processes
Article Long-Term Performance of Anti-Freeze Protection System of a Solar Thermal System
Sebastian Pater
Faculty of Chemical Engineering and Technology, Cracow University of Technology, 31-155 Kraków, Poland; [email protected]
Received: 7 September 2020; Accepted: 12 October 2020; Published: 14 October 2020
Abstract: In a moderate, transitory climate, to prevent freezing of outdoor pipes and collectors in solar thermal systems, anti-freezing fluids are commonly used. There is little experience of using water without any additives as a solar thermal fluid in such a climate. Based on these findings, to fill the knowledge gap this article presents the long-term results of thermal performance and anti-freeze protection of a solar heating system with heat pipe evacuated tube collectors with water as a solar thermal fluid. The operation of this system under real conditions was analysed for five years in southern Poland. The annual value of solar insolation ranged from 839 to almost 1000 kWh/m2. The monthly efficiency of the solar collectors from March to October was usually higher than 25%, and the lowest was between November and January. The anti-freeze protection system consumed annually from 7 to 13% of the heat generated by the collectors in the installation. Supporting the operation of the central heating system in the building during the winter season highly improved the efficiency of the solar collectors. Results show that it is possible to use water without any additives as a solar thermal fluid in a moderate, transitory climate.
Keywords: solar collector; anti-freeze protection; solar fluid; heat pipe; evacuated tube collector
1. Introduction Energy is an essential commodity in many aspects of human life and industrial production, in both developing and developed economies. To reconcile the economic growth with the ever-increasing global demand for energy, energy-efficient and environmentally-friendly solutions are being sought in various branches of human activity. For this reason, equipment using renewable energy sources is becoming more and more important. Commonly used solar collectors (SCs) use one of these sources, i.e., solar energy. These devices are nothing more than a special type of heat exchanger in which solar radiation is converted into heat, transferred to the appropriate medium in the system, or used for different purposes [1]. The literature on the subject provides, as a rule, a division of SCs into stationary (fixed, non-tracking) ones, which include flat plate collectors, evacuated tube collectors (ETCs) and compound parabolic collectors [2,3]. The tracking collectors in turn, which track the position of the sun so that the sun’s rays always fall perpendicularly on them, include a parabolic trough collector, cylindrical trough collector, linear Fresnel reflector, circular Fresnel lens, parabolic dish reflector and central tower receiver [2,4]. SCs can also be classified into concentrating (e.g., compound parabolic collectors) and non-concentrating (e.g., flat plate collectors, ETCs) [2]. Another equally important criterion for the division of SCs is the type of solar fluid used (i.e., liquid or gas) [5]. Flat collectors, due to their simple design and low operating cost, are most often used in countries with high insolation levels and warm climates. However, they offer low efficiency, low outlet temperatures of the fluid and high convective heat loss through the glass cover from the collector
Processes 2020, 8, 1286; doi:10.3390/pr8101286 www.mdpi.com/journal/processes Processes 2020, 8, 1286 2 of 11 plate [5,6]. ETCs are characterised by lower convective heat loss due to the presence of a vacuum in the tube spaces. They are used in countries with moderate climates [7,8]. According to [9], the available types of ETCs can be categorised into two groups: the single-walled glass evacuated tube and the Dewar tube. Many variations of these two basic types can be found on the market: with a U-pipe, heat pipe or direct liquid contact [10]. The heat pipe evacuated tube collector (HPETC) with a Dewar tube consists of vacuum-sealed tubes with the absorber surface on the inner glass tube. Heat generated in the absorber is transmitted via a path through a metal (usually aluminium) insert to the heat pipe installed in it. The heat pipe consists of a high thermal conductance sealed tube (copper), which contains a small amount of a low boiling point fluid. During the operation of the collector, the liquid in the heat pipe evaporates and then condenses in the upper part of the heat pipe, transferring heat to the fluid flowing in the manifold ETC [11]. The selection of the solar fluid mainly depends on the application of the solar system (i.e., the working temperature) which also affects the long-term operation, safety and energy efficiency of the system. Water, oils and glycol solutions are commonly used worldwide [1,4]. Due to climatic conditions in Poland, i.e., a moderate transient climate, the most commonly used solar liquids are 35–50% aqueous solutions of propylene or ethylene glycol. Such mixtures, together with refining additives, effectively protect the solar system from freezing of the working medium at outdoor temperatures reaching around 36 C during the winter. In comparison to water, such solutions have − ◦ several times higher viscosity, which generates greater hydraulic resistance when pumping liquid in the solar system and, consequently, an increased consumption of electricity by the circulating pump [12,13]. Other disadvantages of these fluids compared to water include:
- lower specific heat values in the range of typical operating temperatures of the working fluid; - thermal degradation with precipitation of, among others, lactates, acetates and formates, as well as insoluble ethers depositing on the inner surface of the solar system components, - hindering heat transfer and increasing the freezing point of the solar fluid; - can be highly corrosive, resulting in accelerated clogging of the collectors and in some cases other system components; - risk of poisoning as a result of accidental ingestion; - much higher price per unit volume [12–14].
In solar systems operating in moderate climate conditions, it is possible to use environmentally safe water without the addition of substances reducing the freezing point. It is then necessary to apply a solution that protects the system against the freezing of water. In the literature, several solutions can be found:
- automatic control system equipped with an anti-freeze protection (AFP) function which, by switching on the solar pump, allows for the transfer of heat from the heat storage to the SC; - use of an additional component, e.g., electric heating system (heating tapes), that keeps the water temperature in the part of the system exposed to freezing at a safe level; - a drain-back system in which solar liquid is discharged from elements exposed to freezing to a special tank; - a drain-out system in which the system is emptied completely of water; - use of flexible materials which can withstand the occasional freezing of liquids in system elements, e.g., flexible silicone peroxide pipes insides copper pipes in the SC;
- the use of a phase-change material in the SCs with a transformation temperature <10 ◦C which does not directly protect the pipes of the solar system [11,12,15–19].
The AFP which, by switching on the solar pump, makes it possible to transfer heat from the heat storage to SCs does not require any additional hydraulic changes in currently used solar systems. In this case, the use of an appropriate automatic protection control system is of utmost importance. The use of water as a solar fluid is recommended especially in systems Processes 2020, 8, 1286 3 of 11
- with a short total length of piping exposed to freezing; - in which the majority of piping is located inside heated areas, and piping outside the building is well insulated; - equipped with an SC with a low external heat loss coefficient, e.g., for ETCs.
The purpose of this article is to analyse the thermal performance and AFP system of a solar heating system with HPETCs with water as a solar thermal fluid, while indicating the advantages and disadvantages of such a solution. The installation uses a specially selected automatic control system that switches on the solar pump in order to maintain the appropriate water temperature in the SCs and the pipeline. There are no devices that heat the installation components exposed to freezing of water, e.g., heating tapes. The operation of this system under real conditions was analysed for five years in a residential and retail building located near Kraków in Southern Poland. The HPETCs are mainly used for preparing domestic hot water (DHW), as well as for supporting central heating. They are part of a multivalent hybrid system where, for the production of heat and cold, three different heating devices (brine-to-water heat pump, biomass boiler and gas condensing boiler) are additionally used [20].
2. Description of the System An experimental solar system with HPETCs was set up at the beginning of September 2011 on the roof of a detached, two-storey building located a few kilometres outside Kraków, Poland (latitude 2 50.12◦N and longitude 20.03◦E). The building footprint area is about 268 m . The roof with metal roofing tiles is symmetrical, with an inclination of 35◦ deviated from the south by 10◦ to the easterly direction. The solar system consists of one 12-pipe and five 20-tube HPETCs with a total aperture area of 10.53 m2. The collectors were installed parallel to the roof surface. The basic technical data for the SCs used are summarised in Table1.
Table 1. Basic heat pipe evacuated tube collector (HPETC) data.
Specifications Values or Materials Units Producer Wuxi High-New Tech. Ind. Dev. Co. - Collector name NSC-58-12 NSC-58-20 - Tube number 12 20 - Aperture area 1.200 1.866 m2 Gross area 2.084 3.357 m2 Gross width 1047 1687 mm Gross length 1990 mm Gross height 180 mm Optical efficiency related to aperture area 0.618 - 2 1 Linear heat loss coefficient 1.3767 Wm− K− 2 2 Quadratic heat loss coefficient 0.0184 Wm− K−
The SCs are connected in series and parallel (three collectors for each series) by means of a DN16 corrugated stainless steel tube, insulated with a 25 mm layer of rubber foam. The total length of the piping on the roof of the building is 12 m and 11 m at the unheated attic. The main function of the group of collectors in the system is to supply generated heat for DHW by the lower coil of the combined tank (CT) named SG (K) 800/200 with a total capacity of 800 L (Figure1). DHW is heated in an internal tank with a capacity of 200 L. From the beginning of September 2011 to 24 March 2012, the solar fluid was a 40% aqueous propylene glycol solution. At the end of March 2012, the solar fluid used was replaced with demineralised water. The heat generated by the SCs can also be directed, through appropriate switching of the three-way valve, directly to the buffer tank (BT) with a capacity of 1500 L. This process takes place without an additional heat exchanger, since the same liquid, the deionised water, remains in the solar circuit and the BT. The HPETCs used are characterised by low values of linear and quadratic heat loss coefficients Processes 2020, 8, 1286 4 of 11
(Table1); therefore, it was possible to use deionised water as an intermediate fluid in transferring heat from the SC to the tanks. In addition to a number of benefits resulting from this, the main drawback of such a solution in the installation used throughout the year is the possibility of water freezing in the piping or in the manifold at low outdoor temperatures in winter. Processes 2020, 8, x FOR PEER REVIEW 4 of 11
Figure 1. Scheme of the solarsolar installationinstallation andand photophoto of of the the solar solar collectors collectors (SCs) (SCs) installed installed on on the the roof roof of ofa building.a building.
2.1. TheThe AFP heat System generated Strategy by the SCs can also be directed, through appropriate switching of the three-waySafe operation valve, directly of the to solar the systembuffer tank in the (BT) winter, with withouta capacity the of risk 1500 of L. leaks This or process damage, takes required place withoutactivation an of additional the AFP functionheat exchanger, in the DigiENERGY since the same control liquid, system. the deionised The operation water, ofremains the system in the is solarshown circuit in Figure and 2the over BT. four The selectedHPETCs days used di areffering characterised in insolation by low patterns, values outsideof linear air and temperature quadratic heat loss coefficients (Table 1); therefore, it was possible to use deionised water as an intermediate (Ta) and temperature of water in the lower part of the BT (TBT,low) and the lower part of the CT fluid in transferring heat from the SC to the tanks. In addition to a number of benefits resulting from (TCT,low). The AFP operation consisted of short-term switching of the solar pump when the water this, the main drawback of such a solution in the installation used throughout the year is the temperature measured in one of the SC heads (TSC) was lower than 9.5 ◦C. As a result, the transfer of possibilityheat accumulated of water in freezing the lower in partthe piping of the BTor in followed the manifold on to theat low collectors outdoor on temperatures the roof of the in building, winter. as well as to the piping. This lasted until TSC and the return water temperature rose above 20 ◦C. 2.1. The AFP System Strategy The AFP system was most often activated during the winter period (at low outside temperatures) duringSafe the operation night hours of the and solar during system the dayin the when winter insolation, without was the low risk (Figure of leaks2). or The damage, AFP switching required activationfrequency wasof the dependent AFP function upon Tina :the when DigiENERGY it was around control 0 ◦C, itsystem. was switched The operation on every of 2.5 the h on system average is shown(5–6 January in Figure 2015), 2 over whilst four at selected10 Cit days switched differing on everyin insolation 1.5 h (07 patterns, January outside 2015). Theair temperature result of the − ◦ (Tsystema) and operation temperature was theof water negative in heatingthe lower power part of of the the SCs, BT shown(TBT,low in) and Figure the2 aslower peaks part ranging of the from CT (TC1.5T,low to). The9.5kW. AFP These operation values consisted mean the of heat short-term flux that switching was taken of fromthe solar the bupumpffertank when and the used water to − − temperaturemaintain the measured appropriate in one temperature of the SC ofheads the solar(TSC) was fluid, lower i.e.,water than 9.5 in solar°C. As collectors a result, the and transfer pipelines. of heatIf, during accumulated the day, in the the appropriatelower part of solar the BT radiation followed intensity on to the was collectors applied on to the the roof surface of theof building, the SC, asthe well collectors as to the produced piping. heatThis and lasted returned until T itSC to and the the CT, return which water can be temperature observed as therose increase above 20 of °C. TCT,low . At lowThe radiation AFP system intensity, was only most the increase often ofactivated water temperature during the in thewinter collectors period was (at recorded, low outside so that temperatures)from 8.00 a.m. during to 6.00 p.m.,the night the AFPhours system and during did not the start day the when circulation insolation pump. was low (Figure 2). The AFP switching frequency was dependent upon Ta: when it was around 0 °C, it was switched on every 2.5 h on average (5–6 January 2015), whilst at −10 °C it switched on every 1.5 h (07 January 2015). The result of the system operation was the negative heating power of the SCs, shown in Figure 2 as peaks ranging from −1.5 to −9.5 kW. These values mean the heat flux that was taken from the buffer tank and used to maintain the appropriate temperature of the solar fluid, i.e., water in solar collectors and pipelines. If, during the day, the appropriate solar radiation intensity was applied to the surface of the SC, the collectors produced heat and returned it to the CT, which can be observed as the increase of TCT,low. At low radiation intensity, only the increase of water temperature in the
Processes 2020, 8, x FOR PEER REVIEW 5 of 11 collectorsProcesses 2020 was, 8, 1286 recorded, so that from 8.00 a.m. to 6.00 p.m., the AFP system did not start5 ofthe 11 circulation pump.
FigureFigure 2. 2. SelectedSelected parameters parameters of of SC SC operation operation from from 5 5 to to 8 8 January January 2015. 2015.
2.2.2.2. Data Data Measurement Measurement and and Acquisition Acquisition TheThe visualisation, visualisation, monitoring, monitoring, remote remote maintenance maintenance and and data data recording recording of of the the installation installation in in real real timetime was was provided byby aa DigiENERGYDigiENERGY control control system. system. All All the the measuring measuring apparatus apparatus and and parameters parameters are arelisted listed in Table in Table2. The 2. temperatureThe temperature sensors sensors were mountedwere mounted in an OG-type in an OG-type outer sheath, outer orsheath, in the or TT-532 in the/B TT-532/Bhousing. housing. A secondary A secondary standard standard pyranometer pyranometer was installed was ininstalled parallel in to parallel the surface to the of surface the SCs. of the SCs. Table 2. Basic HPETC data. Table 2. Basic HPETC data. Apparatus Measuring Parameter Accuracy PlatinumApparatus resistance TemperatureMeasuring of media in theParameter installation from Accuracy0.19 to 0.29 C ± ± ◦ thermometer Pt1000 (solar fluid, DHW, ambient air) (in the range 20 to 70 C) from ±0.19− to ◦ Pyranometer CM 21 IntensityTemperature of solar radiationof media in the 2% Platinum resistance ±0.29± °C Water flow meter GSD5installation Water (solar flow fluid, rate DHW, ambient 3% thermometer Pt1000 (in the± range −20 air) to 70 °C) 3. ResultsPyranometer and Discussion CM 21 Intensity of solar radiation ±2% 3.1. SolarWater Insolation flow meter GSD5 Water flow rate ±3%
3. ResultsMeasurements and Discussion of the intensity of solar radiation acting on the SC surface, which are necessary to determine the HPETC work efficiency in real conditions, were carried out from 18 April 2012 using the 3.1.CM21 Solar pyranometer. Insolation Figure3 presents summarised values of monthly and annual insolation for the location of the solar system in question in years under review. Annual solar insolation for 2012 is not Measurements of the intensity of solar radiation acting on the SC surface, which are necessary marked, because full measurement data for this year was not collected. It can be seen that during the to determine the HPETC work efficiency in real conditions, were carried out from 18 April 2012 year, the lowest values of monthly insolation occur in two months: December and January. In turn,
ProcessesProcesses 2020 2020, 8, ,8 x, xFOR FOR PEER PEER REVIEW REVIEW 6 6of of 11 11 usingusing thethe CM21CM21 pyranometer.pyranometer. FigureFigure 33 presentspresents summarisedsummarised valuesvalues ofof monthlymonthly andand annualannual insolationinsolation forfor thethe locationlocation ofof thethe solarsolar systemsystem inin questionquestion inin yearsyears underunder review.review. AnnualAnnual solarsolar Processesinsolationinsolation2020 for, for8, 12862012 2012 is is not not marked, marked, because because full full meas measurementurement data data for for this this year year was was not not collected. collected.6 of 11It It cancan be be seen seen that that during during the the year, year, the the lowest lowest va valueslues of of monthly monthly insolation insolation occur occur in in two two months: months: December and January. In turn, the peak of insolation intensity fell in July. In the period from April theDecember peak of and insolation January. intensity In turn, fellthe inpeak July. of In insolation the period intensity from April fell in to July. September, In the period from 74 from to 79% April of to September, from 74 to 79% of the total annual insolation reached the surface of the SCs. In these theto September, total annual from insolation 74 to 79% reached of the the total surface annual of the insolation SCs. In thesereached months, the surface the greatest of the diSCs.fferences In these in months, the greatest differences in relation to the average insolation value were present (average in relationmonths, tothe the greatest average differences insolation in value relation were to present the average (average insolation in Figure value3). were present (average in FigureFigure 3). 3).
FigureFigure 3. 3. Measured Measured monthly monthly and and annual annual sola solar solar rinsolation insolation from from 2012 2012 to to 2017. 2017.
ForFor comparison, comparison, the the averageaverage monthlymonthly insolationinsolation values values in in the the years years 2004–2014 2004–2014 are are presented presented (average(average forfor for 2004–2014) 2004–2014) 2004–2014) in in Figurein Figure Figure3 for 3 3 Krakfor for Kraków, Kraków,ów, as described as as described described in the in paperin the the [paper 21paper]. Additionally, [21]. [21]. Additionally, Additionally, the average the the valuesaverageaverage of values values insolation of of insolation insolation for the periodfor for the the 2013–2016period period 2013–2016 2013–2016 obtained obtained obtained from the from from PV GISthe the PV databasePV GIS GIS database database [22] are [22] shown.[22] are are Theshown.shown. line The The graphs line line ofgraphs graphs the average of of the the average monthlyaverage monthly monthly insolation insolation insolation for the threefor for the the databases three three databases databases compared compared compared are similar. are are Thesimilar.similar. diff erences TheThe differencesdifferences might have mightmight been have causedhave beenbeen mainly causedcaused by a mainlymainly different byby installation aa diffdifferenterent angle installationinstallation of the pyranometer angleangle ofof thethe andpyranometerpyranometer other device andand installation otherother devicedevice locations, installationinstallation resulting locations,locations, in slightly resulting diresultingfferent weatherinin slightlyslightly conditions differentdifferent (cloudiness, weatherweather 2 2 fog,conditionsconditions dust, etc.). (cloudiness, (cloudiness, Annually, fog, fog, 839 du du kWhst,st, etc.)./ metc.).2 (2013) Annually, Annually, to nearly 839 839 1000kWh/m kWh/m kWh2 (2013) /(2013)m2 (2015) to to nearly nearly reached 1000 1000 the kWh/m kWh/m surface2 (2015) of(2015) the SCreachedreached during the the the surface surface period of underof the the SC reviewSC during during (Figure the the3 period). period On average, under under review inreview Poland, (Figure (Figure annual 3). 3). insolationOn On average, average, per horizontalin in Poland, Poland, 2 surfaceannualannual insolation ranges insolation from per per 980 horizontal horizontal to 1074 kWh surface surface/m2 ranges[ ranges21]. from from 980 980 to to 1074 1074 kWh/m kWh/m2 [21].[21].
3.2.3.2. The The Analysis Analysis of of AFP AFP Operation Operation in in the the Long-Term Long-Term
Figure4 shows the daily energy consumption for the AFP system (Q AFP,d) and the change in the FigureFigure 4 4 shows shows the the daily daily energy energy consumption consumption for for the the AFP AFP system system (Q (QAFP,dAFP,d) )and and the the change change in in the the meanmean daily daily externalexternal external temperaturetemperature temperature over over over a a givena given given period period period of of of time. time. time. Each Each Each year, year, year, the the the AFP’s AFP’s AFP’s operation operation operation began began began in Septemberinin September September and and and ended ended ended in May.in in May. May.
Figure 4. Anti-freeze protection (AFP) system energy consumption and outdoor temperature in the Figure 4. Anti-freeze protection (AFP) system energy consumption and outdoor temperature in the years 2012–2017. years 2012–2017.
From December to February, the average daily air temperature was usually below 4 ◦C. However, the highest air temperatures, exceeding 20 ◦C, occurred between June and July. The peak of the energy ProcessesProcesses 20202020,, 88,, xx FORFOR PEERPEER REVIEWREVIEW 77 ofof 1111 Processes 2020, 8, 1286 7 of 11 FromFrom DecemberDecember toto February,February, thethe averageaverage dadailyily airair temperaturetemperature waswas usuallyusually belowbelow 44 °C.°C. However,However, thethe highesthighest airair temperatures,temperatures, exceedingexceeding 2020 °C,°C, occurredoccurred betweenbetween JuneJune andand July.July. TheThe peakpeak ofdemandof thethe energyenergy for the demanddemand AFP system forfor thethe was AFPAFP naturally systemsystem waswas in the natunatu winterrallyrally inin months, thethe winterwinter with months,months, a low outside withwith aa temperature lowlow outsideoutside temperaturereachingtemperature several reachingreaching kWh. severalseveral kWh.kWh. Figure5 shows the relationship between Q AFP,dand the average daily outside temperature. For an FigureFigure 55 showsshows thethe relationshiprelationship betweenbetween AFP,dQQAFP,d andand thethe averageaverage dailydaily outsideoutside temperature.temperature. ForFor averagean average daily daily air air temperature temperature below below3.0 −3.0C, °C, the the value value of of Q QAFP,d rangedranged fromfrom 1.51.5 toto even 6 kWh. an average daily air temperature below− −3.0◦ °C, the value of QAFP,dAFP,d ranged from 1.5 to even 6 kWh. Above 0 C, Q AFP,dwas below 4.5 kWh. As can be seen at a given average daily outside air temperature, AboveAbove 00◦ °C,°C, AFP,dQQAFP,d waswas belowbelow 4.54.5 kWh.kWh. AsAs cancan bebe seenseen atat aa givengiven averageaverage dailydaily outsideoutside airair temperature,atemperature, wide range ofaa widewide energy rangerange consumption ofof energyenergy by consumptionconsumption the AFP occurred. byby thethe There AFPAFP occurred.occurred. are several ThereThere reasons areare for severalseveral such reasons areasons result: for such a result: for- suchthe averagea result: daily outside temperature, by definition, does not show the temperature spread over -- thea the given averageaverage day; dailydaily outsideoutside temperature,temperature, byby defidefinition,nition, doesdoes notnot showshow thethe temperaturetemperature spreadspread - overtheover chart aa givengiven does day;day; not take into account the influence of insolation; -- thethe chart chart doesdoes notnot showtaketake intointo the patternaccountaccount of thethe fluid influenceinfluence temperature ofof insolation;insolation; in the BT from which energy was drawn -- thefor the thechartchart AFP doesdoes system notnot toshowshow run. thethe patternpattern ofof fluidfluid temperaturetemperature inin thethe BTBT fromfrom whichwhich energyenergy waswas drawndrawn forfor thethe AFPAFP systemsystem toto run.run.
FigureFigure 5. 5. DependenceDependence of of the the AFP AFP system system energy energy consumption consumption on on outdoor outdoor temperature. temperature.
FigureFigure 6 66 shows showsshows thethethe results resultsresults of ofof monthly monthlymonthly energy energyenergy consumption consumptionconsumption bybyby thethethe AFP AFPAFP system systemsystem (vertical (vertical(vertical boxesboxesboxes underunder 0 0 kWh) kWh) and and heat heat production production by by the the SCs SCs in in particularparticular years. years. InIn MayMay andand SeptemberSeptember eacheach year,year, thethe monthlymonthlymonthly heat heatheat consumption consumptionconsumption by byby the thethe AFP AFPAFP system systemsystem was waswas just just ajust few aa kWh.fewfew kWh.kWh. However, However,However, heat quantities heatheat quantitiesquantities above above50above kWh 5050 were kWhkWh consumed werewere consumedconsumed in January inin JanuaryJanuary (from 22(from(from to 33% 2222 toto of 33%33% the totalofof thethe annual totaltotal annualannual heat consumption heatheat consumptionconsumption for AFP), forfor AFP),asAFP), well asas as wellwell in December asas inin DecemberDecember (18–23%). (18–23%).(18–23%).
Figure 6. Heat production and energy consumption by the AFP system from 2013 to 2017. Figure 6. HeatHeat production production and energy consumptio consumptionn by the AFP system from 2013 to 2017.
Processes 2020, 8, 1286 8 of 11 Processes 2020, 8, x FOR PEER REVIEW 8 of 11
DueDue to to the the monthly monthly amount amount of of energy energy from from inso insolationlation (available (available to to SCs), SCs), the the monthly monthly heat heat productionproduction distributiondistribution patternpattern was was similar similar to theto monthlythe monthly insolation insolation (Figure (Figure3). The 3). greatest The greatest amount amountof heat forof heat the AFPfor the system AFP system was used was in used January in January 2017 (113 2017 kWh), (113 kWh), due to due the to very the lowvery values low values of Ta of(Figure Ta (Figure4). From 4). MayFrom to May August, to August, the SCs the produced SCs produc overed 350 over kWh 350 of heatkWh inof the heat installation in the installation monthly. monthly.In December In December and January and each January year, moreeach year, energy more was energy spent on was the spent AFP systemon the thanAFP thesystem SCs werethan ablethe SCsto generate. were able to generate.
3.3.3.3. HPETC HPETC Long-Term Long-Term Operation Operation Effects Effects InIn the the years years under under review, review, the the monthly monthly efficiency efficiency of of the the collectors collectors (calculated (calculated as as the the ratio ratio of of monthlymonthly heat heat production production of of SCs SCs and and the the value value of of mo monthlynthly solar solar radiation radiation multiplied multiplied by by the the aperture aperture areaarea of solarsolar collectors)collectors) from from March March until until October October was was usually usually higher higher than than 25% (Figure25% (Figure7). The 7). lowest The lowestmonthly monthly efficiency efficiency was recorded was recorded between between November November and January. and January. Supporting Supporting the operation the operation of the ofheating the heating system system in the buildingin the building in the winterin the winter season highlyseason improvedhighly improved the efficiency the efficiency of the SCs of (Octoberthe SCs (Octoberand December and December 2013; February, 2013; February, March, October March, andOctober November and November 2014). In 2014). September In September 2014, the 2014, SCs thereturned SCs returned part of thepart heat of the produced heat produced to the BT, to fromthe BT, which from the which energy the wasenergy transferred was transferred to the ground to the groundprobes ofprobes a vertical of a heatvertical exchanger heat exchanger of a heat pumpof a heat for thermalpump for regeneration thermal regeneration of the soil surrounding of the soil surroundingthree boreholes. three This boreholes. process This also process caused also an increasecaused an in increase the average in the monthly average emonthlyfficiency efficiency of SCs to ofabove SCs to 40%. above 40%.
FigureFigure 7. 7. TheThe monthly monthly efficiency efficiency of of HPETCs. HPETCs.
TheThe data data shown shown in in Figure Figure 7 could suggest that thethe eefficiencyfficiency of of the the SC SC decreased decreased in in subsequent subsequent years.years. This This disturbing disturbing tendency tendency was was primarily primarily ca causedused by by the the smaller smaller consumption consumption of of DHW DHW by by the the occupantsoccupants of of the building in subsequentsubsequent years—theyears—the numbernumber of of occupants occupants was was 11 11 in in 2012 2012 and and only only 5 in5 in2017. 2017. Over Over the the years, years, this factthis wasfact particularlywas particularly noticeable noticeable in thesummer in the summer months, whenmonths, the when amount the of amountavailable of solar available energy solar was energy the greatest, was the and greatest the SCs,, and due the to SCs, a lower due DHWto a lower consumption, DHW consumption, operated in operatedtemperatures in temperatures above 60 ◦C, above reaching 60 °C, a lowerreaching effi ciencya lower by efficiency definition. by definition. FigureFigure 88 showsshows thethe relationshiprelationship between between the the daily daily e efficiencyfficiency of of SCs SCs and and the the production production of of heat heat in inthe the years years 2013–2017. 2013–2017. The The daily daily heat heat production production values values of SCs of were SCs determined were determined from the from data the recorded data recordedby the control by the system control of system the installation of the installation on a given on day a given (from day the (from measurements the measurements of the solar of fluidthe solar flow fluidrate, theflow di rate,fference the difference in fluid temperature in fluid temperature at the outlet at and the inletoutlet of and the solarinlet of system, the solar and system, the appropriate and the appropriatevalues of density values and of density specific and heat specific of water). heat On of thewater). other On hand, the other daily hand, SC effi dailyciency SC values efficiency were valuescalculated were as calculated the ratio ofas daily the ratio heat productionof daily heat and production the value and of daily the value solar radiationof daily solar measured radiation by a measuredpyranometer by a converted pyranometer to the converted aperture to area the ofap solarerture collectors area of solar (10.53 collectors m2). If the (10.53 daily m2 production). If the daily of productionheat by SCs of exceeded heat by 24SCs kWh, exceeded their e ffi24ciency kWh, wastheir over efficiency 30%, and was above over 2830%, kWh, and it above ranged 28 from kWh, 38 toit ranged57%. The from daily 38 etoffi 57%.ciency The of daily the SCs efficiency above 60% of th wase SCs achieved above 60% within was 39 achieved days, most within of which 39 days, fell onmost the of which fell on the winter days in which the SCs also produced heat for central heating. Figure 8 demonstrates that the daily efficiency of SCs increased with the amount of heat produced. This can
Processes 2020, 8, 1286 9 of 11
Processes 2020, 8, x FOR PEER REVIEW 9 of 11 winter days in which the SCs also produced heat for central heating. Figure8 demonstrates that the bedaily explained efficiency by of the SCs fact increased that only with thefavourable amount ofconditions heat produced. for the This production can be explained of heat by(adequate the fact that only favourable conditions for the production of heat (adequate insolation, T ,T and T ) insolation, Ta, TBT,low and TCT,low) allow for the production of large amounts aof BT,lowheat, whichCT,low also translatesallow for theinto production high efficiency of large SCs. amounts of heat, which also translates into high efficiency SCs.
Figure 8. DependenceDependence of of the the daily effi efficiency on heat production.
Table3 3summarises summarises the the data data on theon annualthe annual results results of operation of operatio of then solarof the system solar insystem the analysed in the analysedyears. The years. greatest The annual greatest SC eannualfficiency SC figures, efficiency amounting figures, to amounting more than 30%,to more were than achieved 30%, inwere the achievedyears 2013–2015, in the years at the 2013–2015, same time at generating the same time the greatest generating amount the greatest of heat andamount solar of pump heat operatingand solar pumptime. Theoperating annual time. amount The of annual energy amount used for of the energy AFP systemused for was the from AFP 7system to 13% was of the from heat 7 generatedto 13% of thein the heat system. generated In 2017, in the the system. lowest In effi 2017,ciency the of lowest the solar efficiency installation of the wassolar achieved installation due was to low achieved DHW dueconsumption to low DHW in the consumption building (especially in the building during the (e summer)specially andduring the the lack summer) of support and for the the lack central of supportheating systemfor the central in winter. heating system in winter.
Table 3. Summary of annual solar installation results.
SpecificationsSpecifications 2013 2013 20142014 20152015 20162016 2017 2017 Annual solar insolation (kWhm2 −2) 838.8 992.0 997.9 971.5 974.1 Annual solar insolation (kWhm− ) 838.8 992.0 997.9 971.5 974.1 AnnualAnnual heat heat production production (kWh) (kWh) 3298.4 3298.4 3369.5 3369.5 3368.83368.8 2944.92944.9 2633.5 2633.5 AnnualAnnual AFP AFP system system energy energy consumption consumption (kWh) (kWh) 281.6 281.6 323.3 323.3 239.7 239.7 333.7 333.7 337.6 337.6 AnnualAnnual effi ciencyefficiency (%) (%) 37.3 37.3 32.332.3 32.132.1 28.828.8 25.7 25.7
4. Conclusions Conclusions This articlearticle presentspresents several several years years of resultsof results of the of solar the thermalsolar thermal system’s system’s operation operation in a moderate, in a moderate,transitory climate.transitory Because climate. water Because was water used was as a us solared as thermal a solar fluid,thermal the fluid, AFP the system AFP was system applied. was applied.The operation The operation of the HPETSCs of the HPETSCs was analysed was analysed from 2013 from until 2013 2017 until in a 2017 residential in a residential and retail and building retail buildinglocated near located Krak nearów, Kraków, Poland. ThePoland. collected The collected data was data used was to discussused to topicsdiscuss relating topics torelating thermal to thermalperformance, performance, energy production energy production and consumption and consumption of energy of by energy a solar by and a solar AFP and system. AFP system. The functioning of the AFP system in a moderate,moderate, transitory climate is associated with the consumption of heat accumulated in the heat storage during months with low outsid outsidee temperatures and low insolation. In In the the analysed analysed installation, installation, the AFP system worked from September to May and consumed annually from 7 to 13% of the heat generated by the SCs in the installation. The greatest amounts of heat, above 50 kWh/day, were consumed in January and December. As demonstrated in
Processes 2020, 8, 1286 10 of 11 consumed annually from 7 to 13% of the heat generated by the SCs in the installation. The greatest amounts of heat, above 50 kWh/day, were consumed in January and December. As demonstrated in this article, in these months, more energy was spent on running the AFP system than the SCs were able to generate. The AFP system employed was characterised by the minimal investment cost of the project compared to other systems using, among other methods, heating of the piping with electric heating tapes and gravity drainage of the piping into a special tank (so-called drain-back system). Moreover, such a protection solution requires neither additional hydraulic modifications in the system nor a complex control system, as most currently used solar system controllers have a built-in AFP function. It is also worth noting that water without additives that reduce the freezing point is environmentally-friendly, unlike other substances used in solar systems. Traditional methods of producing propylene glycol are based on the processing of crude oil. In these technologies, propylene oxide obtained from the crude oil component (propylene) is subjected to high-temperature and high-pressure hydrolysis. This method of producing glycol has a negative impact on the natural environment. In the years under review, the monthly efficiency of the SCs from March to October was usually higher than 25%, and the lowest efficiency was recorded between November and January. The change in the number of building residents, which translated into reduced DHW consumption, had a significant impact on the all-year efficiency of HPETSCs. The greatest annual SC efficiency figures, amounting to more than 30%, were achieved in the years 2013–2015, at the same time generating the greatest amount of heat and solar pump operating time. During the study, no system failures were recorded which could have caused the water to freeze in the system. Furthermore, there was no overheating of water or stagnation in the SCs. While designing a solar system running with water, it should be noted that the length of piping between the SCs and the heat receiving device should be as short as possible. Similarly, the length of the piping between the collectors and that in the immediate vicinity of the outside air should be kept as short as possible. If these conditions are not met, the temperature of the fluid should be additionally monitored at the appropriate points of the system piping. The experimental results of the AFP system presented in this paper can be used as a basis for optimisation works in terms of the appropriate strategy of switching on and duration of operation of circulation pumps, temperature settings of the AFP system switching on and off, or modifying insulation parameters of solar pipes. Such results have not been comprehensively presented so far in relation to a moderate, transitory climate. Moreover, they can be useful for comparison among different working strategies of AFP solar systems with water as a solar fluid.
Funding: This research received no external funding. Conflicts of Interest: The author declares no conflict of interest.
References
1. Sarsam, W.S.; Kazi, S.N.; Badarudin, A. A review of studies on using nanofluids in flat-plate solar collectors. Sol. Energy 2015, 122, 1245–1265. [CrossRef] 2. Gorjian, S.; Ebadi, H.; Calise, F.; Shukla, A.; Ingrao, C. A review on recent advancements in performance enhancement techniques for low-temperature solar collectors. Energy Convers. Manag. 2020, 222, 113246. [CrossRef] 3. Shamsul Azha, N.I.; Hussin, H.; Nasif, M.S.; Hussain, T. Thermal Performance Enhancement in Flat Plate Solar Collector Solar Water Heater: A Review. Processes 2020, 8, 756. [CrossRef] 4. Suman, S.; Khan, M.K.; Pathak, M. Performance enhancement of solar collectors—A review. Renew. Sustain. Energy Rev. 2015, 49, 192–210. [CrossRef] 5. Daghigh, R.; Shafieian, A. Theoretical and experimental analysis of thermal performance of a solar water heating system with evacuated tube heat pipe collector. Appl. Therm. Eng. 2016, 103, 1219–1227. [CrossRef] 6. Genc, A.M.; Ezan, M.A.; Turgut, A. Thermal performance of a nanofluid-based flat plate solar collector: A transient numerical study. Appl. Therm. Eng. 2018, 130, 395–407. [CrossRef] Processes 2020, 8, 1286 11 of 11
7. Sabiha, M.A.; Saidur, R.; Mekhilef, S.; Mahian, O. Progress and latest developments of evacuated tube solar collectors. Renew. Sustain. Energy Rev. 2015, 51, 1038–1054. [CrossRef] 8. Sakulchangsatjatai, P.; Wannagosit, C.; Kammuang-Lue, N.; Terdtoon, P. Theoretical and experimental investigation of the evacuated tube solar water heater system. Therm. Sci. 2019, 24, 795–808. [CrossRef] 9. Gao, Y.; Zhang, Q.; Fan, R.; Lin, X.; Yu, Y. Effects of thermal mass and flow rate on forced-circulation solar hot-water system: Comparison of water-in-glass and U-pipe evacuated-tube solar collectors. Sol. Energy 2013, 98, 290–301. [CrossRef] 10. Elsheniti, M.B.; Kotb, A.; Elsamni, O. Thermal performance of a heat-pipe evacuated-tube solar collector at high inlet temperatures. Appl. Therm. Eng. 2019, 154, 315–325. [CrossRef] 11. Duffie, J.A.; Beckman, W.A. Solar Engineering of Thermal Processes, 4th, ed.; John Wiley & Sons, Inc.: Hoboken, NJ, USA, 2013. 12. Pater, S. Energy efficiency of solar collectors using water, as solar liquid. J. Civ. Eng. Environ. Archit. 2014, 61, 401–410. [CrossRef] 13. ASHRAE. 2001 ASHRAE Handbook Fundamentals; ASHRAE: Atlanta, GA, USA, 2001; Volume 53, ISBN 9788578110796. 14. Rimar, M.; Fedak, M.; Vahovsky, J.; Kulikov, A.; Oravec, P.; Kulikova, O.; Smajda, M.; Kana, M. Performance evaluation of elimination of stagnation of solar thermal systems. Processes 2020, 8, 621. [CrossRef] 15. Liu, H.; Zhang, S.; Jiang, Y.; Yao, Y. Feasibility study on a novel freeze protection strategy for solar heating systems in severely cold areas. Sol. Energy 2015, 112, 144–153. [CrossRef] 16. Botpaev, R.; Louvet, Y.; Perers, B.; Furbo, S.; Vajen, K. Drainback solar thermal systems: A review. Sol. Energy 2016, 128, 41–60. [CrossRef] 17. Vera-Medina, J.; Lillo-Bravo, I.; Hernández, J.; Larrañeta, M. Experimental and numerical study on a freeze protection system for flat-plate solar collectors with silicone peroxide tubes. Appl. Therm. Eng. 2018, 135, 446–453. [CrossRef] 18. Zhou, F.; Ji, J.; Yuan, W.; Zhao, X.; Huang, S. Study on the PCM flat-plate solar collector system with antifreeze characteristics. Int. J. Heat Mass Transf. 2019, 129, 357–366. [CrossRef] 19. Zhou, F.; Jie Ji, W.Y.; Modjinou, M.; Zhao, X.; Huang, S. Experimental study and performance prediction of the PCM-antifreeze solar thermal system under cold weather conditions. Appl. Therm. Eng. 2019, 146, 526–539. [CrossRef] 20. Pater, S. Field measurements and energy performance analysis of renewable energy source devices in a heating and cooling system in a residential building in southern Poland. Energy Build. 2019, 199, 115–125. [CrossRef] 21. Matuszko, D.; Celi´nski-Mysław, D. Solar conditions in Cracow and their use fulness for helioenergetics. Acta Scientiarum Polonorum. Formatio Circumiectus 2016, 15, 103–111. [CrossRef] 22. PVGIS European Communities 2001–2020. Available online: https://re.jrc.ec.europa.eu/pvg_tools/en/#MR (accessed on 8 October 2020).
Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.
© 2020 by the author. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).