A Comparative Energy and Economic Analysis Between a Low Enthalpy Geothermal Design and Gas, Diesel and Biomass Technologies for a HVAC System Installed in an Office Building

Article A Comparative Energy and Economic Analysis between a Low Enthalpy Geothermal Design and Gas, Diesel and Biomass Technologies for a HVAC System Installed in an Ofﬁce Building

José Ignacio Villarino 1, Alberto Villarino 1,* , I. de Arteaga 2 , Roberto Quinteros 2 and Alejandro Alañón 1

1 Department of Construction and Agronomy, Construction Engineering Area, High Polytechnic School of Ávila, University of Salamanca, Hornos Caleros, 50, 05003 Ávila, Spain; [email protected] (J.I.V.); [email protected] (A.A.) 2 Facultad de Ingeniería, Escuela de Ingeniería Mecánica, Pontiﬁcia Universidad Católica de Valparaíso, Av. Los Carrera 01567, Quilpué 2430000, Chile; [email protected] (I.d.A.); [email protected] (R.Q.) * Correspondence: [email protected]; Tel.: +34-920-353-500; Fax: +34-920-353-501

Received: 3 January 2019; Accepted: 25 February 2019; Published: 6 March 2019

Abstract: This paper presents an analysis of economic and energy between a ground-coupled heat pump system and other available technologies, such as natural gas, biomass, and diesel, providing heating, ventilation, and air conditioning to an office building. All the proposed systems are capable of reaching temperatures of 22 ◦C/25 ◦C in heating and cooling modes. EnergyPlus software was used to develop a simulation model and carry out the validation process. The first objective of the paper is the validation of the numerical model developed in EnergyPlus with the experimental results collected from the monitored building to evaluate the system in other operating conditions and to compare it with other available technologies. The second aim of the study is the assessment of the position of the low enthalpy geothermal system proposed versus the rest of the systems, from energy, economic, and environmental aspects. In addition, the annual heating and cooling seasonal energy efficiency ratio (COPsys) of the ground-coupled heat pump (GCHP) shown is higher than the others. The economic results determine a period between 6 and 9 years for the proposed GCHP system to have lower economic cost than the rest of the systems. The results obtained determine that the GCHP proposed system can satisfy the thermal demand in heating and cooling conditions, with optimal environmental values and economic viability.

Keywords: low enthalpy geothermal system; renewable energy; simulation models; energy analysis; economic feasibility

1. Introduction Geothermal energy is recognized as a source of renewable energy that is environmentally friendly and technically feasible. For this reason, geothermal energy technologies can benefit from any climate mitigation policies. Directive 2009/28/EC [1] on the promotion of the use of energy from renewable sources has been the most significant piece of EU legislation for geothermal energy. The main objectives are focused in the reduction of at least 20% in greenhouse gas (GHG) emissions compared to 1990 levels, with 20% of the final energy consumption to come from renewable sources and the improvement of energy efficiency by 20% compared to 2007 projections. These goals are the main drivers for the growth of geothermal technologies.

Energies 2019, 12, 870; doi:10.3390/en12050870 www.mdpi.com/journal/energies Energies 2019, 12, 870 2 of 16

For this reason, development of low energy consumption in HVAC systems is important. Taking the year 2010 in Spain as reference, the main distribution of energy consumption in buildings is concentrated in HVAC, domestic hot water (DHW), equipment, and lighting. The HVAC and DHW necessities cover 62% of the total energy consumption in buildings [2]. The experimental installation built is composed of a ground-coupled heat pump (GCHP) and radiant floor (RF) system supported by a mechanical ventilation system (MV) [3,4]. The energy is extracted from a ground heat exchanger (GHE) drilled to a depth of 100 m [5–7]. There are many studies that analyze the behavior of each of the elements that make up the geothermal system, focusing on the operation of the GHE from theoretical and simulation models [8,9] or in an experimental installation monitoring different variables [10–13]. Related with control systems of the installation [14], demonstrate that the use of automatic systems to control the circulating pump speed versus classical adjustment case increased the coefficient of performance of the seasonal energy efficiency ratio (COPsys) (COPsys 7–8% higher and 7.5–8% lower CO2 emission level). Regarding the use of simulation tools [15], analyzed the results obtained and compared this with the experimental data, based on the development of two numerical simulation models using transient systems simulation (TRNSYS). The main parameters analyzed were the useful thermal energy and COPsys. The results present a small difference in the COPsys (4.5%) between radiator system and radiant floor. Using the same operating conditions, the radiator system has 10% higher energy consumption and CO2 emissions than the radiant floor system. Using ground heat exchanger software (GLHEPRO) and TRANSYS software [16], analyzed the performance of an office building with heating and cooling necessities. The heat pump model development presented a deviation of 2% with respect to experimental results. The energy consumption of the heat pump unit was well predicted in the TRNSYS simulation model. In relation to economic analysis [17], performed an economic analysis of a residential ground heat system in sedimentary rock formation, demonstrating the feasibility of the system from performance analysis and giving solutions for improvement with regard to a reduction in utility bills. The analysis done between GCHP and natural gas concludes that the data also indicate a marginal reduction in utility bill, and they center the improvements in operations strategies and optimal design, two aspects that are considered in this paper from the beginning. The present study is characterized by analyzing a building located in Madrid, Spain [18], and consists of a GCHP, RF, and mechanical ventilation system to ensure the indoor air quality, not only focusing on energy performance, but also introducing the cost function and comparing the energy and economic results with other available technologies. The results obtained are used to analyze the possible investment for this type of installation from energy, economic, and environmental aspects, three principal factors to consider in any project. This new methodology analyzes the building globally. In addition, the difference to other studies is based on the action of four key points at the same time: decreasing the building energy demand, improving the energy efficiency of the building in the experimental design, developing a model simulation process to ensure the correct functioning of the installation, and validating the model designed in order to have a tool to simulate any building under different conditions. Using this model, a simulation process was done for the available technologies under the same building, obtaining energy results used to compare and analyze the viability of the GCHP system proposed. A new factor in this study is the comparative economic analysis between the technologies’ comments, obtaining economic results which determine the feasibility of the proposed design in a global way. Energies 2019, 12, 870 3 of 16

2. Experimental Building Description

2.1. Building Description The office is located in Madrid, Spain, latitude +40.7◦ and longitude −3.99◦ and altitude of 2 1075Energies meters 2019, 12 above, x sea level in a continental temperature climate (Figure1a). The building has 1893 mof 16 floor area and includes a reception, office room, meeting room, archive room, rest room, bathroom, andand experimentalexperimental showroomshowroom (Figure(Figure1 1b).b). TheThe buildingbuilding isis usedused asas anan office office and and is is occupied occupied by by three three people.people. TheThe officeoffice building building is is located located in in Madrid, Madrid, Spain, Spain, in in a continentala continental temperature temperature climate climate (Figure (Figure1). The1). The indoor indoor temperature temperature design design is 22/25 is 22/25◦C for °C heating for heating and cooling and cooling modes, respectively.modes, respectively. The building The isbuilding equipped is equipped with a vertical with a U-tube vertical GHE, U-tube GCHP, GHE, RF, GCHP, and MV RF, systems. and MV systems.

(a)

(b)

FigureFigure 1.1. ((aa)) ExperimentalExperimental officeoffice building;building; ((bb)) OfficeOffice floorfloor plan.plan.

AsAs shownshown inin FigureFigure2 2,, the the main main parts parts of of the the system system are are BHE, BHE, heat heat pump pump unit, unit, circulating circulating water water pumps,pumps, radiantradiant ﬂoor,floor, data data acquisition acquisition instruments, instruments, and and auxiliary auxiliary parts. parts.

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Figure 2. Ground-coupled heat pump (GCHP) system schematic. FigureFigure 2. 2. Ground-coupledGround-coupled heat heat pump pump (GCHP) (GCHP) system system schematic. schematic. The monthly energy demand for the building in heating and cooling modes is illustrated in TheThe monthly monthly energy energy demand demand for for the the building building in in heating heating and and cooling cooling modes modes is is illustrated illustrated in in Figure 3. FigureFigure 33..

DEC DEC NOV NOV OCT OCT SEP SEP AUG AUG Heating JUL Heating JUL Cooling JUN Cooling JUN MAY Month MAY

Month APR APR MAR MAR FEB FEB JAN JAN -1,000 -500 0 500 1,000 1,500 2,000 2,500 -1,000 -500 0 500 1,000 1,500 2,000 2,500 Energy [kWh] Energy [kWh]

Figure 3. Monthly energy demand. Figure 3. Monthly energy demand. 2.2. Borehole Heat Exchanger 2.2. Borehole Heat Exchanger 2.2. BoreholeThe GHE Heat is Exchanger a vertical borehole with double U-shaped HDPE 100 pipes. The borehole was The GHE is a vertical borehole with double U-shaped HDPE 100 pipes. The borehole was completelyThe GHE backﬁlled is a vertical with bentonite,borehole with cement, double and U-shaped sand. The HDPE average 100 thermal pipes. conductivityThe borehole of was the completely backfilled with bentonite, cement, and sand. The average thermal conductivity of the completelyground was backfilled 1.80 W/mK. with The bentonite, main thermal cement, parameters and sand. are The listed average in Table thermal1. conductivity of the ground was 1.80 W/mK. The main thermal parameters are listed in Table 1. ground was 1.80 W/mK. The main thermal parameters are listed in Table 1. Table 1. Soil features. Table 1. Soil features. Table 1. Soil features. Soil Contents GRAVEL + CLAY + GRANITE Soil Contents GRAVEL + CLAY + GRANITE Soil ContentsThermal GRAVEL conductivity + CLAY (W/mK) + GRANITE 1.8 Thermal conductivity (W/mK) 1.8 ThermalThermal conductivity diffusivity (m (W/mK)2/d) 0.065 1.8 Thermal diffusivity (m2/d) 0.065 AverageThermal Thermal diffusivity resistivity (m (K/(W/m)2/d) 0.065 0.059 Average Thermal resistivity (K/(W/m) 0.059 Average Thermal resistivity (K/(W/m) 0.059 2.3. Ground-Coupled Heat Pump Unit 2.3. Ground-Coupled Heat Pump Unit 2.3. Ground-Coupled Heat Pump Unit Due to the importance of electricity consumption in the COPSYS of the installation, it is necessary Due to the importance of electricity consumption in the COPSYS of the installation, it is necessary to selectDue to the the adequate importance heat of pump electricity capacity consumption and ensure in the that COP it canSYS of supply the installation, the thermal it is loads necessary of the to select the adequate heat pump capacity and ensure that it can supply the thermal loads of the tobuilding select the under adequate analysis. heat The pump installed capacity GCHP and is anensure SD VWS that 61/2it can model supply manufactured the thermal byloads SAUNIER of the building under analysis. The installed GCHP is an SD VWS 61/2 model manufactured by SAUNIER building under analysis. The installed GCHP is an SD VWS 61/2 model manufactured by SAUNIER DUVAL. The VWS series uses a scroll compressor unit with R-407c as a refrigerant. The COP for DUVAL. The VWS series uses a scroll compressor unit with R-407c as a refrigerant. The COP for

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heating is 4.2 and an EER of 3.92 for cooling. The VWS 61/2 model has a nominal heating capacity of DUVAL. The VWS series uses a scroll compressor unit with R-407c as a refrigerant. The COP for 6 kW and a nominal cooling capacity of 4.9 kW (a rated power of 1.25 kW). An ethylene glycol heating is 4.2 and an EER of 3.92 for cooling. The VWS 61/2 model has a nominal heating capacity aqueous solution is circulated between the heat pump and the BHE with a WILO TOP-S 30/4EM of 6 kW and a nominal cooling capacity of 4.9 kW (a rated power of 1.25 kW). An ethylene glycol water circulating pump with a maximum flow capacity of 4 m3/h. The same model of water pump is aqueous solution is circulated between the heat pump and the BHE with a WILO TOP-S 30/4EM water used for the radiant floor system. The total annual energy consumption of the water recirculating circulating pump with a maximum ﬂow capacity of 4 m3/h. The same model of water pump is used pumps is 608 kWh, which represented the 25.65% of the total energy consumption of the GCHP for the radiant ﬂoor system. The total annual energy consumption of the water recirculating pumps is system. 608 kWh, which represented the 25.65% of the total energy consumption of the GCHP system. Figure 4 shows the power consumption and heating and cooling capacities, and they were used Figure4 shows the power consumption and heating and cooling capacities, and they were used to model the heat pump in EnergyPlus. The nominal rates have been calculated using hot water at to model the heat pump in EnergyPlus. The nominal rates have been calculated using hot water at 35 °C and cold water at 7 °C for cooling capacity, and cold water at 5 °C and hot water at 45 °C for 35 ◦C and cold water at 7 ◦C for cooling capacity, and cold water at 5 ◦C and hot water at 45 ◦C for heating capacity. heating capacity.

Figure 4. Heat pump performance data. Figure 4. Heat pump performance data. 2.4. Mechanical Ventilation 2.4. Mechanical Ventilation The installed VMC is a SIBER DF MAX double flow equipment, with a maximum flow rate of 220 mThe3/h. installed The outside VMC airis a is SIBER filtered DF and MAX heated double before flow being equipment, blown with into thea maximum rooms in flow winter rate and of 3 cooled220 m /h. in summerThe outside with air a 92% is filtered heat recovery. and heated before being blown into the rooms in winter and cooled in summer with a 92% heat recovery. 2.5. Radiant Floor System 2.5. Radiant Floor System The RF system is distributed in four areas and with seven hydraulic circuits in order to ensure hydronicThe RF balance. system The is distributed heat in each in four area areas is provided and wi byth seven one variable hydraulic circulating circuits in pump orderand to ensure other variablehydronic circulation balance. The pumps heat for in theeach other area three is provided zones. Toby achieve one variable higher circulating performances pump of theand HVAC other system,variable thecirculation thermostat pumps controls for the the other speed three of the zone variables. To achieve circulation higher pump performances depending of on the the HVAC heat carriersystem, demand the thermostat in the zone.controls Figure the 5speed shows of thethe distributionvariable circulation of RF circuits pump overdepending the ground on the floor. heat Thiscarrier system demand guarantees in the zone. a homogeneous Figure 5 shows indoor the temperaturedistribution of distribution RF circuits and over increases the ground the COPfloor.SYS Thisof thesystem GCHP guarantees [19–21]. Thea homogeneous main characteristics indoor temp are listederature in Tabledistribution2. and increases the COPSYS of the GCHP [19–21]. The main characteristics are listed in Table 2.

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Table 2. Characteristics of the radiant floor.

Active Radiant Floor Layer Pipe wall conductivity (W/mK) 0.37 Pipe wall thickness (m) 0.002 Pipe outside diameter (m) 0.016 Pipe spacing (m) 0.15 Energies 2019, 12, 870 Specific heat coefficient fluid (kJ/kgK) 4.18 6 of 16

Figure 5. Radiant floor system. Figure 5. Radiant floor system. Table 2. Characteristics of the radiant floor. 2.6. Data Acquisition and Experimental Procedure Active Radiant Floor Layer A temperature and humidity sensors system were installed in the building, connected to a Pipe wall conductivity (W/mK) 0.37 thermostat model EXACONTROL E7RPipe C/SH, wall thicknessSaveris V2.0 (m) 0.002converter, with a range of 0 to 80 °C and humidity range of 0% to 100%, withPipe accuracy outside diameter±0.15 °C (m) and 0.016 3%, respectively. These temperature sensors are Pt100. A Siemens Coriolis massPipe spacingflow meter, (m) 0.15 model SITRANS FC MASS 6000 with an accuracy <0.1% was installed. TheSpecific power heat meters coefficient are from fluid (kJ/kgK)Fluke, model 4.18 1736/EUS, with accuracy ±1.4 of the nominal value and used to measure the power consumption of the HP, MV, internal/external pumps,2.6. Data PCs, Acquisition and lighting. and Experimental The temperature Procedure sensors for the RF system are Orkli V05, with a range of 0–100 °C with an accuracy of ±0.3 °C. A temperature and humidity sensors system were installed in the building, connected to a thermostat model EXACONTROL E7R C/SH, Saveris V2.0 converter, with a range of 0 to 80 ◦C and 3. Experiment Results humidity range of 0% to 100%, with accuracy ±0.15 ◦C and 3%, respectively. These temperature sensors are Pt100.The study A Siemens was carried Coriolis out massconsidering flow meter, the two model most SITRANS unfavorable FC weeks MASS in 6000 heating with and an accuracy cooling modes,<0.1% was obtained installed. from The the power weather meters data arefile. from The Fluke,period model corresponds 1736/EUS, to the with months accuracy of January±1.4 of and the Julynominal of 2011. value The and daily used analysis to measure period the power corresponds consumption to the oftime the HP,band MV, from internal/external 09:00 hs to 18:00 pumps, hs. AnnualPCs, and indoor lighting. and The outdoor temperature temperatures sensors were for themeas RFured system for areeach Orkli zone V05, and withits evolution a range ofduring 0–100 the◦C yearwith anis presented accuracy of in± 0.3Figure◦C. 6. It is observed that in heating and cooling modes, the average temperature for each zone reaches the defined setpoints of 22 °C in heating and 25 °C in cooling operation.3. Experiment Results The study was carried out considering the two most unfavorable weeks in heating and cooling modes, obtained from the weather data file. The period corresponds to the months of January and July of 2011. The daily analysis period corresponds to the time band from 09:00 hs to 18:00 hs. Annual indoor and outdoor temperatures were measured for each zone and its evolution during the year is presented in Figure6. It is observed that in heating and cooling modes, the average temperature for each zone reaches the defined setpoints of 22 ◦C in heating and 25 ◦C in cooling operation. Figures7 and8 show the comparison between indoor/outdoor ambient air temperature and surface average floor temperature reached in heating and cooling modes. The results show that the GCHP system can maintain the indoor temperature during all the processes. Table3 shows how the comfort temperature and RH parameters are reached in heating and cooling modes in a period of one year. In order to optimize the energy consumption, the DHW values are maintained at a low step. The COPsys in cooling operation is higher than in heating. This is due to the MV in night periods exerting a cooling effect over the system, minimizing the thermal demand. Energies 2019, 12, x 7 of 16

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Figure 6. Recorded annual indoor and outdoor temperature.

Figures 7 and 8 show the comparison between indoor/outdoor ambient air temperature and surface average floor temperatureFigureFigure 6.6. Recorded reached annualannual in heating indoorindoor andand outdoor outdoorcooling temperature.temperature. modes. The results show that the GCHP system can maintain the indoor temperature during all the processes. Figures 7 and 8 show the comparison between indoor/outdoor ambient air temperature and surface average floor temperature reached in heating and cooling modes. The results show that the 30 GCHP system can maintain the indoor temperature during all the processes. 25

2030

1525

1020

155 Temperature [ºC] Temperature Temperature [ºC] Temperature 100 5 Temperature [ºC] Temperature 0 tint [º C ] text [º C ] tsr [º C ]

Figure 7. Indoor temperature in heating. tint [º C ] text [º C ] tsr [º C ]

40 Figure 7. Indoor temperature in heating.

35 40 30 35 25 30 20 25 Temperature [ºC] Temperature Temperature [ºC] Temperature 15 20 9:00 11:0013:0015:0017:0019:0021:0023:00 1:00 3:00 5:00 7:00

Temperature [ºC] Temperature tint [º C ] text [º C ] tsr [º C ] 15 9:00 11:0013:0015:0017:0019:0021:0023:00 1:00 3:00 5:00 7:00 Figure 8. Indoor temperature in cooling. tintFigure [º C 8.] Indoor temperaturetext [º C in ] cooling. tsr [º C ]

Figure 8. Indoor temperature in cooling.

Energies 2019, 12, x 8 of 16 Energies 2019, 12, x 8 of 16 Table 3 shows how the comfort temperature and RH parameters are reached in heating and coolingTable modes 3 shows in a howperiod the of comfort one year. temperature In order toand optimize RH parameters the energy are consumption, reached in heating the DHW and valuescooling are mode maintaineds in a period at a low of step. one year.The COP In ordersys in cooling to optimize operation the energyis higher consumption, than in heating. the This DHW is duevalues to arethe mainMV tainin nighted at periodsa low step. exerting The COP a coolinsys in gcooling effect operationover the system, is higher minimizing than in hea theting. thermal This is Energiesdemand.due to2019 the , 12MV, 870 in night periods exerting a cooling effect over the system, minimizing the thermal8 of 16 demand. Table 3. Average values of experimental results. HVAC. Table 3. Average valuesvalues ofof experimentalexperimental results.results. HVAC.HVAC. തതതതതത (°C) COPsys COP/ERRܟܐ܂ (തതതതത (°Cܚܛ܂ (%) തതതത܀C) ۶°) ܜതതതതതܖ଍܂C) ത°) ܜതതതതതതܠ܍܂HVAC System ത ◦ ◦ ◦ ◦ HVACHeatingHVAC System mode System 퐓퐞퐱퐭̅̅̅Text̅̅ 6.69̅̅ (° ( CC)) 퐓퐢퐧퐭̅Tint̅̅̅ 21.7̅̅ ((°C)C) HR퐇퐑̅̅̅ 36.8̅ (%)(%) 퐓퐬퐫̅Tsr̅̅ 26.89̅̅ ((°C)C) Thw퐓퐡퐰̅̅̅̅ 30.24̅̅ ( (°C)C) COPCOP 3.86syssys COP/ERRCOP/ERR 4.85 CoolingHeatingHeating mode mode 25.576. 6.6969 21.724.821.7 36.8 40.136.8 26.8926. 21.389 30.24 28.6230.24 3.86 5.293.86 4.85 6.654.85 CoolingCooling mode mode 25. 25.5757 24.824.8 40.140.1 21.321.3 28.6228.62 5.295.29 6.656.65 Figure 9 shows the non-existence of condensation. Figure9 9 shows shows the the non-existence non-existence of of condensation. condensation.

Figure 9. Radiant ﬂoorfloor (RF) system temperatures. Figure 9. Radiant floor (RF) system temperatures. 4. Numerical Simulation 4. NumericalEnergyPlus Simulation was the software used for the designdesign ofof thethe simulationsimulation model.model. Figure 10 shows the modelEnergyPlus of the building was the createdcreated software inin SketchUp,SketchUp, used for necessarynecessarythe design toto of characterizecharacterize the simulation thethe building.model.building. Figure 10 shows the model of the building created in SketchUp, necessary to characterize the building.

Figure 10. SketchUp plan. Simulation Models in EnergyPlus Figure 10. SketchUp plan.

EnergyPlus was the software selected to simulate the thermal operation of the ofﬁce building. Using software speciﬁcations and building characteristics data, the calculation model is implemented. Three principal models are used in the simulation process. The heat exchanger was done using GLHEPRO program [22]. Based on physical parameters generated from the catalog data of the pump Energies 2019, 12, x 9 of 16

Simulation Models in EnergyPlus EnergyPlus was the software selected to simulate the thermal operation of the office building. Energies 2019, 12, x 9 of 16 Using software specifications and building characteristics data, the calculation model is implemented.Simulation Models Three in principal EnergyPlus models are used in the simulation process. The heat exchanger was done using GLHEPRO program [22]. Based on physical parameters generated from the catalog data EnergyPlus was the software selected to simulate the thermal operation of the office building. of the pump unit, a second model was used to describe the heat pump unit [23,24]. For modeling the Using software specifications and building characteristics data, the calculation model is transientimplemented. heat conduction Three principal through models the radiantare used floor in the system, simulation the conductionprocess. The transferheat exchanger method was (CTFs) was doneused. using GLHEPRO program [22]. Based on physical parameters generated from the catalog data ofEnergiesFigure the pump2019 11, 12 unit,,shows 870 a second the modelnode wasscheme used towhich describe represents the heat pump the unitconfiguration [23,24]. For modeling of the9 ofthesystem 16 implementedtransient heat in conductionEnergyPlus through to integrate the radiant all floorthe process system, theand conduction elements transferto model method the (CTFs)simulation process.wasunit, used. a second model was used to describe the heat pump unit [23,24]. For modeling the transient heat conductionFigure through11 shows the radiantthe node ﬂoor scheme system, which the conduction represents transfer the methodconfiguration (CTFs) of was the used. system implementedFigure 11 inshows EnergyPlus the node to scheme integrate which all represents the process the and conﬁguration elements ofto the model system the implemented simulation process.in EnergyPlus to integrate all the process and elements to model the simulation process.

Figure 11. System nodes scheme. Figure 11. System nodes scheme. Figure 11. System nodes scheme. FiguresFigures 12 and12 and 13 13show show the the comparison comparison between thethe simulated simulated and and experimental experimental values values using using Figures 12 and 13 show the comparison between the simulated and experimental values using the statisticalthe statistical method method coefficient coefﬁcient of of multiple multiple determinations (R (R2).2). the statistical method coefficient of multiple determinations (R2).

Energies 2019, 12, x 10 of 16 22 FigureFigureFigure 12. 12. 12. RRR2 graphgraphgraph resultsresults results in inin heatingheating heating mode. mode. mode.

Figure 13. R2 graph results in cooling mode. Figure 13. R2 graph results in cooling mode.

5. Results and Discussion In this section, the main results from real and simulated process are shown and analyzed. First, the most significant results of the GCHP systems are discussed, comparing the experimental and simulated results for the monthly average temperature of each of the four zones. The energy consumption of the system is illustrated comparing the experimental and simulated values obtained in the process. Finally, the heat flow transfer to the geothermal field and radiant floor system is analyzed and discussed. In Figure 14, the comparison between the real and simulated average monthly zone temperature values are represented. It is observed that the HVAC system supplies the energy necessary to maintain the comfort temperature conditions of 22 °C in heating and 25 °C in cooling mode in all zones.

Figure 14. Experimental and simulated average monthly temperature for each zone.

The statistical method of coefficient of multiple determinations (R2) was used to compare simulated and real values. The R2 values for the different zones and the entire building are shown in Table 4. Zone 1 and 4 have lower R2 values than other areas because these areas have peak demand to be areas that are not continually used (test room and bathroom), which translates into greater variability in the peak value of internal loads, therefore, the time response is longer such that the difference between the real and simulated values is higher. The other two zones have a higher linearity in their internal loads to be continuous use areas. As is shown in Figure 14, the zones 2 and 4 have R2 values similar to the entire building.

Table 4. R2 values for four zones and entire building.

Tª R2 Tª average, zone 1 0.9841 Tª average, zone 2 0.9913 Tª average, zone 3 0.9914

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Energies 2019, 12, 870 10 of 16 Figure 13. R2 graph results in cooling mode.

5. Results and Discussion In this this section, section, the the main main results results from from real real and and simulated simulated process process are shown are shown and analyzed. and analyzed. First, theFirst, most the mostsignificant significant results results of the of GCHP the GCHP system systemss are discussed, are discussed, comparing comparing the experimental the experimental and simulatedand simulated results results for forthe themonthly monthly average average temperat temperatureure of ofeach each of of the the four four zones. zones. The The energy consumption ofof thethe systemsystem isis illustrated illustrated comparing comparing the the experimental experimental and and simulated simulated values values obtained obtained in inthe the process. process. Finally, Finally, the heatthe heat flow transferflow transfer to the to geothermal the geothermal field and field radiant and floorradiant system floor is system analyzed is analyzedand discussed. and discussed. In Figure 1414,, thethe comparisoncomparison betweenbetween the realreal andand simulatedsimulated averageaverage monthlymonthly zonezone temperaturetemperature values are represented. It is observed that the HVAC system supplies the energy necessary to maintain ◦ ◦ the comfort comfort temperature temperature conditions conditions of of 22 22 °C Cin inhe heatingating and and 25 25°C inC cooling in cooling mode mode in all in zones. all zones.

Figure 14. Experimental and simulated average monthly temperature for each zone.

The statistical method of coefﬁcientcoefficient of multiplemultiple determinationsdeterminations (R2)) was used to compare simulated and real values. The R2 valuesvalues for for the the different different zones and the entire building are shown in Table4 4.. ZoneZone 11 andand 44 have have lower lower R R22 values than other areas because these areas have peak demand to be areas that areare notnot continuallycontinually usedused (test(test roomroom and bathroom),bathroom), which translates into greater variability in the peak value of internal loads, therefore,therefore, the time response is longer such that the difference betweenbetween the the real real and and simulated simulated values values is higher. is higher. The otherThe twoother zones two havezones a higherhave a linearity higher linearityin their internal in their loads internal to be loads continuous to be continuous use areas. use As areas. is shown As inis shown Figure 14in ,Figure the zones 14, the 2 and zones 4 have 2 and R2 values4 have similarR2 values to similar the entire to the building. entire building.

Table 4.4. R22 values for four zones and entire building.

Tª Tª R22 Tª average,Tª zone average, 1 zone 1 0.9841 0.9841 Tª average,Tª zone average, 2 zone 2 0.9913 0.9913 Tª average,Tª zone average, 3 zone 3 0.9914 0.9914 Tª average, zone 4 0.9871 Tª average, building 0.991

The consumption of the heat pump unit, circulating pumps, and auxiliary elements are considered to have the real value of the energy consumption of the building. In Figure 15, the experimental and simulated values of the total energy consumption are presented. The results are in accordance with monthly energy demand presented in Figure3. The highest energy consumption corresponds to the heating period and is higher than in cooling period. Using the same statistical method, an R2 value of 0.9841 is obtained for the comparison of the energy consumption of the GCHP system in real and simulated conditions. Energies 2019, 12, x 11 of 16

Tª average, zone 4 0.9871 Tª average, building 0.991

The consumption of the heat pump unit, circulating pumps, and auxiliary elements are considered to have the real value of the energy consumption of the building. In Figure 15, the experimental and simulated values of the total energy consumption are presented. The results are in accordance with monthly energy demand presented in Figure 3. The highest energy consumption corresponds to the heating period and is higher than in cooling period. Using the same statistical 2 method, an R2 value of 0.9841 is obtained for the comparison of the energy consumption of the Energies 2019 12 GCHP system, , 870 in real and simulated conditions. 11 of 16

Figure 15. Energy consumption of the GCHP system. Figure 15. Energy consumption of the GCHP system. Figure 16 shows the average monthly energy exchanged between the soil and the heat pump unit. Figure 16 shows the average monthly energy exchanged between the soil and the heat pump It should be noted that this energy is more important in the period in which the temperature difference unit. It should be noted that this energy is more important in the period in which the temperature between the demand and the soil temperature is higher. These results guarantee a better performance difference between the demand and the soil temperature is higher. These results guarantee a better of the heat pump and, therefore, a decrease of the energy consumption. performance of the heat pump and, therefore, a decrease of the energy consumption.

Figure 16. Monthly energy transferred to geothermal ﬁeld.field.

In thisthis case,case, thethe radiantradiant ﬂoor floor can can handle handle the the sensible sensible load load of of the the different different zones. zones. In particular,In particular, we canwe can see see that that in the in heatingthe heating period, period, the energythe energy exchanged exchanged is higher is higher than than in cooling in cooling mode, mode, due due to the to thermalthe thermal demand demand being being more importantmore important in this period.in this period. The values The shown values in shown Figure 17in areFigure in accordance 17 are in withaccordance the thermal with the demand. thermal demand. The simulations results are closer to the actual results in heating and cooling mode. The COPsys in heating and cooling mode for experimental and simulated conditions is shown in Table5.

Table 5. COPsys real/simulated for heating and cooling operation.

COPSYS COPSYS COPSYS COPSYS Heating_Real Heating_Sim Cooling_Real Cooling_Sim GCHP system 3.86 3.80 5.29 5.18 Energies 2019, 12, 870 12 of 16 Energies 2019, 12, x 12 of 16

Figure 17. Monthly energy transferred to radiant ﬂoorfloor system.

5.1. ComparisonThe simulations of COP results and CO are2 Emissions closer to the with actual Other results Proposed in Technologiesheating and cooling mode. The COPsys in heatingIn order and to cooling know themode positioning for experimental of the GCHP and simulated system designed, conditions a comparison is shown in with Table natural 5. gas, diesel, and biomass was conducted. The three systems are composed of a boiler for each technology and radiant ﬂoor forTable distribution. 5. COPsys Anreal/simulated additional for heat heating pump and unit cooling is installed operation. in order to satisfy the cooling demand and maintainCOPSYS the same conditionsCOPSYS for the analysis.COP TheSYS model validated theCOP calculatedSYS COPSYS and CO2 emissions.Heating_Real In Table 6, theHeating_Sim primary energy coefﬁcientsCooling_Real are listed in orderCooling_Sim to obtain the COGCHP2 emissions. 3.86 3.80 5.29 5.18 system Table 6. Energy step factors.

2 5.1. Comparison of COP andEnergy CO Emissions with Other Proposed To CO 2TechnologiesEmissions (kgCO 2/kWhEF) In order to know theElectricity positioning of the GCHP system designed, 0.331a comparison with natural gas, diesel, and biomass was conducted.Diesel The three systems are composed 0.311 of a boiler for each technology Natural Gas 0.252 and radiant floor for distribution. An additional heat pump unit is installed in order to satisfy the Biomass 0.018 cooling demand and maintain the same conditions for the analysis. The model validated the calculated COPSYS and CO2 emissions. In Table 6, the primary energy coefficients are listed in order In Table7, annual COP and CO emissions values are shown. to obtain the CO2 emissions. sys 2

Table 7.TableCOP sys6. Energyand CO step2 emission factors. results.

HVAC SystemEnergy COP HeatingTo Mode CO2 Emissions COP Cooling (kgCO Mode2/kWhEF) CO2 Emissions (Kgs) GCHPElectricity 3.860.331 5.29 985 Natural GasDiesel 1.030.311 2.80 2741 Diesel 0.9 2.80 3715 Natural Gas 0.252 Biomass 0.8 2.8 475 Biomass 0.018

The results show that the GCHP system is the technology with the highest COP in heating and In Table 7, annual COP sys and CO2 emissions values are shown. sys cooling modes, and least CO2 emissions, with the latter surpassed only by biomass.

Table 7. COPsys and CO2 emission results. 5.2. Comparative Economic Analysis

ForHVAC the comparative System COP economic Heating analysis Mode of COP the Cooling four technologies Mode CO proposed,2 Emissions the (Kgs) cost of the installations,GCHP maintenance, and energy3.86 source for each technology 5.29 is presented. An 985 economic lifetime of 15 yearsNatural is considered. Gas The cost 1.03data were obtained in 2015 2.80 through different companies 2741 specializing in these kindsDiesel of systems. Using the0.9 data of energy price 2.80 forecast from the hydrocarbon 3715 geoportal of the SpanishBiomass industrial and energy0.8 ministry, and combined 2.8 with the average annual 475 fuel/electricity consumption obtained from real/simulated data, the annual fuel cost is calculated. A ﬁxed percentage The results show that the GCHP system is the technology with the highest COPsys in heating and cooling modes, and least CO2 emissions, with the latter surpassed only by biomass.

Energies 2019, 12, x 13 of 16 Energies 2019, 12, x 13 of 16 5.2. Comparative Economic Analysis 5.2. Comparative Economic Analysis For the comparative economic analysis of the four technologies proposed, the cost of the installations,For the comparativemaintenance, economic and energy analysis source of fother eachfour technologiestechnology is proposed, presented. the An cost economic of the installations,lifetime of 15 maintenance,years is considered. and energy The cost source data werefor each obtained technology in 2015 isthrough presented. different An companieseconomic lifetimespecializing of 15 in years these is kinds considered. of syst ems.The costUsing data the were data obtainedof energy in price 2015 forecast through from different the hydrocarbon companies specializinggeoportal of in the these Spanish kinds industrialof systems. and Using energy the data ministry, of energy and pricecombined forecast with from the the average hydrocarbon annual geoportalfuel/electricity of the consumption Spanish industrial obtained and from energy real/simul ministry,ated anddata, combined the annual with fuel thecost average is calculated. annual A Energies 2019, 12, 870 13 of 16 fuel/electricityfixed percentage consumption of the initial obtained installation from ofreal/simul each systemated data,is considered the annual for fuel determining cost is calculated. the annual A fixedmaintenance percentage cost. of The the result initials areinstallation shown in of Table each 8.system is considered for determining the annual ofmaintenance the initial installationcost. The result of eachs are system shown isin considered Table 8. for determining the annual maintenance cost. The results are shown in Table8.Table 8. Data used for economic analysis. Table 8. Data used for economic analysis. Table 8. DataGeothermal used for economic analysis.Biomass Natural gas Diesel Total Investment (€) Geothermal 16,475 Biomass 13,275 Natural 12,875 gas Diesel 13,275 Total GCHPInvestment (€) (€) Geothermal 16,475 5800 Biomass 13,275 Natural gas 12,875 Diesel 13,275 Total InvestmentGCHPBoiler (€) (€) 16,475 5800 13,275 2800 12,875 2400 13,275 2800 GCHPBoilerSplit ( €(€)) 5800 2800 4000 2400 4000 2800 4000 Boiler (€) 2800 2400 2800 BoreholeSplit (€) (€) 4325 4000 4000 4000 Split (€) 4000 4000 4000 RadiantBoreholeBorehole Floor (€ (€)) (€) 4325 4325 6350 6475 6475 6475 AnnualRadiantRadiant maintenance Floor Floor (€ )(€) cost (%) 6350 6350 4.7% 6475 6475 4.0% 6475 6475 4.0% 6475 6475 4.0% AnnualAnnualCost maintenancemaintenance effective (€/kWh) cost cost (%) (%) 4.7% 0.1764 4.7% 4.0% 0.0507 4.0% 4.0% 0.0839 4.0% 4.0% 0.0905 4.0% CostCost effective effective (€ /kWh)(€/kWh) 0.1764 0.1764 0.0507 0.0507 0.0839 0.0839 0.0905 0.0905 As Figure 18 shows, GCHP systems are, for the first time, not the economically preferred investmentAs Figure as 18a HVACshows, system GCHP for systems buildings. are, for On thethlye after ﬁrstfirst time,some notyears the of economicallyeconomically functioning preferredwere the investmenteconomic results asas a a HVAC HVACfor this system system technology for buildings.for buildings.better Only than On after allly the someafter other yearssome options. ofyears functioning Theof functioning accumulated were the economicwere savings the resultseconomic(positive for values) thisresults technology fromfor this GCHP bettertechnology versus than natural all better the other gas,than biomass, options. all the Theotherand accumulateddiesel options. are shownThe savings accumulated in Figure (positive 19. savings values) from(positive GCHP values) versus from natural GCHP gas, versus biomass, natural and gas, diesel biomass, are shown and indiesel Figure are 19 shown. in Figure 19.

Figure 18. Initial investments for the technologies proposed. Figure 18. Initial investments for the technologies proposed.

Figure 19. Annual net savings for a 15-year period.

After 6.5 years’ functioning of the installation, the GCHP overcame the cost of diesel, and in the seventh year, natural gas. Only after 9 years did the proposed system reach costs equivalent to using biomass. In Figure 20, the accumulated cost for each technology during a 15-year period is illustrated. Energies 2019, 12, x 14 of 16

Figure 19. Annual net savings for a 15-year period.

Figure 20. Accumulated cost for each technology. Figure 20. Accumulated cost for each technology. 6. Conclusions 6. Conclusions The use of low enthalpy geothermal systems in buildings with low energy demand is a recommendedThe use of option low toenthalpy satisfy HVACgeothermal and DHWsystems necessities. in buildings This with paper low shows energy the demand position is of a therecommended system proposed option against to satisfy other HVAC technologies—natural and DHW necessities. gas, This diesel, paper and shows biomass—from the position energy, of the economic,system proposed and environmental against other aspects. technologies—nat The parameter selectedural gas, to analyzediesel, theand comfort biomass—from of the installation energy, iseconomic, the mean indoorand environmental temperature in eachaspects. area. The EnergyPlus parameter was selected the software to analyze used to createthe comfort the simulation of the model,installation in order is the to describemean indoor the operation temperature of the in installation.each area. EnergyPlus was the software used to create the Thesimulation results model, obtained in order show to that describe the indoor the operation temperature of the in installation. each area is closed to the objective. The differencesThe results were obtained only show of 0.3/0.2 that the◦C inindoor heating temperat and coolingure in each modes area respectively. is closed to Thethe objective. average temperatureThe differences reached were in only cooling of 0.3/0.2 and heating°C in heating modes and was cooling 24.8/21.7 modes◦C, consideredrespectively. to The be aaverage good performancetemperature for reached the designed in cooling HVAC and system. heating modes was 24.8/21.7 °C, considered to be a good performanceA comparison for the between designed experimental HVAC system. and simulation results was done in order to validate the modelA designed. comparison The between R2 results experimental obtained representedand simulation a good results approximation was done in oforder the modelto validate to real the 2 operation.model designed. The performance The R results of the obtained system represented is represented a good by the approximation COPsys, which of wasthe 3.86/5.89model to forreal heatingoperation. and The cooling performance modes, and of represents the system a goodis represented performance. by the COPsys, which was 3.86/5.89 for heatingNatural and cooling gas, diesel, modes, and and biomass represents were a good the technologies performance. selected for the comparison done. The COPNaturalsys obtained gas, diesel, in heating and biomass modes was were 1.03/0.9/0.8 the technologies respectively, selected which for show the comparison the high performance done. The ofCOP thesys GCHP obtained compared in heating with modes the rest, was being 1.03/0.9/0.8 higher respectively, between 3.7 andwhich 4.8 show times. the In high cooling performance operation, of thethe COP GCHPsys was compared two times with higher the rest, because being MV higher ensures between the indoor 3.7 and air 4.8 quality times. and In exertscooling a coolingoperation, effect the overCOP thesys was system, twominimizing times higher the because thermal MV demand. ensures the indoor air quality and exerts a cooling effect overThe the economic system, minimizing analysis illustrates the thermal that thedemand. GCHP systems have a higher initial cost than the rest of the availableThe economic technologies. analysis After illustrates six and that seven the years, GCHP the sy GCHPstems have maintains a higher HVAC initial conditions cost than at the lower rest costof the than available diesel and technologies. natural gas, After respectively, six and andseven reaches years, thethe same GCHP level maintains as biomass HVAC in the conditions ninth year. at lower cost than diesel and natural gas, respectively, and reaches the same level as biomass in the Authorninth year. Contributions: Conceptualization, J.I.V. and A.V.; methodology, J.I.V. and A.V.; software, J.I.V. and A.V.; validation, I.d.A. and R.Q.; formal analysis, J.I.V.; investigation, J.I.V. and A.V.; resources, A.A.; data curation, I.d.A. andAuthor R.Q.; Contributions: writing—original conceptualization, draft preparation, J.I.V. I.d.A. and and A.V.; R.Q.; methodol writing—Reviewogy, J.I.V. and and editing, A.V.; A.V.;software, visualization, J.I.V. and A.V.;A.V.; supervision, validation, J.I.V.;I.d.A. project and R.Q.; administration, formal analysis, A.A.; fundingJ.I.V.; investigation, acquisition, A.A. J.I.V. and A.V; resources, A.A.; data curation, I.d.A. and R.Q; writing—original draft preparation, I.d.A. and R.Q; writing—Review and editing, Funding: This research received no external funding. A.V.; visualization, A.V.; supervision, J.I.V.; project administration, A.A.; funding acquisition, A.A. Acknowledgments: Authors would like to thank Energies and the anonymous reviewers. Also, the assistance providedFunding: by This the research University received of Salamanca. no external Authors funding. would also like to thank Department of Construction and Agronomy, Construction Engineering Area of High Polytechnic School of Avila. Conﬂicts of Interest: The authors declare no conﬂict of interest. Energies 2019, 12, 870 15 of 16

Nomenclature

BHE borehole heat exchanger CO2 fossil carbon dioxide emissions (kg) COPSYS seasonal energy efﬁciency ratio Cps factor for electricity (kgCO2/kWh) CTFs conduction transfer method DHW domestic hot water GCHP ground-coupled heat pump GHE ground heat exchanger GLHEPRO ground heat exchanger software HVAC heating, ventilating, and air-conditioning MV mechanical ventilation system ◦ Pr dew point temperature ( C) RF radiant ﬂoor R2 coeff of multiple determinations ◦ text outdoor air temperature ( C) ◦ thw hot-water temperature ( C) ◦ tint indoor air temperature ( C) TRNSYS transient systems simulation ◦ tsr surface average temperature ( C)

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