe,e ,as as estimated estimated through through the the preliminary preliminary simulation simulation with with TRNSYS TRNSYS software software [28,29]. [28,29]. In In the the first first versionaversion component of of TRNSYS TRNSYS used to model, simulatemodel, the the the following amountfollowing of components heatcomponents transferred were were from included: included: the boiler Type Type to the5 5 (heat (heat oil circuit), exchanger—a exchanger—aType 637 component(steamcomponent generator—a used used to to simulate simulate component the the amount usedamount to simulateof of heat heat transferred transferred the operation from from of the thethe boiler evaporatorboiler to to the the andoil oil circuit), superheater),circuit), Type Type 637637 (steam(steam generator—agenerator—a componentcomponent usedused toto simulatesimulate thethe operationoperation ofof thethe evaporatorevaporator andand

Energies 2020, 13, 3909 13 of 16

Type 598 (condenser), Type 4 (water tank), Type 3 and Type 618 (pumps), Type 31 (pipeline), Type 647 (diverting valve), Type 2 (controllers) and other components. The above-mentioned components have been parameterized using measurement data. On the other hand, a dedicated component was created to simulate the operation of the steam engine. The actual version of the TRNSYS model does not simulate the process of thermal utilization of straw in the batch boiler. Instead, the heat source was parameterized based on measurement data (i.e., temperature and oil flow measured during combustion processes in laboratory conditions). Assuming the electricity generation at the level of ca. 10 kWe, it will be possible to provide self-sufficient operation of the system (energy consumed by pumps, fans and other components is ca. 5 kWe). Additional power (ca. 5 kWe) will be available to power, e.g., domestic appliances and other electric devices. Taking into account simulation results, the estimated differences in CO2 emissions during the combustion of coal, gasoline and natural gas compared to straw combustion are presented in Table2. The analysis was conducted for five variants of daily work time (8, 12, 16, 20 and 24 h), assuming that the installation works 95% of the time in a year. As it can be observed, straw is characterized by lower CO2 emission only compared to coal.

Table 2. Diﬀerences in CO2 emission during primary fuel combustion compared to straw combustion (“+” means higher emission, “ ” means lower emission compared to straw combustion). Calculations − based on emission factors given in [30].

CO2, t/a Daily Work Time, h Coal Gasoline Natural Gas 8 +72.6 1.9 31.6 − − 12 +109.1 2.9 47.4 − − 16 +145.5 3.8 63.1 − − 20 +181.7 4.8 79.0 − − 24 +218.2 5.8 94.8 − −

Taking into account previously assumed electricity generation at the level of ca. 10 kWe (only 5 kWe available for any use), and taking into account an average electricity price of 0.20 EUR/kWhe, potential electric energy cost savings were calculated (see Table3). Calculations were made for five variants of daily work time (8, 12, 16, 20 and 24 h), assuming that the installation works 95% of the time in a year. When analyzing the economic aspects of heat and power generation in the straw-fired micro-cogeneration system, it was assumed that straw was a free and local available fuel (as a residue of agricultural production). Investment costs will depend, among other things on the finally used solutions (materials and components) and production volume. It may be estimated that the payback period will be less than five years.

Table 3. Potential electricity generation and energy cost savings.

Electric Power Generation, Energy Cost Savings, Daily Work Time, h MWh/a EUR/a 8 ~20.8 4160 12 ~34.7 6940 16 ~48.5 9700 20 ~62.4 12,480 24 ~83.2 16,640 Energies 2020, 13, 3909 14 of 16

4. Conclusions This paper shows two examples of biomass-fired micro-cogeneration systems: a system with a wood-fired stove and market-available thermoelectric generators, and the system operating according to the modified Rankine cycle using a straw-fired batch boiler as the heat source. These two cogeneration technologies were selected, taking into account construction parameters of the considered stove and boiler, and available temperature and power, which may be obtained from the analyzed devices. The approach presented is original—the analyzed devices are not widely studied in the context of combined heat and power generation and include several novelties compared to other available solutions. Thermoelectric generators are promising electricity generating units, which may cooperate with home heating devices (stoves, boilers, etc.). However, although it seems to be a relatively simple task, problems with proper selection and complicated use of thermoelectric generators do arise. As was shown, the power generated by tested TEGs was significantly lower than their nominal power. The TEG1 operated at 22.5 W at the maximum power point (50% of its nominal power), while the TEG2 operated at 31.2 W (31.2%). Such a situation was caused mainly by the fact that non-uniform surface temperature reduced the heat flux to the hot side of the TEGs and consequently the obtained power. Simply calculated payback time (185.2 years for TEG1 and 133.7 years for TEG2) contraindicates the use of the TEGs from a purely economic point of view. On the other hand, the economy of the usage of TEGs is not the only argument. By analyzing the environmental impact of the stove used, the lowest CO emission was obtained for combustion of the wood in the stove without an accumulation layer. The average CO emission was 1916 mg/m3 in the combustion phase. This value was higher than Eco-Design requirements, which limit CO emission for room heaters to 1500 mg/m3. The operational parameters of the developed micro-cogeneration system with a wood-fired stove and thermoelectric generator may be improved. In general, there are two possible approaches:

Modification of the heating device’s structure to provide a flat area, improvements in the air • distribution system to provide high temperature inside the furnace and sufficient heat flux, etc.; Design and development of a dedicated TEGs structure characterized both by proper operation • parameters and acceptable investing costs.

Additionally, the electrical efficiency of the second described system—a prototypical micro-cogeneration installation with a biomass-fired boiler and modified Rankine xycle—was not satisfying. When analyzing the percentage dissipation of the power generated in the boiler in the oil circuit, it was observed that ca. 71% of heat was transferred to the condensate and steam, while 20.5% was transferred to water (mainly in the post-combustion phase). Ca. 8.5% of the heat was lost. As was stated, the most important steam parameters from the standpoint of power generation were pressure, temperature and flow. In time, when the steam engine operated, the average steam temperature at the superheater outlet was 195 ◦C, while the average temperature at the steam engine inlet was 155 ◦C. The maximum steam pressure (7.5 bar) was observed during the initial part of the evaporating process. When the steam engine was switched on, steam pressure dropped to 3.5 bar at the outlet from the superheater and 3.0 bar at the inlet to the steam engine. As a result, the maximum power was only ca. 825 We (when a constant electric load was set) or ca. 1145 We (when the various electric load was used). To achieve a higher level of power generation, some modifications should be introduced, including:

Extension of the heat exchange surface in the evaporator, • Reduction of the length and dimension of pipes in the steam bus, • Replacement of actually used power generator (due to too low installed power), • Fully automation of the system operation. • Energies 2020, 13, 3909 15 of 16

Introducing the above-mentioned improvements should allow one to reach electricity generation at a level of ca. 10 kWe (this value was estimated based on a preliminary dynamic simulation using TRNSYS software).

Funding: This work was carried out under Subvention from the Faculty of Energy and Fuels, AGH University of Science and Technology in Krakow. Experimental studies have been conducted using the infrastructure of the Center of Energy AGH. The data obtained during the fulfillment of the project “BioORC: Construction of cogeneration system with small to medium size biomass boilers” was used. Conflicts of Interest: The author declares no conflict of interest.

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