Combined Solar and AC Mains Powered Lighting System

BOEKE Ulrich

Combined Solar and AC Mains Powered LED Lighting System Ulrich Boeke, Matthias Wendt, Lennart Yseboodt Philips Research Laboratories Europe High Tech Campus 37 5656AE Eindhoven, The Netherlands Tel.: +31-40-2794868 Fax: +31-40-2746276 E-Mail: [email protected] URL: www.research.philips.com

Keywords « Hybrid power integration » , « Photovoltaic », « Lighting, »

Abstract An experiment is presented that uses solar direct current (DC) in an AC mains powered 48 V DC power grid to reduce the AC power consumption of lighting systems in professional buildings. Measured annual AC mains power has been reduced by 15 % for lighting systems with a combined DC solar and AC mains supply compared with a pure AC mains supplied lighting reference system.

1. Introduction Prices of new solar systems and value of solar electricity have been continuously reduced by the great success of the German renewable energy law and other support schemes [1]. In parallel utility electricity rates increased in Germany by about 5 %/year. Figure 1 illustrates these two trends. A continuation of both trends will result in cost competitive solar electricity from 2013 - 2015 onwards for different types of utility customers such as consumers, small and medium professional customers. Consequentially it will become an interesting technical and commercial challenge how to use solar electricity in buildings most profitable e.g. by avoiding electricity purchasing for higher rates than by using solar feed-in tariffs. €ct/kWh

60

History Trend

50 40 30 20 10 0 2000

2005

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2015

Figure 1: Comparison of utility rates and feed-in tariffs for solar electricity in Germany Green line: Blue line: Red line: Orange line:

EPE 2011 - Birmingham

Feed-in tariff for new solar systems up to 30 kW in Germany RWE basic consumer utility tariff "RWE Klassik Strom" Utility tariff for small commercial customers (E < 0.1 Mio. kWh/a) Utility tariff for medium commercial customers (E~ 3 Mio. kWh/a)

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Combined Solar and AC Mains Powered Lighting System

BOEKE Ulrich

Most energy efficient professional buildings today use only electricity as energy carrier [2, 3]. Up to 90 % of all loads in these buildings make use of DC in end-applications such as lighting and variablespeed drives in freezers, air-conditioner, heating with heat-pumps and ventilation units. Since solar systems on roofs also generate DC it is straight forward to feed DC solar electricity with DC grids in DC powered applications. Hereby power losses can be avoided in solar inverters and rectifiers of applications. This feature of efficient use of electric energy is also investigated by partners of the EMerge Alliance industry consortia [4, 5]. The authors expect that especially lighting and cooling applications in supermarkets and shopping centers are an interesting application for the self consumption of solar electricity. Power levels of such lighting installations are in a range of 10 kW till 10 MW.

1.1. Electricity Distribution Systems The self-consumption of solar electricity in buildings can be realized using different electricity distribution systems. The comparison of these different systems will become an interesting R&D topic to identify most cost and energy efficient electricity distribution systems. State-of-the-art systems use 230 V single-phase or 400 V 3-phase AC distribution. These systems require solar inverters to convert solar DC current in utility AC current. The grid frequency is an important grid quality parameter. Latest grid standard developments in Germany will require a reduction of the actual solar inverter output power and an operation of the solar system outside the maximum power point (MPP) if the grid frequency rises above 50.2 Hz [6]. Thus local electric loads in buildings can be supplied to less extend from a building integrated solar system if external grid conditions like a low total grid load requires that. These events will increase in the German electricity grid with about 78 GW peak load if the total power level of grid connect solar systems will increase from actually 18 GW to 52 GW in 2020 [7]. Consequentially solar electricity distribution inside a building will be less influenced from AC power grid unbalance conditions and mains frequency disturbances by using a local DC power grid. Various DC power grid systems with different nominal voltages are currently discussed to distribute DC in buildings. These DC power grids are proposed as hybrid grids with a dual supply from at least an AC mains rectifier to control and regulate the DC bus. Photovoltaic solar systems are an often named second source of power. Extensions with wind power systems, combined-heat-power (CHP) and energy storage modules are also discussed [8]. Most cost effective DC power grids will be realized without energy storage units e.g. batteries. That limits the rated power level of a solar system to the always present peak load at noon if no solar power shall be unused. This is considered in the experiment discussed below. The EMerge Alliance industry consortium promotes 24 V DC grids to comply with US save class 2 voltage level [9]. The authors have selected 48 V DC for a first experiment since it is a higher voltage level with lower currents than 24 V DC within the range of extra low voltage according to EN 50178 [10]. Additionally various electronic modules are available for 48 V DC voltage that is a standard voltage of telecom and IT systems. 380 V DC grids are discussed as rated voltage for data centers. This 380 V DC grid is more precisely proposed with an IT earthing system (French: Isolé Terre) using ±190 V split DC [11, 12]. The isolation of split DC sources from ground most likely increases the effort of AC mains rectifier circuits. A lower effort and higher efficiency of hybrid supplied DC power grids is expected from the use of non-isolated 380 V AC mains rectifiers and the generation of ±380 V split DC voltages using a TN-S earthing system (French: Terre Neutre Séparé). A global agreement on standardized earthing systems of DC power grids is seen as one challenge for an industrialization of DC power grids [13].

EPE 2011 - Birmingham

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Combined Solar and AC Mains Powered Lighting System

BOEKE Ulrich

2. DC LED Lighting Experiment Lighting systems in professional buildings are one potential application that can make use of locally generated solar electricity. A test bed has been realized to get hands-on experience on hybrid supplied DC LED lighting systems that started operation at Philips Research in Aachen, Germany end of 2009. The lighting system consists of 12 Philips LED luminaries installed in a 48 m2 office corridor depicted in Figure 2. Philips Fugato down-light luminaries with 2000 lumen Fortimo LED engines have been used. This luminary from 2009 has now a successor named "LuxSpace Compact" [14]. These 12 luminaries have been divided in 3 groups each with four luminaries. The first group is used as reference load that includes the original LED lamp driver supplied only from 230 V AC mains. The second and the third group of luminaries have been equipped with new LED drivers to supply these luminaries from hybrid supplied 48 V DC grids. All LED drivers generate equal regulated DC currents in LED strings of luminaries that results in an average illuminance level of 375 Lux in the corridor. Figure 3 illustrates measured luminance levels on the corridor ground measured every meter without daylight contribution and without light from poster spots. Figure 4 illustrates the AC supply of all three system. The AC power of the corridor lighting systems is centrally turned-on at 6 AM and turned-off at 8 PM. The operation time of the AC grid is monitored with an operating hour counter. After that counter the AC grid is split in three lines each addressing one luminary system. Each AC line has an AC power wattmeter [15] and a separated energy meter [16]. Separated class 1 energy meters have been used since the selected wattmeters with large data storage have a specified power accuracy but no specification for energy metering accuracy.

Figure 2: Photos of a corridor with LED down-light luminaries and solar modules on building roof Illuminance (Lux) 500 400 300 200 100 0 0

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Figure 3: Measured illuminance levels in the middle of the corridor ground excluding daylight

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Combined Solar and AC Mains Powered Lighting System

BOEKE Ulrich

Energy Meter 1

.

Time controlled switch

Operating hours counter

AC LED luminary 1

W

AC LED luminary 2

SD card

AC LED luminary 3 AC LED luminary 4

Energy Meter 2

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.

230 V AC

Wattmeter 1

Wattmeter 2 W

AC/DC Power Supply

48 V DC

AC/DC Power Supply

48 V DC

SD card Energy Meter 3

.

Wattmeter 3 W SD card

Figure 4: Drawing illustrating the use of meters in the AC supply grid

2.1 AC supplied reference system Figure 5 offers details of the AC supplied LED reference system. The monitored AC power is feed to four LED luminaries. Each LED luminary has an LED driver that converts AC mains power in a regulated DC LED string current with an efficiency of 90%. This lamp driver consists of two power converters in series. A mains rectifier and power factor correction circuit converts AC mains in an internal DC bus voltage. The second power converter is a DC/DC converter with mains insulation generating a regulated DC current in a string of series connected LEDs. 4x 44 W LED Luminaries 230 V AC grid 177 W

Energy Meter

230 V AC Distribution

AC - DC DC - DC Rectifier Converter 90 % efficiency

Figure 5: Block diagram of the pure AC mains powered reference lighting system

2.2 Hybrid supplied 48 V DC systems The hybrid supplied 48 V DC grid system in Figure 6 has been realized two times to study the selfconsumption of solar power avoiding solar inverters and reducing power losses mains voltage rectifiers. Each system uses an industrial power supply to control the voltage of a 48 V DC bus insulated from AC mains [17]. A maximum power point (MPP) tracker controls the operation point of connected solar modules and feeds a controlled power level in the DC bus [18]. Solar modules are only connected to systems if relays are powered with AC mains voltage to avoid an island operation of the systems. The 160 W rated power of the solar module under standard test conditions is slightly smaller than the 168 W load of 4 LED luminaries connected to the DC bus. In that way the power supply can always controls the DC bus voltage levels that requires a load current from this supply. For safety reasons a “40CPQ060” diode has been added at the power supply output that also costs 1 % efficiency to avoid interferences when power supply load current should reach zero. In that case the diode separates the two control loops of power supply and solar MPP converter. The battery charging function of the MPP converter limits the DC bus voltage to maximum 56 V in this case.

EPE 2011 - Birmingham

ISBN: 9789075815153

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Combined Solar and AC Mains Powered Lighting System

BOEKE Ulrich

Four LED luminaries have been connected to each 48 V DC bus using modified LED drivers for a supply from a 40 V...60 V DC input. These LED drivers makes use of an isolated load resonant DC/DC converter that generates the same controlled DC current in LED strings of the luminary than the AC powered LED driver in Figure 5. The hybrid supplied lighting system has been realized two times. Hybrid system number one makes use of two 80 W solar modules in parallel each equipped with 36 polycrystalline solar cells [19]. Hybrid system number two has been combined with two 81 W silicon thin-film solar modules in parallel [20]. All solar modules have been orientated towards south, Figure 2. The parallel operation of two solar modules was given by the input voltage range of the solar MPP converter module. The fluctuating power generation of solar modules also results in fluctuating power in a wide range of power supplies to supply constant loads. Thus power supplies and MPP converters are operated not at fixed operation points but at different efficiency levels indicated with the data in Figure 6. Principally the system could be extended with a 48 V chargeable battery. The current system design, however, has focused on minimum system cost and thus does not use a battery. Power Supply 230 V AC grid 15 W...179 W

Energy Meter Relay 12 V...20 V

160 W peak

Diode 40CPQ060

AC - DC DC - DC Rectifier Converter 85% ...94% efficiency Maximum Power Point DC/DC Converter

48 V DC grid

92%...96% efficiency

4x 42 W LED Luminaries DC - DC Converter 94 % efficiency

Figure 6: Block diagram of the combined solar and mains powered lighting system

2.3 Measurements The complete system is in operation since September 2009 and is under regular monitoring since beginning 2010. The system operated 4980 hours, 350 days from about 6 AM till 8 PM in 2010. The two hybrid powered systems have consumed 14.6 % less energy on average from AC mains than the reference system documented in Table I. The hybrid system number 2 has achieved even 16.3 % AC mains power reduction. Figure 7 depicts relative monthly AC power consumption reductions of the two hybrid supplied lighting systems in relation to the energy consumption of the reference system. Hybrid system number two has a higher reduction of AC mains power than system one. We can not explain the differences with measured parameters of electronic modules and loads. Most likely the difference is due to the different performance of the used solar module types and due to measurement tolerances. Nominal power levels of both solar modules have a tolerance of ±5 %. The Schott silicon thin-film module has a power temperature coefficient of only 0.2 %/°C that is lower than the power temperature coefficient of 0.5 %/°C of the BP solar module. Thus the Schott modules should perform better at module temperatures above 25°C of standard test conditions. The relative and absolute annual AC power savings due to the self consumption of solar power of the two hybrid powered systems in kWh/(kWp·year) in Table 1 could have been higher for the following reasons. First, the generation of the internal 400 V DC supply voltage from AC mains of the LED driver of the reference system has an efficiency of 95 % that is 2 % higher than the generation of the 48 V DC grid voltage from AC mains of the two hybrid supplied systems. That results in about 16 kWh/year higher annual AC power consumption of the hybrid supplied systems. This 2 % efficiency drawback of the hybrid supplied systems is visible in Figure 7 for the month December where solar systems in the region of the installations have generated nearly zero solar power due to winter weather conditions end of 2010. This efficiency drawback of the presented hybrid supplied 48 V DC power system can be avoided by distributing DC power on higher voltage levels of e.g. 380 V discussed in [11-13].

EPE 2011 - Birmingham

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Combined Solar and AC Mains Powered Lighting System

BOEKE Ulrich

Second, the hybrid systems have not used solar power 15 days in 2010 when the complete system was turned-off. That does not influence the relative AC power consumption saving in relation to the reference system power consumption. But the ratio of AC power reduction to solar panel peak power could have been additionally about 35 kWh/(kWp·year) higher when the system would have been operated 365 days instead of 350 days in 2010. The AC mains power savings of the system also depends on the local solar radiation offer. The German Weather Information Office informs on an annual solar radiation level of 1060 kWh/m2 on a horizontal surface for the city of Aachen in 2010 that is of interest for a comparison with other systems [21].

Table I: Measured data of the 3 LED lighting sub-systems

AC Reference System Hybrid System 1 Hybrid System 2

AC power consumption 2010 800 kWh 697 kWh 670 kWh

AC power reduction / solar panel peak power

AC power reduction

No solar power

Reference 13.0 % 16.3 %

649 kWh/(kWp·year) 805 kWh/(kWp·year)

Reduced AC Mains Power Consumption

35% 30%

Hybrid System 2 Hybrid System 1

25% 20% Annual Average

15% 10% 5% 0% -5%

Figure 7: Measured relative AC mains power reduction of luminary groups 2 and 3 compared with the AC mains power consumption of luminary group 1 in 2010

3. Outlook Further reduction steps of governmental defined solar feed-in tariffs motivate alternative technologies to use solar electricity. The self-consumption of solar power in DC grids of professional buildings is one opportunity. Electric powered cooling is an other interesting application [22]. The combination of power electronic modules and applications from different manufacturers in one DC grid requires technical standards that are investigated by the European Telecommunications Standards Institute (ETSI), the EMerge Alliance industry consortia and the International Electrotechnical Commission (IEC) [12, 13, 23]. The extension of existing standards by new dedicated DC power grid standards benefits from practical experiences with pilot installations. The European academic and industrial R&D community can contribute with technical innovations in this field to cost effective future netzero energy buildings to obtain or better outperform Europe's 20-20-20 targets.

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4. Conclusion Further cost reduction of solar system upfront investments will motivate new technologies and applications of solar power self-consumption. Solar power self-consumption can be realized especially in "all electric buildings" that benefit from controlled electric appliances like HVAC and lighting. Hybrid supplied electricity distribution from AC mains and solar systems can be realised with AC or DC power grids. A 48 V DC power grid is presented in this paper that avoids solar inverters and solar power rectification loss in end-applications targeting solar power usage with higher efficiency. The presented test bed has realised a reduction of AC mains power consumption of 15 % by the use of solar power in a local 48 V DC power grid considering solar radiation in Germany 2010. DC Voltage levels between 24 V... ±380 V are currently under discussion for future DC power grids.

5. Appendix 5. 1 References [1] Federal Ministry for the Environment, Nature Conservation and Nuclear Safety: Erfahrungsbericht 2011 zum Erneuerbare-Energien-Gesetz (EEG-Erfahrungsbericht), Tab. 3-34: Entwicklung der PV-Vergütung bei Rückführung des Zubaus auf den Ausbaupfad (Seite 131), in German, http://erneuerbareenergien.de/files/pdfs/allgemein/application/pdf/eeg_erfahrungsbericht_2011_entwurf.pdf [2] REWE: REWE Green Building - a sustainable end-to-end concept, http://www.rewe.de/image/web09/gruenstrom/REWE_Green_Building_Broschuere_englisch_2010_final.p df [3] Marche Restaurants Schweiz AG: Fact Sheet Support Office Marche International - The first net-zero energy office building in Switzerland (in German), http://www.energieagenda.ch/files/Fact%20Sheet%20B%C3%BCrogeb%C3%A4ude%20Marche%20Int% 20MINERGIE-P-Eco.pdf [4] Emerge Alliance, http://www.emergealliance.org [5] Nextek Power Systems Inc., www.nextekpower.com [6] SMA Technology AG: Technik Kompendium 3.1 - PV-Netzintegration, in German, http://download.sma.de/smaprosa/dateien/10040/TECHKOMP-ADE111211W.pdf [7] Roland Berger Strategy Consultans & Prognos AG: Wegweiser Solarwirtschaft PV Roadmap 2020, in German, http://www.solarwirtschaft.de/fileadmin/content_files/wegweiser_sw_pvrm-lang.pdf [8] I. Cvetkovic et al: CPES Initiative on sustainable buildings and nanogrids, IEEE Applied Power Electronic Conference 2011, special presentation session 1.5.7 on power electronics and alternative energy, http://www.apec-conf.org/2011/free-downloads/2011-sp [9] Emerge Alliance, http://www.emergealliance.org/en/standard/standard_faq.asp#04 [10] CENELEC: Electronic equipment for use in power installations, EN 50178 [11] Lawrence Berkeley National Laboratory: DC power for data centers of the future, http://hightech.lbl.gov/dc-powering [12] D. Symanski, B. Fortenbery: DC power standards, IEEE Applied Power Electronic Conference 2011, special presentation session 1.5.3 on power electronics and alternative energy, http://www.apecconf.org/2011/free-downloads/2011-sp [13] European Telecommunications Standards Institute: Draft standard ETSI EN 300 132-2 “Environmental Engineering (EE); Power supply interface at the input to telecommunications equipment; Part 3: Operated by rectified current source, alternating current source or direct current source up to 400 V”, http://hightech.lbl.gov/dc-powering/pubs/european-std.pdf [14] Philips LED Specification Cataluogue: LuxSpace compact, Volume 1 - 2010/2011, page 236, http://www.lighting.philips.co.uk/connect/tools_literature/Downloads.wpd [15] Conrad: VOLTCRAFT® Energy Logger 4000 Energiekosten-Messgerät Datenlogger mit SD-Karten-Slot, LCD 0,001 - 9999 kWh 4320 h, in German [16] Finder: kWh Energy meter 1-phase with mechanical display, type 7E.13.8.230.00x0, http://www.finder.de/comuni/pdf/S7EEN.pdf [17] XP Power: CCM250 series power supply type “CCM250PS48”, www.xppower.com [18] MSTE Solar: MPT 1.170-48 Solar 48 V battery charger, www.mste-solar.de

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[19] BP Solar: BP 380 solar module datasheet, www.bp.com/genericcountryjump.do?categoryId=9070&contentId=7038143 [20] Schott Solar: Schott ASITM 81 W thin film solar module data sheet, www.schottsolar.com/global/home [21] Deutscher Wetterdienst: Globalstrahlung in der Bundesrepublik Deutschland – Jahressummen 2010, in German [22] Steca: Steca PF 166Solar-Kühl-/Gefriertruhe, http://www.stecasolar.com/index.php?Archiv_KuehlGefriertruhe [23] International Electrotechnical Commission: Standardization Management Board Strategic Group 4 on "LVDC distribution systems up to 1500 V DC", http://www.iec.ch/dyn/www/f?p=103:85:0::::FSP_ORG_ID,FSP_LANG_ID:6019,25

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