Feasibility of active solar water heating systems with evacuated tube collector at different operational water temperatures

Feasibility of active solar water heating systems with evacuated tube collector at different operational water temperatures

Energy Conversion and Management 113 (2016) 16–26 Contents lists available at ScienceDirect Energy Conversion and Management journal homepage: www.e...

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Energy Conversion and Management 113 (2016) 16–26

Contents lists available at ScienceDirect

Energy Conversion and Management journal homepage: www.elsevier.com/locate/enconman

Feasibility of active solar water heating systems with evacuated tube collector at different operational water temperatures Fernando R. Mazarrón ⇑, Carlos Javier Porras-Prieto, José Luis García, Rosa María Benavente Technical University of Madrid, School of Agricultural, Food and Biosystems Engineering, Agroforestry Engineering Department, 28040 Madrid, Spain

a r t i c l e

i n f o

Article history: Received 26 October 2015 Accepted 17 January 2016

Keywords: Active solar water-heating system Evacuated-tube collector Operational temperature Feasibility

a b s t r a c t With rapid advancements in society, higher water temperatures are needed in a number of applications. The demand for hot water presents a great variability with water required at different temperatures. In this study, the design, installation, and evaluation of a solar water heating system with evacuated tube collector and active circulation has been carried out. The main objective is to analyze how the required tank water temperature affects the useful energy that the system is capable of delivering, and consequently its profitability. The results show how the energy that is collected and delivered to the tank decreases with increasing the required temperature due to a lower performance of the collector and losses in the pipes. The annual system efficiency reaches average values of 66%, 64%, 61%, 56%, and 55% for required temperatures of 40 °C, 50 °C, 60 °C, 70 °C, and 80 °C. As a result, profitability decreases as temperature increases. The useful energy, and therefore the profitability, will decrease if the demand is not distributed throughout the day or focused on the end of the day. The system’s profitability was determined in two cases: considering maximum profitability of the system, assuming 100% utilization of useful energy (scenario 1); assuming a particular demand, considering that on many days all the useful energy the system can supply is not used (scenario 2). The analysis shows that through proper sizing of the system, optimizing the number of solar collectors, the investment in the solar system can be profitable with similar profitability values in the two contemplated scenarios. In scenario 2, a combined-delivery system (solar and diesel boiler) generates savings of between 23% and 15% compared to a single-delivery system of diesel, with a reduction in consumption of diesel close to 70%. The number of collectors that maximizes the profitability depends on the required temperature; therefore, in designing this kind of installation water temperature requirements must always be taken into account. From an environmental point of view, CO2 emissions can be reduced between 392 and 325 kg CO2 per m2 of collector, depending on the required temperature. The results of this study can be very useful in determining the feasibility of using such systems to supply a part of demand for hot water. Ó 2016 Elsevier Ltd. All rights reserved.

1. Introduction In the search for renewable and clean energy/technology, solar collectors have been proven to have promising applications in water heating, refrigeration, water desalination, space heating/ cooling, heat pumps, combined-energy supply, and so on [1]. Solar water heaters (SWHs) are the most popular way to use solar energy, a consequence of their technological feasibility and the economic benefits they afford [2]. The report Renewable Energy Policy Network for the 21st Century [3] on the global state of renewable energies notes that solar thermal systems contribute ⇑ Corresponding author. Tel.: +34 913365670. E-mail addresses: [email protected] (F.R. Mazarrón), [email protected] (C.J. Porras-Prieto), [email protected] (J.L. García), [email protected] (R.M. Benavente). http://dx.doi.org/10.1016/j.enconman.2016.01.046 0196-8904/Ó 2016 Elsevier Ltd. All rights reserved.

significantly to the production of hot water in many countries. In 2012, the world added 55.4 GW (over 79 million m2) of solarheat capacity and the increase in the cumulative capacity installed of all types of collectors was more than 14% for a total of 283.4 GW by year’s end. Depending on whether they require pumps or not to run, SWHs can be grouped into two basic categories: passive-circulation systems and active-circulation systems [4]. Passive-circulation systems refer to the thermosyphonic method in which the density difference induces the circulation of the fluid naturally. On the other hand, active circulation employs a pump to effect forced circulation of the working fluid [5]. Active circulation eliminates the restriction that the collector be located under the supply tank, allowing its positioning away from the supply tank: therefore, active circulation offers greater flexibility for adaptation in case of high demand, such as in industrial processes.

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Nomenclature Ac Cp G _ m Qc Qd Q pl T1 T2 T3

useful area of the ETC (m2) specific heat capacity of the collector heat-transfer fluid (J kg1 °C1) solar radiation (W m2) mass flow rate of the heat transfer fluid (kg s1) useful heat collected by the ETC (W m2) useful heat delivered to the tank (W m2) circuit pipe losses (W m2) temperature of the heat-transfer fluid inside the ETC (°C) temperature of the heat-transfer fluid when leaving the ETC (°C) temperature of the heat-transfer fluid on entering the tank’s heat exchanger (°C)

Solar collectors are the main component of any solar-energy system. They gather solar radiation and transfer heat to a fluid [6]. Three types of stationary collectors are commonly used in SWH systems: flat plate collectors (FPCs), evacuated tube collectors (ETCs), and compound parabolic collectors (CPCs). FPCs and ETCs are the collectors most widely used for small-scale water heating applications [2]. Heat-pipe ETCs and U-tube-glass ETCs are the two most widely used solar collectors in domestic water heating [7]. Conventional FPCs were developed for use in sunny and warm climates. Their benefits, however, are greatly reduced when conditions become unfavorable during cold, cloudy, and windy days [8]. Condensation and moisture can also cause their deterioration, resulting in reduced performance and eventual system failure. ETCs have the advantage that their vacuum envelope reduces convection and conduction losses [8]; they can, therefore, perform even in cold weather when flat-plate collectors perform poorly due to heat losses [9]. With an optimum design of the number of tubes and flow rate, FPCs can reach maximum-output water temperatures close to 80 °C in a context of high solar irradiation [10]. Nevertheless, according to several authors, ETCs show better performance and efficiency than FPCs at high-heat-transfer-fluid temperatures. Thus, for example, under the same weather conditions in Dublin (Ireland), the annual average collector efficiencies were 46.1% and 60.7% while the system efficiencies were 37.9% and 50.3%, for a FPC and a ETC, respectively [2]; ETC technology is more suitable than FPC from a technical point a view in Morocco [11]; ETCs can operate at higher temperatures than FPCs [8]. This type of unit (ETCs) is the other form of solar collector that is typically more efficient at higher temperatures than flat-plate collectors [9]; evacuated tubes are usually found to be more efficient for hightemperature applications than flat-plate collectors [12]. Also, recent studies have demonstrated that an evacuated-tube collector coupled with parabolic trough can produce instant hot water at temperatures between 40 °C and 68 °C under low solar radiation (between 200 W m2 and 600 W m2) [13]. With rapid advancements in society, higher water temperatures are needed in a number of applications, such as air conditioning, refrigeration, building heating, seawater desalination, and industrial heating [14]. The demand for hot water in the industrial sector is highly variable, often requiring temperatures above 60 °C [15]. For example, in the meat industry water is required at 60 °C throughout the day for the production process, as are large volumes of water at 80 °C late in the day to clean the facilities. To date, most studies on SWHs focus on the production of water at a lower temperature than that required in many industrial

T4 T5 T6 T7 T8 T9 T10 Ta

gs

temperature of the heat-transfer fluid on leaving the tank’s heat exchanger (°C) temperature of the heat-transfer fluid upon entering the ETC (°C) temperature of the water at the bottom of the tank (°C) temperature of the water in the middle part of the tank (°C) temperature of the water at the top of the tank (°C) temperature of the water leaving the tank (°C) temperature of the water entering the tank (°C) ambient temperature (°C) system efficiency (–)

processes, usually for domestic use in homes. The study of the relationship between tank-water temperature and the performance of the system needs to be undertaken differently depending on whether the heating system is active or passive. In the case of heating systems with natural circulation by thermosyphon, the temperatures values and the mass flow rate increase with solar intensity and reach their maximum in the middle of the day and then decrease with the falling of the sun and reach their minimum at night [16]; the natural convection flow rate in the tube is high enough to disturb the tank’s stratification; additionally it is high enough that the tank temperature strongly affects the circulation flow rate through the tubes [17]; the circulation flow rate can be correlated in terms of solar input, tank temperature, collector inclination, and tube aspect ratio [18]. In this type of system it is the temperature difference that induces the movement of the fluid and enables the heating of the water tank. Thus, the conclusions drawn are not applicable to active systems. In active systems, precedents are scarce, focusing on the relationship between different required tank-water temperature (rTWT) and system performance. Among them, the performance of SHWs fitted with flat-plate and vacuum collectors has been analyzed for specific mean water temperatures in the storage tank of 37 °C, 45 °C, and 55 °C, although they are at low temperature [19]. For higher temperatures (>60 °C), the recent work by Gang et al. should be highlighted, which analyzes the performance in meeting higher-temperature requirements of a compound parabolic concentrator (CPC) solar water-heater system experiment rig with a U-pipe in winter [14] and a comparison without a mini-CPC reflector [20]. In these studies, the water in the tank was heated from 26.9 °C to 55 °C, 65 °C, 75 °C, 85 °C, and 95 °C, obtaining the average thermal and exergy efficiencies. Due to its objective, in this work an analysis of a wide range of daily solar radiation values was not carried out. Recently, Alfaro-Ayala el al. [21] have obtained via CFD numerical simulation the thermal and hydraulic performance of a low-temperature water-in-glass evacuated-tube solar collector, using different temperatures at the inlet and outlet of the tubes. Regarding the economic aspect, there are numerous precedents in the study of the profitability of SWHs under different conditions and locations, such as the economic and environmental comparison of two different types of solar collectors (i.e. flat-plate and evacuated-tube solar collectors) with different auxiliary systems (i.e. natural gas and electricity), and different locations [22]; the performance and the life-cycle perspective of domestic solar water-heating systems with FPCs and with ETCs [23]; the lifecycle environmental sustainability of solar water-heating systems (FPC and ETC) in regions with low solar irradiation [24]; the

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comparative field performance study of flat-plate and heat-pipe evacuated-tube collectors for domestic water-heating systems in a temperate climate [2]; the financial evaluation of SWH in Malaysia, which takes into account cost benefit and payback period [25]; the development of a simplified method for optimizing the key parameters of solar water-heating systems based on lifecycle energy analysis [26]; the technoeconomic feasibility of retrofitting solar domestic hot-water-heating systems in the Canadian housing stock [27], and so on. Profitability varies according to the studies and circumstances, although payback usually takes place after between six and fifteen years. However, there is a gap in the study surrounding how the required water temperature affects the useful energy that the system is capable of delivering in a real scenario, and consequently its profitability. An end user should determine the economically optimal solar collector area of an SWH according to the hot-waterconsumption pattern [28]. Therefore, in this study the design, installation, and evaluation of SWHs with ETCs and active circulation for hot-water supply at different temperatures has been carried out. For this end, besides the data from the annual monitoring of the SWH, the useful energy provided by the SWH for different operational water temperatures (40 °C, 50 °C, 60 °C, 70 °C, and 80 °C) has been calculated, allowing determination of the annual system efficiency. The system’s profitability has been determined in two scenarios: scenario 1 considering maximum profitability of the system, assuming 100% utilization of useful energy; scenario 2 assuming a particular demand, considering that on many days all the useful energy the system can supply is not used. The results, based on experimental data, are related to the location of the solar facility in Madrid, Spain, where the aggregate annual irradiation in the level of the collector is close to 2000 kW h m2. 2. Material and methods It was planned to determine the profitability of SWHs with active circulation, under the assumption of carrying out the investment in an existing facility that had a system of energy supply. Therefore, the solar system would work as a complementary system of the existing system, covering a portion of demand. To quantify the influence of rTWT on the profitability of the system, it is necessary to determine the useful energy that SWHs are capable of supplying throughout the year for each temperature. For this end, an experimental SWH was designed reproducing currently marketed systems of active circulation. 2.1. Solar water heating system The system allows varying the rTWT (40 °C, 50 °C, 60 °C, 70 °C, and 80 °C), monitoring different variables essential to carry out an energy analysis of its performance (see Section 2.1.4. Monitoring

system), and quantifying the useful energy captured. The experimental SWH was installed on the roof of a building in Madrid (Spain) (40°260 31.1100 N, 3°430 39.7100 W and altitude 600 m above sea level). This installation required the connection of different subsystems: a solar-energy capture subsystem, a fluid distribution and accumulation subsystem, and a control subsystem and a monitoring system (Fig. 1). 2.1.1. Solar energy capture subsystem The selected vacuum-tube solar collector is model SP-S58/ 1800–24 of WesTech Solar Co. (China). This collector has 24 vacuum tubes, a net collection area of 2 m2, and a flow of 200 L h1. The annual solar gain is related to the site latitude, tilt, azimuth angles of collectors, and local climatic conditions. To maximize annual energy collection in the northern hemisphere, collectors should be south-facing mounted and inclined at a yearly optimal tilt-angle from the horizon [29]. To ensure good annual performance, the ETC was mounted on a south-facing metallic structure (azimuth 180°) held at an angle of 41° from the horizon. 2.1.2. Distribution and accumulation subsystem The accumulation and exchange system allows the system to transfer the energy collected in the ETC through the heat transfer fluid of the primary circuit to the hot water tank. The accumulator installed is of the brand Thermor (France), model IAV 80/100 with exchange system through coil and 80 L capacity. It presents highdensity thermal insulation and a glazed stainless-steel container. The hydraulic circuit is made of copper, according to ISO/TR 10217 and UNE EN 806-1. The pipes have an inner diameter of 20 mm and thickness of 1 mm. The horizontal sections have a minimum slope of 1% in the direction of the flow. The pipes are coated with 13 mm-thick insulation material with a thermal conductivity of 0.040 W m1 °C1 at 20 °C. The system has an expansion vessel of 8 L with fixed membrane. The pump used was the Wilo-Star-ST 15/6 ECO-3 submerged rotor model of the brand Wilo (Germany). The circuit was provided with a safety valve to correct possible pressure surges. Water has been used as a heat transfer fluid. 2.1.3. Control subsystem The role of this subsystem was to regulate the energy flow between the ETC, the distribution and accumulation subsystem, and the discharge of hot water from the tank. The system installed consists of two regulatory subunits: a controller Allegro 453 of the firm Sonder (Spain), responsible for controlling the operation of the pump of the collector circuit and an automat PLC logo of the brand Siemens (Germany) that controls the discharging of the tank. Control probes (SC1–SC3) are of type PTC 1000, temperature range 50 + 150 °C and accuracy of ±0.15 °C at 0 °C (Fig. 2).

Fig. 1. The experimental solar water heating system.

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Fig. 2. Outline of the main components of the active SWHs-ETC.

The water pump in the primary circuit was configured following the recommendations of the ETC manufacturer in order to reproduce normal operating conditions. Controller 1 turns the pump of the circuit of the collector when there is a temperature difference of 4 °C between the sensor in the collector (SC1) and the sensor at the bottom of the tank near the exchanger (SC2). Controller 1 stops the pump when the difference drops below 2 °C. In order to determine the influence that the rTWT has on the system, controller 2 was configured to maintain a constant rTWT throughout each day, varying from one day to another for cycles of 5 days in the following sequence: 40 °C, 50 °C, 60 °C, 70 °C, and 80 °C. It was intended to determine the maximum production potential of the system. For this end, controller 2 opens the discharging electrovalve when it detects that sensor SC3 (in the top of the tank near the exit) registers a temperature of 1 °C higher than the rTWT of that day. The discharging ends when controller 2 detects that the temperature sensor SC3 has fallen 1 °C below the rTWT, closing the electrovalve. This condition ensures obtaining a volume of water approximate to the desired temperature. The hot-water outlet at the top is produced by the pressure of the supply network, with the input of the same volume of cold water through the connection to the supply pipe at the bottom thereof, provided with a diffuser to avoid breaking the stratification of the water in the tank. The temperature of the cold water of the supply network oscillates between 19 °C in winter months and 26 °C in summer months. Moreover, every day starts with the water in the tank at the temperature of the supply network. For this end, controller 2 opens the drain valve at 7:15 am, producing complete discharge equivalent to the discharge when the rTWT temperature is reached. The

controller keeps the valve open for 15 min, sufficient time to completely evacuate the volume of the tank and replace it with the same volume of cold water from the supply network. Thus, the temperature recorded by the three sensors located inside the tank (T6, T7, and T8) tends to equalize. The complete filling of the tank ensures daily similar starting conditions for the experiment during consecutive days, allowing a more objective comparison between different rTWTs. 2.1.4. Monitoring system Sensors and data loggers have been installed to obtain and store all the necessary information about the collection, accumulation, and control subsystems. In order to monitor the temperature we used 10 probes (T1 T10) PTC 100 type with temperature range from 50 °C to 150 °C and accuracy of ±0.15 °C at 0 °C. The probes were placed at strategic points in the installation (Fig. 2), directly in contact with the fluid through holes designed for this purpose, hermetically sealed. For flow monitoring, two mechanical flowmeters with digital readouts were installed: one near the inlet of the collector, which measures the flow of the primary circuit; and one on the hot water outlet of the tank, which measures the flow of the secondary circuit, used to calculate the volume of hot water discharged from the tank. To record data from the previously mentioned parameters two data acquisition equipment Datataker (Australia) mod DT50s were used, placed in a sealed box. The meteorological parameters were recorded using a Micro Weather Station HOBO of the brand Onset (USA). Solar radiation was measured using a pyranometer for total solar radiation (R).

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This self-powered sensor measures the radiation between 300 and 1100 nm. The pyranometer has a measuring range between 0 and 1280 W m2 with accuracy of ±10 W m2 or ±5%. The pyranometer was installed with the same angle of inclination as the vacuum tubes. In addition, other parameters were monitored, such as ambient temperature (Ta), wind velocity (W), and relative humidity (Hr). The ambient temperature and relative humidity were monitored by sensors housed in a plastic capsule, with precision of ±0.2 °C and range from 0 °C to 50 °C for temperature and ±2.5% and range from 5% to 95% of relative humidity. To monitor wind speed a cup anemometer was used with a measuring range of 0–45 m s1, a resolution of 0.38 m s1, and a maximum deviation ±1.1 m s1. The system was programmed setting a data capture interval of 1 min, monitoring the SWHs for a period of one year. 2.2. Quantifying the useful energy of the SWHs Equivalent to other previous studies analyzing the efficiency of SWHs [15,30–32] the fundamental variables for the characterization of the system have been calculated, namely: (a) Energy collected by the ETC The energy captured by the ETC and delivered to the heattransfer fluid was calculated from the difference in water temperature at the ETC water entry and exit points, using the following equation:

_ p ðT 2  T 5 Þ=Ac Q c ¼ mC

ð1Þ

(b) Energy delivered to the tank The energy transferred to the tank through the coil was determined as:

_ p ðT 3  T 4 Þ=Ac Q d ¼ mC

ð2Þ

The energy delivered to the tank is considered useful energy of the SWHs, as the complementary delivery system would harness this energy, increasing the temperature of the water if the rTWT is not reached. (c) Supply pipe losses in the primary circuit These were determined as the difference between the energy captured by the ETC and that delivered to the tank:

Q pl ¼ Q c  Q d

ð3Þ

(d) System efficiency System efficiency was calculated as:

_ p ðT 3  T 4 Þ=ðAc GÞ ¼ Q d =G gs ¼ mC

2.3. Financial evaluation 2.3.1. Profitability indicators The system’s profitability has been quantified taking into account the variations in cash flow originating from the high initial investment necessary for SWHs versus traditional supply. As an alternative supply, a diesel boiler is proposed with an efficiency of 96% and calorific value of diesel of 10 kW h L1. The investment cost of the diesel boiler has not been taken into account, considering that it will always be necessary because of the limited production capacity of SWHs in winter. The system’s profitability was determined in two cases. First we have determined the maximum profit that the system could achieve, assuming a hypothetical case in which 100% of the useful energy of the SWHs was harnessed. In this way, we limit the maximum profitability of the system per m2 of collector. In the second case, we have analyzed a scenario with a particular demand as an example of how to optimize the SWH and calculate its profitability considering that on many days all the useful energy the SWH can supply is not used. The cash flows for each year (CFj) are calculated as differences in payments before investment (Pb)—generated by the diesel boiler—and payments after the investment (Pa)—generated by the SWHs. These cash flows will allow quantifying the profitability of the installation through the following indicators: (a) Net Present Value (NPV): expresses the present value of all financial returns generated by the investment (K), considering a discount rate (r) and a useful life of ‘‘n” years.

NPV ¼ K þ

j¼n X Pbj  Paj j¼0

ð1 þ rÞ j

¼ K þ

j¼n X

CFj

j¼0

ð1 þ rÞ j

ð5Þ

(b) Internal Rate of Return (IRR): discount rate for which the NPV takes a zero value.

K þ

j¼n X Pbj  Paj j¼0

ð1 þ IRRÞ

j

¼ K þ

j¼n X

CFj

j¼0

ð1 þ IRRÞ j

¼0

ð6Þ

(c) Discounted payback period (DPP): the time an investment takes to recover the initial outlay, with the cash flows generated by the same date.

K

j¼DPP X

CFj

j¼0

ð1 þ rÞ j

¼0

ð7Þ

ð4Þ

The calculated energy values have been expressed with respect to the collector area, that is dividing the values by 2 m2 of effective area. Although in some cases the variable does not depend directly on the surface of the collector, as in the case of losses in the pipes, these units allow us to make a comparative analysis of energy flows in the different elements of the system. These values in turn can be easily comparable with those obtained by other authors with different surfaces of collector. The daily values of useful energy were calculated from the analysis of the monitoring data obtained for a period of one year, from 1 July 2014 to 30 June 2015. Regression equations that relate the energy delivered to the tank and the solar irradiation for each rTWT have been calculated. Although this methodology is an approach based on regression curves, the coefficients of determination obtained (>0.95) ensure a reduced associated error. The regression equations obtained could be used to roughly estimate the useful energy a system of similar features could deliver, using the values of daily irradiation of different locations.

To calculate profitability indicators the following parameters have been assumed:  The investment of the installation, including assembly, amounts to about 1000 € m2.  SWH-electricity consumption due to the pump of the primary circuit has an average value of 0.7 W h m1 of pump operation. The product of the annual energy consumption of the pump (kW h m2) for the price of electricity will be used as payment after investment (Pa). The average price of electricity was set at 0.12 € kW h1, which is the average price of electricity in 2014 for industrial consumers in Spain, and also the EU-28 average electricity price for industrial consumers during the second half of 2014, according Eurostat.  A constant value of the price of heating diesel in Spain during the first half of 2014 has been used, without considering the possible variation of such prices during the lifetime of the installation; specifically 0.89 € L1. The quantity of diesel

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needed to provide the same useful energy multiplied by the price of the diesel will be defined as payment before investment (Pb).  The useful life of the facility has been set at 20 years.  We used a discount rate of 4% (average rate of the last five years of a Spanish 10-year government bond). The assumed parameters pose a relatively conservative scenario. The profitability of the analyzed system would increase in situations that are more unfavorable for the alternative supply through diesel; an example of such an unfavorable situation would be if heating diesel prices rise (and in the last few years, there has been a notable rise) and reach an adjusted rate lower than 4% (it currently ranges between 1% and 2%). 2.3.2. Scenario 1: Maximum profitability of the SWHs, assuming 100% utilization of useful energy To determine the profitability of SWHs without the limitations imposed by a specific demand, the scenario of full potential of the SWHs has been considered, in which all the useful energy that the SWHs can store in the tank during the year is exploited. How much it would cost to produce all the energy of the SWHs through a traditional alternative (diesel boiler) has been determined, calculating the differences in payments between the electric cost of the SWHs and the cost of the alternative. So some benchmarks are obtained that will quantify, per square meter of collector, the return on investment of the solar system to replace some of the demand of conventional systems. It should be stressed that this is the setting for maximum profitability. Profitability will decline in any real facility as it is complicated to take advantage of all the useful energy throughout the year. Under the baseline scenario, a sensitivity analysis has been carried out, quantifying the profitability with changes in each of the previously defined parameters, determining the threshold of profitability; specifically:  Change in investment cost, assuming different assembly costs or the price of the components.  Variation in the price of diesel.  Variation of the useful life of the installation.  Variation of the discount rate, assuming a change in economic conditions that implied a change equivalent to that variation. 2.3.3. Scenario 2: Profitability of optimized SWHs in a scenario with specific demand A fictional scenario is presented as an example of how to apply the results to determine the profitability of SWHs in an existing facility, optimizing the dimensions of the system. For this end, it is assumed that the small-scale behavior of the monitored system will be extrapolated to a larger system. In this scenario the profitability is calculated considering that on many days all the useful energy that the SWHs can provide is not used. This stage will determine whether maximum potential profitability of the system can be approximated with actual demand. This scenario considers an industry with a constant daily demand of 100 kW h. This facility has a diesel boiler capable of supplying that demand. Moreover, it is assumed that water demand is not concentrated early in the morning. Whether demand is spread throughout the day or concentrated at the end of the day, a proper sizing of the accumulation system will allow the solar system to capture energy whenever there is enough radiation. This fictional case is an example of the demand for hot water to clean pipelines and facilities daily at the end of the working day in agribusiness. Under the assumptions described above, the surface of the collectors of SWHs necessary to minimize the total energy

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expenditure (SWH and diesel boiler) over the life of the solar installation has been optimized. To do so, we have calculated 100 different scenarios for each rTWT: Consider an installation with a single collector of 2 m2 up to a scenario of 100 collectors with the same surface. For each rTWT, the optimal scenario will be that involving the lowest total payment up to date, taking the investment made into account. In each of the 100 proposed scenarios we calculated the percentage of demand covered by SWHs for each day of the year, depending on the sunlight. The remaining energy to reach the industry demand is supplied by the diesel boiler. On days when the solar system can supply more than the demand of the industry, only the demand is counted; the extra energy it could supply is discarded, just as would happen in the real world. In contrast, on cloudy days on which the solar system is not capable of supplying power, all the demand is supplied by the diesel boiler. Following this methodology, we calculated the total annual energy supplied by each of the systems, and with these values the annual payment to cover demand—the sum of the diesel payment plus the payment of the solar system pump consumption. The annual payment has been used as annual cash flow of the joint facility, which together with the investment in the solar system has allowed calculating the total payment made for 20 years adjusted at a rate of 4%. Since this is a scenario with a specific daily demand, the constraint of having to use all the useful energy that the solar system can provide is removed. The optimal solution for the entire period includes not using the full potential of high radiation days because the full potential is unnecessary on these days. In contrast, on days with little radiation, the solar system will not meet the required daily demand and will have to resort to high consumption of the complementary system. 3. Results and discussions 3.1. Experimental data The more than 10 million pieces of data obtained by the sensors during monitoring were processed and analyzed. Fig. 3 shows the change in the weather data over the course of the year through a daily summation of solar irradiance and the daily averages for wind, ambient temperature, and relative humidity from 08:00 to 20:59. VBA (Visual Basic for Application) was used to process the data. From the data of the temperature, irradiance, and flow sensors, as well as from the pulse flow meter, the energy and finance variables described in the materials and methods chapter were calculated; the salient data is shown in the chapters that follow. 3.2. Useful energy provided by the SWHs The useful energy delivered to the tank increases as solar radiation increases and assumes a linear relationship with coefficients of determination ranging from 0.95 to 0.96 depending on the rTWT (Fig. 4). The differences between the energy extracted from the ETC and the energy supplied to the tank are due to losses in the pipes, which increases as the rTWT grows. The greater the solar irradiation, the greater the differences in energy supplied among the different rTWTs; for solar irradiation values near 8000 W h m2 d1, these differences can exceed 1300 W h m2 d1 between 40 °C and 80 °C. From the regression lines previously calculated, which describe the energy delivered to the tank (Fig. 4), we can estimate the values of useful energy throughout the year at every rTWT. Using as input values the 365 values of solar irradiance of a full year, we obtain

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Fig. 3. Annual change of the weather data through summations and daily averages from 08:00 to 20:59.

the daily values of energy delivered to the tank (Fig. 5). The biggest differences between the different rTWTs are obtained in the spring and summer, when they can exceed 30 kW h m2 month1 between 40 °C and 80 °C (Fig. 6). The sum of all daily delivered energy values is used as an estimate of the annual useful energy that the SWHs can supply: 1351 kW h m2, 1307 kW h m2, 1242 kW h m2, 1155 kW h m2, and 1119 kW h m2 for 40 °C, 50 °C, 60 °C, 70 °C, and 80 °C, respectively. Given that added solar radiation amounts to 2049 kW h m2, annual mean efficiency values would reach 66%, 64%, 61%, 56%, and 55% for 40 °C, 50 °C, 60 °C, 70 °C, and 80 °C. If water consumption is concentrated during the early morning hours, useful energy decreases drastically. Although the system continues capturing energy, an important part of this energy is lost at night from the tank [15]. Thus, for example, if demand finalizes at 15:00, as in the case of certain industries and public institutions, the annual useful energy that the SWH is capable of capturing up to that time is 826 kW h m2, 796 kW h m2, 754 kW h m2, 763 kW h m2, and 748 kW h m2 for 40 °C, 50 °C, 60 °C, 70 °C, and 80 °C, respectively, which means reductions between 33% and 39%. 3.3. Scenario 1: Maximum profitability of the SWHs, assuming 100% utilization of useful energy For each rTWT, the annual energy consumption (L of diesel) the diesel boiler would have in order to provide the same useful energy

Fig. 4. Energy extracted from the ETC and delivered to the tank at different rTWTs.

extracted by the SWHs has been calculated. From these values, we calculated the annual equivalent cost by multiplying consumption with the price of diesel. Finally, we have calculated the annual cash flow as the difference between the annual cost of diesel (payment before the investment) and the cost involved in the operation of SWHs, corresponding to the drive pump (payment after investment) (Table 1). The pump energy consumption (Table 1) depends on the times at which the pump functions. The temperature of the water tank at the end of the afternoon influences the thermal rise existing with the collector, making the pump stop sooner if the tank temperature is high. In general, the lower the rTWT, the greater the number of water discharges; it is more probable that the water is colder later in the afternoon and that the pump works longer; nevertheless, this was not always the case since the experiment was carried out on different days and the difference among the rTWTs were small. The values of the differences of payments received have been used as cash flow for the calculation of profitability indicators (Table 2). The investment in the solar system would be highly profitable for all the rTWT if 100% of the energy delivered to the tank were successfully exploited. However, the profitability decreases as the rTWT increases, reaching differences in the NPV of 300 € m2 and more than two years of investment recovery from 40 °C to 80 °C. In the present scenario, the system would be profitable as long as the useful energy exceeds the threshold of 880 kW h m2 year1 (Fig. 7). Given that system performance is relatively constant throughout the year, these values can be of

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Fig. 5. Daily energy delivered to the tank along with irradiation, for each rTWT.

Table 2 Profitability indicators of SWHs – for m-2 – as a complementary system of an existing diesel boiler, meaning 100% utilization of useful energy.

NPV IRR DPP

Units

rTWT 40 °C

50 °C

60 °C

70 °C

80 °C

€ m2 % years

594 10.0 10.6

537 9.5 11.1

458 8.7 11.9

350 7.7 13.1

308 7.2 13.7

Fig. 6. Monthly energy delivered to the tank along with irradiation, for each rTWT.

use for making decisions about a potential investment, based on the irradiation of the area where it is to be performed. From an environmental point of view, assuming an emission factor of 2.786 kg CO2 L1 diesel [33], the SWHs present a reduction potential of annual GHG emission of 392 kg CO2 m2, 379 kg CO2

Fig. 7. Variation in profitability according to the useful energy delivered to the tank.

Table 1 Summary table of the calculation of payments differences (Pb–Pa) from the equivalent energy consumption of the diesel boiler. Units

rTWT 40 °C

50 °C

60 °C

70 °C

80 °C

1351 66 8

1307 67 8

1242 66 8

1155 64 8

1119 63 8

SWHs Energy delivered to the tank Pump energy consumption Cost pump consumption (Pa)

kW h m2 year1 kW h m2 year1 € m2 year1

Diesel boiler Energy consumption equivalent Equivalent cost (Pb)

L diesel m2 year1 € m2 year1

141 125

136 121

129 115

120 107

117 104

Difference of payments (Pb–Pa)

€ m2 year1

117

113

107

99

96

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Fig. 8. Variations in profitability (over NPV) due to the investment cost change, discount rate, useful life, and price of diesel.

Fig. 9. Total updated payment (r = 4%) over 20 years, depending on the number of solar collectors. Installation with a demand of 100 kW h d1 and a combined supply through diesel boiler and SWHs.

m2, 360 kg CO2 m2, 335 kg CO2 m2, 325 kg CO2 m2 for 40 °C, 50 °C, 60 °C, 70 °C, and 80 °C. The results of the sensitivity analysis posed allow determination of the variations of profitability before the change of each of the fixed variables, assuming that there are no changes in the others (Fig. 8). Thus, the breakeven point for the cost of the investment would range from 1600 € m2 for 40 °C to 1300 € m2 for 80 °C; the threshold for the discount rate from 10% for 40 °C to 7.2% for 80 °C; the threshold for the useful life from 10.6 years

for 40 °C to 13.7 years for 80 °C; the threshold price of diesel from € 0.58 L1 for 40 °C to 0.70 € L1 for 80 °C. 3.4. Scenario 2: Profitability of optimized SWHs in a scenario with specific demand By removing the constraint of having to use all the useful energy that the solar system can provide, profitability is analyzed realistically. The proposed scenario presents a daily demand of

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F.R. Mazarrón et al. / Energy Conversion and Management 113 (2016) 16–26 Table 3 Summary table of the calculation of total payments over the life of the installation with daily demand of 100 kW h, diesel boiler and optimized SWHs. Units

Diesel boiler Annual energy supply Annual payment diesel Full payment (20 years) updated at 4% SWHs + diesel boiler No. solar collectors 2 m2 Initial investment solar installation Energy supply by SWHs Energy supply by diesel boiler Annual payment electricity SWHs Annual payment diesel Total annual payment Full payment (20 years) updated at 4%

rTWT 40 °C

50 °C

60 °C

70 °C

80 °C

kW h year1 € year1 €

36,500 3802 45,988

36,500 3802 45,988

36,500 3802 45,988

36,500 3802 45,988

36,500 3802 45,988

units € kW h year1 kW h year1 € year1 € year1 € year1 €

9 18,000 24,143 12,357 143 1146 1289 35,506

10 20,000 25,510 10,990 161 1019 1180 36,032

10 20,000 24,710 11,790 156 1093 1249 36,975

11 22,000 25,139 11,361 169 1053 1222 38,611

11 22,000 24,590 11,910 166 1104 1270 39,266

Table 4 Profitability indicators of the optimized solar system for a scenario with daily demand of 100 kW h and a diesel boiler.

NPV IRR DPP

Units

rTWT 40 °C

50 °C

60 °C

70 °C

80 °C

€ % years

10,482 9.9 10.7

9956 9.1 11.5

9012 8.6 12.0

7377 7.5 13.3

6722 7.2 13.7

100 kW h, concentrated at the end of the day, covering this demand with a diesel boiler. The optimization of the sizing of the installation was carried out by calculating the total payment updated in 20 years to meet the demand, which includes the initial investment in the solar installation and annual cash flows—the sum of the payment in fuel plus the payment for electric energy consumption of the pump of the SWHs—for a total of 100 scenarios (from 1 to 100 2 m2 collectors). Fig. 9 shows the total payment for only the first 20 cases since the other data is not relevant due to skyrocketing costs. The total annual demand of the proposed installation amounts to 36,500 kW h year1. The number of 2 m2 solar collectors that minimizes the total updated payment ranges from 9 to 11, depending on the rTWT (Fig. 9). Although the initial investment is high—between 18,000 € and 22,000 €—a significant reduction in annual payments over the sole supply of diesel is achieved. The solar system provides between 66% and 70% of the demand. The energy supplied by the boiler is reduced to values close to 11,000–12,000 kW h year1, with a marked decline in diesel consumption of the installation (Table 3). Thus, the difference of updated total payments (20 years, r = 4%) would be reduced from approximately 46,000 € in the case of single supply by diesel, to values ranging between 35,500 € and 39,000 € with the combined supply, which represents reductions of between 23% and 15% compared to the expenditure with a single supply. The differences in values of annual payments obtained have been used as cash flow for the calculation of the profitability indicators. Table 4 summarizes the performance values obtained in the case studied. In view of the results, the investment in SWHs would be advisable at any rTWT, with NPV between values close to 6700 € at 80 °C and 10,500 € at 40 °C. The internal rate of return ranges from values close to 7% at 80 °C and 10% at 40 °C. The investment will be recovered in a period ranging between 10 and 13 years. It is found that by properly sizing the installation (considering the rTWT), the profitability values are very close to those obtained at the stage of maximum profitability (using 100% of useful energy).

4. Conclusions In this study the design, installation, and evaluation of an SWH with ETC and active circulation for hot water supply has been carried out. The main objective is to analyze how the rTWT affects the useful energy that the system is able to supply, and consequently its profitability. The results, based on experimental data, are related to the location of the solar facility in Madrid, Spain, where the aggregate annual irradiation in the level of the collector is close to 2000 kW h m2. The results show how the energy extracted by the ETC that is delivered to the tank decreases as the rTWT increases, due to losses in pipes and a performance drop of the collector. The average annual usable energy oscillates between 1351 kW h m2 at 40 °C and 1119 kW h m2 at 80 °C. Thus, the annual average values would reach efficiencies of 66%, 64%, 61%, 56%, and 55% for 40 °C, 50 °C, 60 °C, 70 °C, and 80 °C. In addition, given that the system performance is relatively constant throughout the year, the calculated regression equations could be useful to estimate the useful energy in other locations from the irradiation values. As a consequence of the reduction of useful energy, profitability decreases as the rTWT increases. In scenario 1 (SWHs as a complementary system of diesel boiler), differences are achieved in the NPV of approximately 300 € m2 and more than two years of payback going from 40 °C to 80 °C. However, the investment would be profitable at any rTWT as breakeven is reached at 880 kW h m2 year1. The profitability of the SWHs is conditioned by demand being spread throughout the day, or focused at the end of the day, like in many cleaning facilities after working hours. The useful energy, and, therefore, profitability will decrease if the demand is concentrated in the early hours of the day, as the energy that the system might capture in the last hours of the day is not used. From an environmental point of view, CO2 emissions can be reduced by between 392 and 325 kg CO2 per m2 of collector, depending on the temperature required. The sensitivity analysis performed allows analysis of the variations in profitability in less conservative scenarios. The breakeven point for the cost of the investment would range between 1600 € m2 for 40 °C and 1300 € m2 for 80 °C; the threshold for the discount rate from 10% for 40 °C and 7.2% for 80 °C; the threshold for the useful life of between 10.6 years for 40 °C and 13.7 years for 80 °C; the threshold price of diesel between 0.58 € L1 for 40 °C and 0.70 € L1 for 80 °C. The profitability of SWHs has also been evaluated considering a facility with a specific demand and diesel boiler, in which not all

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the energy that the SWHs can supply is exploited (scenario 2). The analysis shows that through proper sizing of SWHs and through optimizing the number of solar collectors, the investment in the solar system can be profitable, with a combined delivery system savings of between 23% and 15% compared to a single system using diesel. The number of collectors that maximizes profitability depends on the rTWT; so in the design of this type of installation, the rTWT requirements must always be considered. It is found that by correctly sizing the installation, profitability values are very close to those obtained in the stage of maximum profitability. The results of this study can be useful in determining the feasibility of using similar systems to supply the demand of hot water. In each case, energy efficiency and profitability will be determined by the irradiation of the area, the rTWT, and the demand curve of the specific processes that are carried out.

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