Monitoring of solar cogenerative PVT power plants: Overview and a practical example

Sustainable Energy Technologies and Assessments 10 (2015) 90–101

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Sustainable Energy Technologies and Assessments journal homepage: www.elsevier.com/locate/seta

Original Research Article

Monitoring of solar cogenerative PVT power plants: Overview and a practical example G.M. Tina a,⇑, A.D. Grasso a, A. Gagliano b a b

Dipartimento di Ingegneria Elettrica Elettronica e Informatica, University of Catania, Viale A. Doria n.6, Catania 95125, Italy Dipartimento d’Ingegneria Industriale, University of Catania, Viale A. Doria n.6, Catania 95125, Italy

a r t i c l e

i n f o

Article history: Received 11 October 2014 Revised 20 March 2015 Accepted 22 March 2015

Keywords: Monitoring system Photovoltaic-thermal system Stand-alone system Energy efficiency

a b s t r a c t This paper introduces the topic of management and monitoring of photovoltaic/thermal (PVT) systems, highlighting the differences respect to photovoltaic (PV) and solar thermal (ST) systems. A PVT module is a collector that produces at the same time both electrical and thermal energy. Thereby, the monitoring architecture of a non concentrating PVT system, realised with crystalline silicon cells liquid-cooled, is described. In addition, a web application allows the on-line monitoring and control of that PVT system. Thus, daily reports of the remote PVT installations are elaborated and sent to the system operator. These data can be analysed off-line to calculate both energy performance indices and statistical values. Historical data analysis is useful not only to optimize the operation of the PVT system but also to design its retrofit. To check the effectiveness of the proposed remote monitoring system, the performances of two twin stand-alone systems a PVT plant, based on a patented PVT collector named TESPI, and a PV plant have been evaluated through an experimental campaign of measurements. The two twin systems (one PV and the other PVT) are installed in Enna (Italy), where typical Mediterranean climate is present. During the survey both electrical and thermal critical operating conditions have been detected such as: deep discharge of batteries, optical losses and stagnation. Ó 2015 Elsevier Ltd. All rights reserved.

Introduction In the last years, renewable energy sources are continuing to grow robustly all around the world, albeit from a low base, but not uniformly, in fact the importance of policy in the strength of renewable forms of energy in different countries has affected relevantly. In 2013 renewables account for more than 5% of global power output and nearly 3% of primary energy consumption. The challenge of sustaining expensive subsidy regimes, however, has become visible where penetration rates are highest, namely the below-average growth of Europe’s leading renewable producers, who are grappling with weak economic growth and strained budgets [1,2]. Specifically, in the EU renewables represented the majority of new electric generating capacity for the sixth consecutive years. In this dynamic context, the possibility of building a solar hybrid system merging thermal and PV modules presents a strong practical interest [3–9]. The integration of the two systems increases the global efficiency (the efficiency of a PV module is

⇑ Corresponding author. E-mail address: [email protected] (G.M. Tina). http://dx.doi.org/10.1016/j.seta.2015.03.007 2213-1388/Ó 2015 Elsevier Ltd. All rights reserved.

10–20% whereas a common flat plate collector can harvest up to 60% of the sun’s energy) exploiting the same available surface and a reduction of the costs (compared with two separated systems that produced the same global energy). A solar hybrid collector (PVT hereafter) typically consists of a PV module on the back of which an absorber plate (a heat extraction device) is attached. The purpose of the absorber plate is twofold. Firstly, to reduce the heating of the PV module, thus improving its electrical performance, and secondly for harnessing the thermal energy produced, which otherwise would be lost into the outdoor environment. The main parameters that affect the global PVT performance are the number of covers, the shape of the absorber plate and the operative design configurations (e.g. connections with thermal storage, mass flow rate, design temperature and so on). Anyway, the role of the fluid temperature is fundamental. Indeed, the domestic hot water (DHW) production requires thermal levels at least of 4050 °C that surely penalize the electric performance of the PV cells. On the contrary, lower temperatures would enable higher electrical efficiency but also would significantly reduce the thermodynamic quality of the recovered heat. The thermal hybrid collectors faces two main problems: (1) a high thermal efficiency requires good thermal insulation that unavoidably increases the

G.M. Tina et al. / Sustainable Energy Technologies and Assessments 10 (2015) 90–101

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Nomenclature Ap Apv Cpw DET Isol mws Pel Pref Pth Ta Tc Tref Tout

surface area of the absorber plate, m2 surface area of the PV module, m2 specific heat of the fluid under constant pressure, J/kg K difference of primary energy, kWh solar irradiation power, W/m2 mass flow rate, kg/s electric power, W nominal electric power, W thermal power, W ambient temperature, °C cell temperature, °C standard reference temperature, 25 °C water outlet temperature, °C

PV cell temperature, as a consequence, the thermal drift decreases the conversion efficiency of PV system and (2) the heat transfer to the thermal fluid vector can be hindered by the presence of the PV cells. Over the last 35 years, a large amount of research on PVT collectors has been carried out [7,8]. Limiting our analysis to PVT collectors that use liquid heat-transfer media, the main technique is to glue either PV cells or an entire commercial PV laminate to the absorber of a commercial thermal collector. The drawback of gluing PV cells is the fact that the PV will not be sufficiently protected from the ambient (in particular from moisture), which makes this technique problematic for commercial application. In addition, problems may result due to insufficient electrical insulation. The hybrid electrical and thermal energy generation implies a tradeoff between the electrical and the thermal energy production. An appropriate monitoring and control system allows an assessment of such a tradeoff. Although data on PVT tested in laboratories are available, experimental tested on PVT plants are nearly missing: only a few PVT systems have been tested under real operating condition and for a relatively long times. Further, nowadays, the web can tell you the instantaneous watt output of an individual PV module, so it shouldn’t be too hard to do something similar for solar thermal energy, but it is surely harder to monitor a PVT due to the interaction of both electrical and thermal parts [9,10]. What follows is an appeal to advance toward modern and efficient PVT systems by implementing monitoring and control technologies. This will be the key to the continued development of PVT as one of the most effective means of displacing fossil fuel use, driven in part by the prospect of monetizing electrical and thermal output. The paper is organized in three main sections (excluding introduction and conclusions): in the first section, entitled solar technology diffusion, some figures about the increase of solar installations in Europe are reported and discussed. The aim is to prove the rapid and not uniform spread that such systems can cause, in an urban or suburban context, to a competition in the exploitation of the sites that have the best solar potential. The second section, entitled Monitoring systems for solar power plants, develops an overview of the main features, standards and references of PV, ST and PVT systems. This section has been divided in three subsections each treating such solar systems. The aim is also to highlight the overlapping points and differences of monitoring strategies applied to PV, ST and PVT systems. In this context the different definitions of efficiency for PVT systems are remarked. Finally, in the third section, named Experimental PVT power plant, a modular and general purpose monitoring and control system architecture developed for a PVT system is presented. To check

Tin Tred

water inlet temperature, °C reduced temperature, °C W1 m2

Greek symbol b power temperature coefficient, °C1 gel electric efficiency gth thermal efficiency gpower electric power generation efficiency gPVT overall energetic efficiency of a PVT module gIIPVT efficiency of a PVT module according 2nd law of Thermodynamics gex exergy efficiency of PVT module PVT

the effectiveness of the proposed remote monitoring system, the performance of an experimental stand-alone PVT power plant based on a patented PVT collector named TESPI [11] (Thermal Electric Solar Panel Integration) has been evaluated.

Solar technology diffusion In 2012, the International Energy Agency (IEA) evaluated an installed capacity of 269.3 GWth, which corresponds of about 384.7 million square meters of collector area, installed in 58 countries (an estimated 95% of the worldwide market). The vast majority of the total capacity in operation was installed in China (180.4 GWth) and Europe (42.8 GWth) [12]. Of this, 88.3% comprised flat-plate collectors (FPC) and evacuated tube collectors (ETC), 11% unglazed water collectors and 0.7% glazed and unglazed air collectors [13]. The estimated total capacity of solar thermal collectors in operation worldwide by the end of 2013 is 330 GWth, or 471 million square meters of collector area. This corresponds to an annual collector yield of 281 TWh [12]. Compared with other forms of renewable energy, the contribution of solar heating in meeting global energy demand is, besides the traditional renewable energies, like biomass and hydropower, second only to wind power. Moreover, solar thermal is the leader in installed capacity. In 2013, the solar thermal sector registered a slowdown with installed collector surface of 3,027,532 m2, i.e. 13.2% less than in 2012 [13]. The solar thermal market contraction was particularly serious in the key European markets: Germany (11.1%), Italy (18.2%), France (19.1%), Greece (13.1%), Austria (13.5%) and Portugal (37.0%). The cumulated solar thermal collector surface area in service in the European Union was about 44.8 million square meters at the end of 2013, equating to 31.4 GWth of capacity [14]. Germany (17,222,000 m2), Austria (5,045,000 m2), Greece (4,164,025 m2) and Italy (3,700,000 m2) are the four countries with the highest collector surface area in service. Using the per capita surface indicator, Cyprus is the European country leader with 0.787 m2/p.c. ahead of Austria (0.598 m2/p.c.), Greece (0.376 m2/p.c.) and Germany (0.214 m2/p.c.). Italy is in twelfth position with 0.062 m2/p.c. The solar thermal operators underline that the solar thermal technology has become less fashionable because its return on investment time is seen very unfavorably when compared with that of other renewable energy sources, especially respect to the PV technology. Another limit to a higher expansion of the solar market is that the public information and recommendation campaigns on heating systems are not high on the public agenda. In this context, it is interesting to underline that the heat production from the solar thermal sector reached

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2.0 Mtoe in 2013. It is 2245 ktoe just the 30% of the NREAP (National Renewable Energy Action Plans) 2020 target (6,348 ktoe). Deployment of technical potential is mainly limited by land and/or roof space availability and by the proximity of heating and cooling demand. The Energy Technologies Perspectives ETP 2012 2DS scenario [14] envisages development and deployment of solar collectors for hot water and space heating in buildings that could reach an installed capacity of nearly 3500 GWth, satisfying annually around 8.9 EJ of energy demand for hot water and space heating in the building sector by 2050. In this vision, solar hot water and space heating in buildings will annually increase by 7.1% on average between 2010 and 2050, while the total energy used for water and space heating will increase only by 1.3% (or 0.8 EJ). As regards the PV technologies, the 24 IEA–PVPS countries represented 123.2 GW of cumulative PV installations together, mostly grid-connected, at the end of 2013. Additional countries that are not part of the PVPS program represent at least 10.8 additional GW, mostly in n Europe. At present, it seems that 134 GW represents the minimum installed by end 2013 with a firm level of certainty. Remaining installations account for some additional GW installed in the rest of world (non-reporting countries, off-grid installations, etc.) that could bring the total installed capacity to more than 136 GW in total [15]. Whereas in [16] EPIA evaluated by 2013, almost 138.9 GW of globally installed PV capacity, an amount capable of producing at least 160 TWh of electricity very year. Furthermore, Europe remains the world’s leading region in terms of cumulative installed capacity, with 81.5 GW as for 2013; this represents about 59% of the world’s cumulative capacity.

Monitoring systems for solar power plants There are many reasons to monitor a system as relatively expensive and long-term as a solar installation, such as: to follow up on the energy yield, assess the solar system performance and timely identify design flaws or malfunctions. These needs for monitoring fall into three main groups: user feedback, performance verification, system evaluation/diagnostic.

The monitoring of solar plants should: work permanently and fully automated, calculate standardized key parameters; detect system failures and malfunctions as quickly and as precisely as possible; be as cost efficient as possible; offer optimal support to service and maintenance activities; analyze all the system parts that are relevant for the solar plant; adapt to the available measurement equipment. An integrated control/monitoring system is the most powerful tool available to efficiently manage PV and solar thermal operations. Once the domain of expensive industrial supervisory control and data acquisition (SCADA) systems, these capabilities are now available in lower-cost, easy-to-use controllers. It has to be stressed the importance of real-time, live interaction with the PV and ST systems, with a complete and remotely accessible view of device status and history. From the monitoring point of view, a PVT plant can be viewed as two separate systems: a PV system and an ST thermal system. In the following the main features of the two separated systems (PV and ST) are summarized and then the relevant characteristics of a monitoring system for a hybrid PVT system are drawn.

Photovoltaic systems Common reference documents for monitoring of PV systems are the standard IEC 61724 [17], entitled ‘‘Photovoltaic system performance monitoring – Guidelines for measurement, data exchange and analysis’’, and the guidelines of the European Joint Research Centre in Ispra, Italy [18,19]. From the parameters listed in [17– 19], those concerning grid-connected PV systems are summarized below in Table 1. The required accuracies and check procedures for data quality are detailed in [17–19]. Careful considerations should be given to the purpose behind the monitoring before developing a specification. The ethos should be to measure only those variables that are necessary using the minimum acquisition rate required to get meaningful results for a period of time over which new information will be produced. Guidelines reported in [17] recommend that parameters, which vary directly with irradiance, shall be sampled with 1 min or less interval. Parameters with larger time

Table 1 Parameters of the PV plants to be measured according to [17–19].

Ambient

PV array

Utility grid

Load

Energy storage

Parameter

Unit

Uncertainty

Notes

In-plane irradiance

W/m2

<5%

Air temperature Wind speed

°C m/s

Output voltage Output current Output power

V A W

<1 °C <0.5 m/s for Speeds < 5 m/s, and < 10% of the reading for Speeds > 5 m/s <1% <1% <2%

Use pryanometers if performance is compared with Energy Yield Prediction. Use reference cells for issues related to plant STC power and to inverter. Wide range of uncertainties depending on calibration (indoors using a sphere vs. outdoors, how mounted, and if corrections for temperature and angular response applied) Optional, useful in case of failure detection Optional, useful in case of failure detection

Module temperature

°C

<1 °C

Utility Voltage Phase Current to/from grid Power to/from grid Load voltage Load current Load power

V V W V A W

<1% <1% <2% <1% <1% <2%

Operating voltage Current to/from storage

V A

<1% <1%

Power to/from storage

W

<2%

DC power calculated based upon instantaneous and not averaged readings or directly measured with wattmeter Measured on back of 1 or more modules in representative of location. Fix sensor in the center of the module behind one solar cell

AC power accounts for power factor and harmonic distortion

DC power calculated based upon instantaneous and not averaged readings or directly measured with wattmeter AC power accounts for power factor and harmonic distortion DC power calculated based upon instantaneous and not averaged readings or directly measured with wattmeter AC power accounts for power factor and harmonic distortion

G.M. Tina et al. / Sustainable Energy Technologies and Assessments 10 (2015) 90–101

constants, an arbitrary interval may be specified between 1 min and 10 min. Special consideration for increasing the sampling frequency shall be given to any parameters, which may change quickly as a function of system load. However, with the cost of hardware and storage decreasing there is no reason to avoid 1min (or sub-minute) data collection. In the case of utility-scale PV plants, monitoring typically serves for comparison of the current plant performance with an initial energy yield assessment. In order to be able to distinguish the performance of the PV system from the variability of the solar resource, monitoring should always include both a measurement of the energy generated and the incoming irradiation. For electricity yield measurements, energy meters or true-rms power meters should be used. The inverter-integrated measurements are usually not sufficiently precise [20]. Nevertheless, they may prove useful for identifying relative changes over time [21]. For more advanced monitoring the power or current on the junction box level or the string currents should be measured. The additional cost for advanced monitoring depends on the PV plant layout and capacity. According to current standards [17,18], the in-plane irradiance should be measured with a crystalline silicon reference device, which should be calibrated and maintained in accordance with IEC 60904-2 or IEC60904-6. The use of silicon reference devices for PV performance evaluation can lead to uncertainties that cannot easily be quantified in utility-scale PV plants, the use of a thermopile pyranometer is recommended [20]. The main drawback of a thermopile pyranometer is its slow response, which is in the range of 5–30 s. Therefore, they react to changing irradiance conditions much more slowly than the PV modules [21]. While, this effect is negligible in the monitoring of utility scale PV plant, for small PV systems photodiode pyranometers are a better (and much cheaper) solution. In order to reduce the uncertainty of the measurement, either a first class or a secondary standard pyranometer should be installed and in any case, it should be asked for a traceable calibration and the associated calibration certificate. The sensor should be installed in a place were no near or far shading can affect the measurement, even if parts of the plant are affected by shading. If two sensors are installed and constantly compared, a recalibration every two years is reasonable. If only one sensor is installed, a yearly recalibration should be considered [20,21]. As part of the European Commission-funded project PERFORMANCE, new PV monitoring guidelines have been recently developed in order to take into account the system performance over its lifetime [22,23]. Based on these new guidelines, a failure detection routine (FDR) for comparing the monitored energy yield with the simulated one for a given period was presented in [24] and [25]. For this method, failure patterns for 12 characteristic failures have been pre-defined. Another example of automatic failure detection from PV monitoring data is the so-called sophisticated verification method [26]. This method allows identifying six kinds of system losses using basic information and four simple quantities to be measured. In the last years many progress have been made in this field, consequently a second revision of IEC 61724 is under development. The main changes are: – monitoring system classification introduced: user may select classification (A, B, or C) according to PV project size (Utilityscale, Commercial scale, residential and small commercial) or monitoring objectives (System performance assessment, Documentation of a performance guarantee, Forecasting performance, Electricity network interaction assessment, Monitoring integration of distributed generation, storage, & loads, System losses analysis, PV technology assessment, PV system degradation measurement); – measurement parameters and sensor requirements to be specified according to monitoring system class.

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– new measured parameters include additional irradiance values, soiling (soiling ratio is the ratio of PV array output power to the power that would be obtained if the PV array were clean and free of soiling), and power quality. – new metrics include temperature-corrected performance ratios. – addressing curtailment & clipping. Inverter clipping is often considered a loss of the system due to design limitations. But many systems now intentionally designed with high DC/AC ratio for more stable output to grid. For these system types, considering additional performance metric based on system AC power rating instead of DC rating. Periods of reduced grid/load demand or availability should not count against PV system performance. Standard notes that irradiation and yield sums should be calculated with such periods excluded for purposes of performance assessments and performance guarantees (while still documenting complete sums). In case of stand-alone PV systems where there is an energy storage system (normally batteries), more parameters have to be monitored, specifically related to the batteries. In this context there are specific guidelines [27] written by the IEA. Equitable assessment of the performance of ‘‘Stand Alone Systems’’ is complicated by system configuration options, application possibilities and cultural variables [28]. The guidelines recommend individual sets of performance parameters that reflect the use for which the data might be intended.

Solar thermal systems The design of the monitoring system of a solar thermal plant must preliminarily define the hydraulic scheme, the measurement equipment and the control of the system. Once the hydraulic configuration has been defined, the sensors for data logging, the algorithms for data analysis, failure detection and failure identification can be defined. The performance of a solar system can be monitored by temperature sensors that measures the heat being added to the heat storage, the solar collector temperature and the tank temperature. Moreover, current transducers measure when the circulation pump is running, as well as when the auxiliary heating is on. An example of monitoring system is reported in [29,30]. Larger solar thermal applications are becoming more prevalent, as power purchase agreement (PPA), models are being applied to thermal output, and states are beginning to incorporate renewable thermal energy in their renewable portfolio standards (RPS). Accurate and reliable monitoring of thermal output is a basic requirement for larger commercial systems, particularly those that benefit of Renewable Heat Incentive (RHI) for the generation of renewable forms of heat. However, controlling and balancing the flow of energy is of utmost importance for cost conscious users. These systems must adhere to higher standards, and industry has recognized that greater credibility will come with a common standard that addresses heat metering in fact, financial institutions will demand it. The current European Standards for heat metering, BS EN 1434-1 to 6, covers the general performance requirements and operational characteristics for heat meter equipment and instrumentation to measure the heat that is either absorbed or released by a heat conveying liquid across a heat exchange circuit. ASTM International has organized subcommittee E44.25 to address this need, and many control/monitoring-system providers and instrument manufacturers are actively involved in the standard’s development, to be released in 2014. These refer to the general requirements, data exchange, installation, commissioning, operational monitoring and maintenance of heat meters.

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Promising new approaches to heat metering are being developed (e.g., enthalpy-based measurement of storage volumes), but the most common form of heat metering employs data from a fluid flow meter and two matched temperature sensors (positioned before and after the heat exchanger) to calculate a heat transfer rate. While there is quite a variety of flow metering technology available over a wide range of accuracy specifications (and cost), some basic principles that should be adhered to for the proper measurement of heat produced can be highlighted. Accurate temperature measurements require excellent heat transfer between the fluid and the temperature sensors that must be suitable for the specific application. Thermistors and resistance temperature detectors (RTDs) are commonly used for this purpose or, active silicon sensors can be adopted alternatively. It is crucial that the sensors are placed near the point of use (immediately upstream and downstream of the heat exchanger) and must accurately represent the temperature of the heat-transfer fluid used. It is worth emphasizing that temperature sensors are placed in thermal wells immersed in the fluid at a location where there is turbulent flow. The use of thermal paste insures a good heat transfer. Moreover, since in large pipes it is difficult to accurately measure low flow rates and, the turbulence can throw things off, it is necessary: verify that the flow meter works in the range expected, reduce the pipe sizing in the vicinity of the flow meter, and include in calculation the precise characteristics of the heat-transfer fluid. Table 2 reports the technical data of commercial heat meters [31]. PVT systems The performance analysis of a PVT system raises a number of issues that do not appear in the separate monitoring of solar thermal and PV systems. Indeed, the thermal yield is influenced by the electrical yield; a high PV performance lowers the amount of energy available for conversion to thermal energy. Thereby, the monitoring systems should be able to detect the efficiency of both the electrical and thermal component of the co-generative system. A high thermal efficiency requires a good solar absorption and thermal insulation and a very effective heat transfer from the absorber to the fluid. Commonly, the overall performance of a PVT system could be evaluated in a simplistic way by the sum of the electrical energy efficiency and thermal energy efficiency. This approach can lead to a coarse mistake since it does not take into account the different quality of the electric and thermal energy, as stated by the Second Law of Thermodynamics. Indeed, while electric energy is pure exergy, the thermal energy has an exergy content that depends on the temperature at which the heat is available. This issue is particularly relevant in PVT systems, where the energy yield is highly influenced by the operative temperature of the system. The electric and the thermal power produced by a PVT are defined as:

Pel ¼ Pref ½1  bðT c  T ref Þ

ð1Þ

Table 2 Technical data of commercial heat meters. Max flow Pipe rate qmax size (mm) (m3/h)

Nominal flow rate qp (m3/h)

Min flow rate qmini (m3/h)

Max temp. hmax (°C)

Min Min temp. temp. difference hmin (°C) Dhmax (°C)

15 15 20

0.6 1.5 2.5

0.006 0.01 0.02

105 105 105

15 15 15

1.2 3 5

3 3 3

Pth ¼ mws Ap C pw ðT out  T in Þ

ð2Þ

The thermal energy efficiency gth and the electric energy efficiency gel of the collector are defined as follows:

gel ¼

Pel Isol Apv

ð3Þ

gth ¼

Pth Isol Ap

ð4Þ

when a PVT module is considered the Apv and Ap are the same, that is APVT = Apv = Ap. Some studies [32,33] propose an overall energetic efficiency, gPVT, for evaluating PVT systems given by

gPVT ¼

Pth þ Pel ¼ gth þ gel Isol APVT

ð5Þ

Thereby, this overall efficiency is founded only on the First Law of Thermodynamics without any reference to the thermodynamic quality of the two forms of energy, thermal and electric. Otherwise, the approach of the Second Law of Thermodynamics allows a proper evaluation of the quality of each energy flux. Thus the overall performance of the system may be presented either in term of energy or exergy efficiency. The net energy output of the system is calculated converting the electrical output into equivalent thermal energy and added to the thermal output, whereas the net exergy is calculated converting the thermal energy into equivalent electrical energy, using the Carnot theorem, and then adding it to the electrical energy of the system. Since the electrical energy is more valuable than thermal energy, the different quality of the thermal and electrical energy must be taken into account when the overall efficiency of the PVT collector is calculated [34]. II PVT

g

gel

¼

gpower

! þ gth

ð6Þ

PVT

Furthermore, the exergy efficiency is calculated as follows:

gex PVT ¼ gel þ CF  gpower

ð7Þ

where gpower is the electric power generation efficiency for a conventional power plant and CF is Carnot Coefficient [11]. Therefore, the comparison between the performance of a PVT and a traditional PV collector has to be more correctly proposed, taking into account the difference between the primary energy produced by the two systems

DET ¼

Z 0

("

t

APVT

gel gpower

!

# þ gth  PVT

"

gel gpower

! #)  Isol dt

ð8Þ

PV

Since the thermal energy has an exergy content that depends on the temperature at which the thermal output is available, the control system must optimizes both the energy and exergy fluxes. In this way, the variations on the electric efficiency of the PVT collector due to the temperature of the thermal circuit can be carefully evaluated. As regards the electrical efficiency, it depends on the cell temperature (Tc), whereas the thermal efficiency, expressed by means of Bliss equation [35] depends on the ‘‘reduced’’ temperature (Tred), which is defined as:

T red ¼ ðT in  T a Þ=Isol

ð9Þ

Thereby, as electrical and thermal efficiencies are function of two different temperatures it is quite, complex for more complex monitoring a PVT system. However, the thermal losses of conventional solar thermal collectors can be expressed as a function of the

G.M. Tina et al. / Sustainable Energy Technologies and Assessments 10 (2015) 90–101

absolute temperature (e.g. radiation and convection). Additional issues such as edge shading and the thermal resistance between collector fluid and PV cells may also affect the PVT performance. It should be desirable to identify a temperature for the evaluation of the energy efficiency without ambiguity. For PVT, no such temperature exists. The ambient temperature, which is often used in power matrix measurements, is not useful for PVT, since the PVT module temperature is largely determined by the temperature of the collector fluid. The collector fluid temperature and even the absorber rear temperature are suffering from the fact that a substantial temperature gradient may exist between the fluid, the rear and the actual PV cell temperature. This temperature gradient is not constant but depends on the thermal resistance between PV cells, the collector rear and the collector fluid and on the heat flow through the absorber, which is a function of thermal efficiency and irradiance. For sheet-and-tube absorbers, the PVT absorber rear temperature is normally available. Since it is the temperature that most closely resembles the PV cell one (more closely than e.g. the average collector fluid) and therefore allows a more accurate efficiency evaluation. For fully wetted absorbers it is not possible to measure between the tubes and the choice is to use the mean collector fluid temperature as the characteristic temperature. The choice is based on the fact that, among the temperatures that can be measured, this temperature will be closest to the cell temperature. However, in this case, the temperature difference between PV cell temperature and characteristic temperature will be larger than in the case of the sheet-and-tube absorbers, due to the additional thermal resistance of the encapsulant and adhesive layers. The thermal resistance of the channel wall and the channel-to-fluid resistance (especially when the flow is in laminar or turbulent regime) will increase the error of the performance measurements. Moreover, the temperature gradient between the fluid temperature and PV cell temperature depends also on the flow rate of the collector fluid. But, since the flow rate also influences the thermal efficiency, the thermal performance evaluation depend on the flow rate anyway, so this does not create additional problems. One should be aware that the mean collector fluid temperature is determined routinely and unambiguously in the thermal efficiency measurements and that it is also the input used for the thermal annual yield predictions. Moreover, it is very important to avoid situations that can be dangerous for the reliability of the hybrid collector. More specifically, PVT collectors are particularly sensitive to hot spots, since the insulation increases any temperature problems that may occur. In addition, if a hot spot occurs in combination with stagnation conditions, the resulting temperatures can be very high; the temperature may get substantially above the hot-spot temperatures that may occur in a conventional PV laminate.

95

As regards the in-plane irradiance, it can be measured with a pyranometer or with a reference cell that matches the PV-cells of the PVT-module. The EN-12975-2 prescribes a pyranometer for the measurement of the solar irradiance, while for PV testing; the IEC 61215 prescribes a PV reference device. The main aspects in which a reference cell differs from a pyranometer are the spectral sensitivity and the somewhat reduced angle of view. However these two effects will only have a small impact for c-Si cells, since for such cells the performance is largely independent of spectral effects. An additional source of error is introduced if the solar radiation is measured with two different devices for the assessment of the PV performance and the solar thermal performance, especially since the thermal and the electrical performance affect each other. In [9] it is recommended that a pyranometer should be applied for the irradiance measurements on the PVT collector, both for the thermal and the electrical measurements. Experimental PVT power plant The basic idea to design this experimental system is to perform a comparative analysis between a PVT system and a PV system with the same electrical characteristics; to this aim two almost twin stand-alone PV systems have been realized (system A and B) and installed in Enna, Italy (Lat.37.55 N, Long.14.29 E). The two PV strings are south oriented and with 30° tilt angle. In system A (TESPI system) each PV module has been covered by a polycarbonate box where the cooling water flows (Fig. 1). Fig. 2 shows the block diagram of the implemented PVT system (A), where the name and main characteristics of components that belong to the hydraulic and electrical loops are reported. The experimental setup comprises a reference test plant made up by three series-connected 240-W conventional PV modules and a TESPI-based plant, where the cooling fluid flows, as detailed in Fig. 3(a) and (b). The polycarbonate box has the following dimensions (the area is the same of the PV module): 1640  992  40 mm. The total weight of the TESPI panel is 59 kg, so an additional weight of about 35 kg of water is added. This additional weight is perfectly compatible with the mechanical characteristics of the PV module and of their supports. The depth of the layer of water is about 2.5 cm. In this condition, as reported in [11], the water layer absorbs the infrared radiation leaving the visible part almost unaffected and, consequently, good global thermal-electric efficiency can be obtained. This preliminary study has been focused to the monitoring of the variation of the water temperature between the inlet and outlet section of the TESPI system. The output of both the PV and the PVT system is connected to a bank of 180-Ah 12-V batteries through an OutBack FLEXmax 60

Fig. 1. Pictures of the experimental plant: (a) outdoor set-up: A – PVT (TESPI); and B – PV reference; (b) indoor set-up.

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Peristalc pump

Water tank

Watson Marlow 323 Thermal panel (TESPI)

Thermal panel (TESPI)

Thermal panel (TESPI)

PV module (UP-M240P 240 W

PV module (UP-M240P 240 W

PV module (UP-M240P 240 W

Load Lamp: Steca Solsum ESL E27 12 V 10*11 W

Baery charger OutBack: FLExmax60 – 12 V

Baery pack VANTAG 12V/80Ah 2*80Ah 640Ah

Fig. 2. Block diagram of a stand-alone TESPI PVT system.

Fig. 3. (a) Drawing of the polycarbonate box; (b) drawing of the TESPI panel.

MPPT charge regulator [36]. Configurable dummy loads are connected in parallel to the batteries in order to force the plants to work at the maximum power point (MPP). Table 3 summarizes the characteristics of the main components of the pilot PVT plant. The acquisition system architecture relies on the same basic concept already developed by the authors in [37]. The list and description of the variables acquired are reported in Table 4. The data acquisition board is the ET-7017-10 produced by ICPDAS [38]. It comprises 20 analog single-ended channels with programmable input range. Currents are measured trough LEM LA100-P closed-loop Hall-effect transducers [39]. Temperatures are acquired by using National Semiconductors’ LM35 precision integrated-circuit sensors [40]. In particular, sensors with TO-220

Table 3 Electrical characteristic of the experimental system. PV module: polycrystalline type, 60 cells in series, cell area156 mm 156mm Voc; Vm 37.6; 30.2 V Isc; Im 8.40; 7.95 A Pmax (tolerance) 240 (±3%) Wp NOCT 45 ± 2 °C Temperature coefficient of ISC; Im 0.05 ± 0.01; 0.02 ± 0.02 %/°C Temperature coefficient of Vm; Voc 0.42 ± 0.03; 0.32 ± 0.02 %/°C Temperature coefficient of Pm 0.43 ± 0.05 %/°C Battery charger Nominal battery voltage Maximum output current Maximum solar array STC nameplate PV open circuit voltage (Voc) Standby power consumption Power conversion efficiency (60 A @ 48 V dc) Battery Nominal voltage Capacity Load (2–4–57–8–10 lamps in parallel) Nominal power Nominal voltage

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package are exploited for measuring ambient and PV modules temperature, whereas sensor with TO-92 package are used to get the inlet and outlet water temperature. The water flow is set at a given value in the pump. Voltages are directly coupled to the acquisition board trough precision voltage dividers. The Honeywell HIH-4030 sensor measures relative humidity [41]. Lastly, a Davis solar radiation sensor [42] measures the in-plane irradiance. The dummy loads is made up of 10 lamps of 11 W each, which can be selectively paralleled by means of six relays, implemented through an ET-7060 board [43]. More specifically, the lamps have divided in four groups. One group of two lamps is always on, whereas the other three groups can be combined to obtain six load steps (2, 4, 5, 7, 8 and 10 lamps). In the PV system, the lamps have been divided in three groups, made of, respectively; 2, 3 and 5 lamps. Another relay is finally exploited to control the peristaltic pump. The information provided by the data acquisition board is acquired by a web-based application, whose conceptual block diagram is shown in Fig. 4 [37].

Table 4 List of measured variables. IPVpanelA IPVpanelB IbattA IbattB IloadA IloadB VPVpanelA VPVpanelB TA1 TA2 TA3 TB Tin Tout Tamb Irr

PV panels output current of TESPI test plant PV panels output current of reference test plant Battery current of TESPI test plant Battery current of reference test plant Dummy load current of TESPI test plant Dummy load current of reference test plant PV panels output voltage of TESPI test plant PV panels output voltage of reference test plant Temperature of TESPI module 1 Temperature of TESPI module 2 Temperature of TESPI module 3 Temperature of center module of reference test plant Temperature of water incoming into TESPI plant Temperature of water outgoing from TESPI plant Ambient temperature Solar irradiance

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Fig. 4. Block diagram of the software data acquisition setup.

The system can be interfaced to any device through the standard MODBUS TCP/IP protocol. This protocol was adopted for several reasons. First of all, MODBUS it is a royalty-free standard communication protocol widely adopted to connect industrial electronic devices. In modern electronic systems, the Ethernet is one of the most widely used communication protocol. Its main advantages are reliability, speed and good immunity to noise and electromagnetic interference. Moreover, the adoption of the TCP/ IP version of MODBUS protocol allows the connection of practical unlimited number of masters and slaves over local networks and/ or the World Wide Web. Among the main features, the software is capable to: – – – –

store data from the individual sensors on a database plot stored data show the captured data in real time carry out operations on aggregate data

– generate periodic graphical reports and/or using a text file and send it via e-mail – generate e-mail alerts and alarms – give remote access to the database via a web browser. The software system allows the connection of several data acquisition board, thus allowing the measurement of a huge number of parameters. Therefore, several devices can be monitored at the same time, even located in different places. Visualization of real-time and stored data is implemented by exploiting Ajax technology. The system allows the creation of charts of historical information. Charts are updated in real time by retrieving data from the server in background without having to refresh the browser window. Moreover, the system is capable of displaying acquired data through a graphical representation, thus simplifying the real time monitoring task [35]. Fig. 5 shows a screenshot of the implemented dashboard to monitor the system in real time.

Fig. 5. Block diagram of the experimental setup and monitoring system.

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The monitoring system allows two types of measured data analysis, specifically: on-line (web-based analysis) and post-processing. The on-line monitoring system is able to produce graphs showing any combination the directly measured variables stored in the database within an arbitrary time interval. Fig. 10 shows three examples of charts that the user can visualize on the web site. In this case, the time interval is six days. Based on the analysis of these graphs some remarks can be drawn.

In Fig. 6(a) irradiance (W/m2), ambient temperature and PV module temperatures (°C) are compared over six days. It is worth noting that all the variables can be represented on the same plot using the same y-axis. Consequently, in order to facilitate the comparison, the irradiance is divided by 20. It is apparent that the temperature of the PV module A2 in the PVT module, is lagged respect to the irradiance. This is due to the thermal capacity of the water which causes a slower dynamic in the temperature change.

Fig. 6. Examples of graphs provided by the web-based monitoring system; (a) irradiance, ambient temperature and PV module temperatures; (b) currents of the system A (PVT system); (c) battery voltages.

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Whereas the temperature of the PV module in system B (only PV) is in phase with the irradiance, as the thermal constant time is about five minutes. It also can be noted that the ambient temperature shows the classical sinusoidal variation, with the exception of the anomalous peaks of temperatures appearing at the sunset. This happens because the solar radiation directly strikes the temperature sensor (the sensor has to be moved in a more suitable place). Moreover, during the last three days the temperature of module A2 increases significantly, of about 10 °C, indicating a variation of the regime of functioning close to the stagnation conditions. In Fig. 6(b) the measured currents flowing in the battery, the load and the PV string, respectively, of the reference PV plant A are compared. This balance is useful to understand how the energy is used. During the night the load current and the battery current are equal and opposite (load and battery are in parallel). During the day, there is also the contribution of the PV generation. In this case, the currents cannot be directly compared as the voltage on the PV module is about 80 V whereas the voltage on the batteries (and load) is about 12 V. The comparative analysis of Fig. 6(b) and (c) highlights the problem of the inadequate capacity of the batteries that at about midday (in a clear day) reach the full charge, and so the PV charge regulator cannot follow the maximum power point of the PV module but limits the power to the one required by the load. In Fig. 6(c): the comparative analysis of the battery voltages allows pointing out the difference between the two daily balances of energy (positive for the PV system, negative for PVT system). In addition, the increase of the internal resistance in the system A indicates the poor performance of the battery of this system (batteries for automotive applications are used).

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the following where the data of a typical day have been taken into account. Fig. 7(a) shows a daily trend of the electrical power produced by the two system: A (PVT) and B (PV). It is appreciable that the string A produces more or less half power than the string B. This is mainly due to the reduced solar irradiance that strikes the PV cells of string A. In fact, the polycarbonate box and the water, which flow over the PV module, absorb not only the infrared part but also a portion of the visible part of the solar spectrum. But the main problem is due to the imperfect contact between the glass of the PV module and the polycarbonate surface of TESPI module (e.g. presence of condensation of moisture) that determines high optical losses. Fig. 7(b) shows the ratio between PA and PB from sunrise to sunset. In this interval of time (apart from transients), the ratio ranges from 0.5 to 0.8, except at about midday when PA/PB is greater than one. This is justified by the fact that the batteries of system A reach the full charge later than the ones of system B (due to the smaller energy produced by the PV modules of the string B). Finally, the electrical efficiency of the two strings were evaluated. The comparison of the electrical efficiencies, both measured and calculated values during a monitored day, of the two PV strings is shown in Fig. 8(b). It is worth noting that in the central part of the day (grey part of the graph), once again, the efficiency is very low for system B. Such low efficiency is due the operation point far from the maximum power point (batteries full charged), that is clear showed in Fig. 8 (a), when the battery is full the voltage on the PV module moves toward the open circuit voltage. Out of this region, the string B has an efficiency close to the theoretical one of a PV module (dashed green line), whereas the efficiency of the string A is halved compared to the theoretical one (dashed red line) mainly due to optical losses. Then, the monitoring system is able to give the following information on the basis of measured or calculated data, that are mass flow rate; difference of fluid temperature between inlet and outlet; the cumulative electrical energy produced by the PVT array EPVA and the PV array EPVB (see Fig. 9). Moreover three efficiencies of the PVT collector (system A) were calculated according to Eqs. 3–5. Which are respectively the thermal efficiency (gAth), the electrical efficiency (gAe) and, the overall efficiency (gPVT). Fig. 10 shows these efficiencies and the temperature drop of the thermal vector fluid under the following operative conditions: mws = 0.01 [l/s m2]; Cpw = 4.186 [kJ/kg k]; gpower = 45%. It is worth noting that the thermal efficiency of the PVT module is very high; this happens since the mean temperatures are not too high. Instead, the electrical efficiency of the PVT module is lower than the efficiency of the PV module.

In this paper it has been shown as the continuous expansion of installation of solar systems (thermal and electric) can create a completion in the exploiting of the surfaces apt to an optimal exposition of the collectors, especially in urban and suburban areas. A feasible solution is the use of hybrid PVT systems, which use collectors that produce heat and electricity at the same time. Generally speaking, monitoring of renewable non programmable energy systems is essential in order to guarantee reliable operation. Indeed, early recognition of failures or low efficiency operating conditions saves money and improves the reliability of renewable energy power supplies. In this context, the development of a modular and general purpose monitoring and control system architecture developed for solar systems (PV, ST and PVT) is presented. The proposed system relies on a web-based application, allowing the distribution of the data over the Internet to remote users. The main services provided to the user include control, monitoring, notification, reporting and data export. Specifically the monitoring of a PVT system requires many variables to be measured and analysed (thermal and electric) and depending on the solar system type, some variables cannot be measured directly (e.g., PV cell temperature). The on-line monitoring of an experimental PVT system, named TESPI has allowed detecting operation problems (soiling of PV modules, high resistance of the batteries, detaching of the temperature sensors, optical losses and so on). On the other hand, the post processing of measured data has been used to check the initial design and sizing of the system. As an example, the installation of another battery, or the substitution with stationary type batteries, can greatly enhance the energy performances of the studied PVT system. The collected data show that the replacement of the traditional solar collectors with PVT modules allows increasing the primary energy produced for unit of surface thus enabling a better use of the solar resource. However, it is necessary to accurately evaluate the enthalpy levels requested for each specific thermal application. Indeed, the PVT systems can surely satisfy the thermal levels required for low temperature utilisations (e.g. DHW, space heating, etc.) but they are not suitable in applications that require high temperature (e.g. steam production or solar cooling). References [1] BP Statistical Review of World Energy; June 2014. http://www. bp.com/content/dam/bp/pdf/Energy-economics/statistical-review-2014/BPstatistical-review-of-world-energy-2014-full-report.pdf. [2] Renewables 2014 Global status report. http://www.ren21.net/ ren21activities/globalstatusreport.aspx. [3] Kalogirou SA, Tripanagnostopoulos Y. Hybrid PV/T solar systems for domestic hot water and electricity production. Energy Convers Manage 2006;47(18): 3368–82. [4] Tripanagnostopoulos Y. Aspects and improvements of hybrid photovoltaic/ thermal solar energy systems. Sol Energy 2007;81(9):1117–31. [5] Maffezzoni P, Codecasa L, D’Amore D. Multi-physics analysis of a photovoltaic panel with a heat recovery system. In: Thermal Investigations of ICs and Systems, 2008. THERMINIC 2008, 14th International Workshop on, 24–26 Sept. 2008; 2008, p. 93–96. [6] Immovilli F, Bellini A, Bianchini C, Franceschini G. Solar trigeneration for residential applications, In: A feasible alternative to traditional microcogeneration and trigeneration plants, IEEE Industry Applications Society Annual Meeting, 2008. IAS ‘08, 5–9 Oct. 2008, p. 1–8. [7] Chow TT. A review on photovoltaic/thermal hybrid solar technology. Appl Energy 2010;87(2):365–79. [8] Zondag HA. Flat-plate PV-thermal collectors and systems: a review. Renew Sustain Energy Rev 2008;12:891–959.

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