Alternative Energy Solutions For Off Grid Sites

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An Eltek Valere Whitepaper:
Why Solar and Hybrid solutions have become the preferred second source of energy for the Telecom Industry.
For more information, visit www.eltekvalere.com

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  • Great Solution -But it is important to also highlight deepcycle battery life due to discharge interuptions that happens on all hybrid solutions. Better results might have to focus on laboratory test with discharge at different intervals against battery life. Most hybrid designers focus on maximum revenue (Battery Autonomy on load) and forget to also have a component of the effects of the solution on Battery life. This is a hidden fact that most Telecom operators never get to know because the sales engineer puts more focus on the savings. Typical example is in most Solutions deployed on African markets.
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Alternative Energy Solutions For Off Grid Sites

  1. 1. ALTERNATIVE ENERGY SOLUTIONS FOR OFF GRID SITES WHY SOLAR AND HYBRID SOLUTIONS HAS BECOME THE PREFERRED SECOND SOURCE OF ENERGY February 2010 www.eltekvalere.com -1-
  2. 2. LOWER THE OPEX WITH ALTERNATIVE ENERGY Why Solar Power has become the ideal Second Source of Energy, either as Autonomous or Hybrid Solutions The consumer demand for telecommunications services continues to increase worldwide, with some of the highest growth areas being seen in the emerging markets. However, these areas often are not able to provide the clean, reliable electrical energy required by today’s telecommunications equipment. Traditionally, operators would use AC Generator Set (“Gen-set”) to provide either the primary or supplemental site power needs given they are relatively cheap and easy to install. However, given Gen-sets typically run on fossil fuels, it also makes them expensive to run, noisy and harmful to the environment. With the growing focus on reducing OPEX and being more environmentally responsible, operators are now looking for more cost-effective and “cleaner” alternative energy solutions. This is highlighted by the increasing number of installations that are using solar energy to provide power to their telecommunication sites. This white paper has therefore been prepared to describe in detail the technologies involved with Solar Power solutions and how they can be utilized to achieve significant OPEX savings. ALTERNATIVE ENERGY SOLUTIONS – OFF GRID -2- AN ELTEK VALERE WHITE PAPER
  3. 3.   TABLE OF CONTENT LOWER THE OPEX WITH ALTERNATIVE ENERGY ....................................................................................... 2  WHY SOLAR POWER HAS BECOME THE IDEAL SECOND SOURCE OF ENERGY, EITHER AS AUTONOMOUS OR HYBRID SOLUTIONS  ................................................................................................................................................................... 2  TABLE OF CONTENT.................................................................................................................................. 3  TERMINOLOGY ......................................................................................................................................... 4  I.  ABBREVIATIONS.............................................................................................................................. 4  1. SOLAR POWER ..................................................................................................................................... 5  1.1 OFF GRID SITE POWER CONFIGURATION ........................................................................................................ 5  1.2 HARVESTING SOLAR ENERGY ....................................................................................................................... 6  1.2.1  Energy conversion ................................................................................................................... 6  1.2.2  Sun position tracking systems  ................................................................................................ 7  . 1.3 BATTERY TECHNOLOGY .............................................................................................................................. 8  2. HOW IT WORKS .................................................................................................................................... 9  2. 1 DIESEL GENERATOR, GEN‐ SET .................................................................................................................... 9  2.1.1 OPEX intensive ............................................................................................................................. 9  2.1.2 CAPEX favourable ....................................................................................................................... 10  2.2 CYCLED DIESEL GENERATOR ...................................................................................................................... 10  2.2.1 CAPEX intensive .......................................................................................................................... 11  2.2.2 OPEX effects ............................................................................................................................... 11  2.3. ADDING SOLAR ENERGY .......................................................................................................................... 11  2.3.1 Solar data ................................................................................................................................... 11  2.4 SUN PATH – FIXED TILT AND TRACKING SYSTEMS .......................................................................................... 13  2.4.1Fixed tilt ...................................................................................................................................... 13  2.4.2 Fixed tilt – seasonal adjustment................................................................................................. 13  2.4.3 E‐W tracking  .............................................................................................................................. 13  . 2.4.4 Dual axis tracking, adding N‐S tracking ..................................................................................... 14  2.5 PV PANEL CHARACTERISTICS ...................................................................................................................... 15  2.6. CHARGE CONTROLLERS ............................................................................................................................ 16  2.6.1 1st generation, direct charge control .......................................................................................... 16  2.6.2 2nd generation, MPPT charger  ................................................................................................... 16  . 2.6.3. 3rd generation, MPPT charger w/galvanic barrier .................................................................... 16  2.6.4 Eltek Valere Solar charger – Flatpack2 HE Solar ........................................................................ 17  2.7 BATTERIES ............................................................................................................................................. 17  2.7.1 Standby – AGM .......................................................................................................................... 17  2.7.2 OPzV ........................................................................................................................................... 17  2.7.3 OpzS ........................................................................................................................................... 18  2.7.4 NiCd ............................................................................................................................................ 18  3. INTEGRATING PV PANELS IN TO THE POWER SYSTEM ........................................................................ 19  3.1 ADDING SOLAR TO A STANDARD SYSTEM ..................................................................................................... 19  3.2 ADDING SOLAR TO CYCLIC APPLICATION ....................................................................................................... 19  3.3 OPTIMIZING THE HYBRID CYCLIC APPLICATION ............................................................................................... 19  4. CASE STUDY ....................................................................................................................................... 21  REFERENCES ........................................................................................................................................... 23  ALTERNATIVE ENERGY SOLUTIONS – OFF GRID -3- AN ELTEK VALERE WHITE PAPER
  4. 4. TERMINOLOGY Chapter 1 will introduce various site solution configurations for an off grid site and compare the relative cost and performance of each of the major technologies available. For easy understanding, the comparison is displayed in a table matrix format, with the following symbols being used to represent the different expenditure or performance levels. Briefly stated; the more shaded the circle, the better the assessment. Symbol Capex / Opex Performance ● Minimal / No cost Excellent ◕ Cost effective Very Good ◑ Acceptable Satisfactory ◔ Expensive Below Average ○ Very Expensive Unacceptable I. ABBREVIATIONS The following lists the abbreviations used in this paper. ATM : Standard Atmosphere symbol (unit of pressure). BTS : Base Transceiver Station DOD : Depth of Discharge EMC : Electro-Magnetic Compatibility HE : High Efficiency MPPT : Maximum Power Point Tracking PV : Photo Voltaic SOC : State Of Charge STC : Standard Test Conditions ALTERNATIVE ENERGY SOLUTIONS – OFF GRID -4- AN ELTEK VALERE WHITE PAPER
  5. 5. 1. SOLAR POWER The first step in finding out how Solar Power can reduce OPEX is to understand what technologies are used, how they can be combined and what benefits and limitations each option has. 1.1 Off Grid Site power configuration When evaluating the use of Solar Power as an alternative power solution to an off-grid or poor-grid site, there are 4 basic site power configurations to be considered. a) Gen-set only: A stand-alone (or redundant) Gen-set configuration runs as a primary power source in an off-grid site, or as a supplemental power source in a poor grid region. If the DC power system has batteries, they would typically be used only in emergency back-up situations. b) Cycled Gen-set: A stand-alone Gen-set configuration with a large battery bank connected to the DC power system. This solution typically runs the site from the batteries and mainly uses the Gen-set for battery charging only. This method greatly improves both the efficiency and lifetime of the gen-set, whilst at the same time reducing the daily fuel consumption. c) Solar Hybrid Site: A combination of Solar Power and Gen-set onsite, with a large battery bank connected to the DC power system. This solution utilizes the solar power when available to run the site and charge batteries. When solar power is not available, the site would typically be configured to function like the Cycled Gen-set solution. This solution further reduces Opex by powering the load when solar power is available and charging the batteries with any excess power harvested. In doing so, there is less need to use the gen-set for battery charging. d) Autonomous Solar: Solar Power is configured as the primary power source with a large battery bank connected to the DC power system. This topology generally requires over sizing of the solar panels and battery bank due to seasonal variation of the available solar power ALTERNATIVE ENERGY SOLUTIONS – OFF GRID -5- AN ELTEK VALERE WHITE PAPER
  6. 6. The following table indicates the typical assessment of each of the basic site configuration. “Site down” Configuration CAPEX OPEX CO2 Emission Risk Gen set only ◕ ○ ○ ◑ Cycled Gen set ◑ ◑ ◑ ◕ Hybrid ◔ ◕ ◕ ● Solar only ○ ● ● ◔* * This risk assessment can be improved with CAPEX investment. 1.2 Harvesting Solar Energy 1.2.1 Energy conversion The radiated energy from the sun is transformed into electrical energy by using Photo Voltaic (PV) diodes. Each diode can only produce a very small amount of power by itself, therefore PV diodes are connected in various series and parallel combinations that help form different types of PV panels. The specific combination will determine both the panel rated voltage and rated current capacity. As the solar energy is instantaneous in nature, any excess energy not used to power a load must be stored in an energy reservoir; a battery bank for example. There are several different charge control technologies currently available in the market for this purpose. a) 1st generation charge control – PV panels connected/disconnected to batteries by a contactor. b) 2nd generation charge control – PV panels connected to batteries through a non isolated dcdc converter with MPPT functionality. c) 3rd generation charge control - PV panels connected to batteries through an isolated dc-dc converter with MPPT functionality. ALTERNATIVE ENERGY SOLUTIONS – OFF GRID -6- AN ELTEK VALERE WHITE PAPER
  7. 7. d) Eltek Valere – Uses an high-efficient isolated DC-DC converter with MPPT functionality. The following table indicates the performance of each technology as a charge controller for solar power on a telecom site. Conversion Panel energy Surge Telecom Technology efficiency utilization protection specification 1st gen ● ◔ ○ ? 2nd gen ◕ ● ○ ? 3rd gen ◑ ● ● ? Eltek Valere ◕ ● ● 1.2.2 Sun position tracking systems The amount of energy collected by a PV panels depends on the intensity of sun beams hitting the front of the panel. As the sun’s path varies east-west during a day and north- south with the seasons, tracking systems can be used to adjust the panel’s orientation towards the sun, thus helping to maximize the energy harvested by the PV panel. If sun tracking is not used, then the PV panels are fixed into the most optimal position based on the site’s latitude from the equator. Energy Technology CAPEX OPEX Reliability utilization Fixed ● ● ● ◔ Seasonal adjusted ◕ ● ● ◑ 1-axis ◔ ◑ ◑ ◕ 2-axis ○ ◑ ◑ ● ALTERNATIVE ENERGY SOLUTIONS – OFF GRID -7- AN ELTEK VALERE WHITE PAPER
  8. 8. 1.3 Battery Technology Choosing the correct battery technology and size is an important step when dimensioning a Solar Hybrid site. The battery must provide the desired backup time, handle the desired number of discharge cycles to the designed DOD, and give a reasonable lifetime in the operating conditions. The following table evaluates the cost and performance of three battery technologies often used in off-grid applications, against a more-traditional standby battery technology. Technology CAPEX OPEX Cycling Temperature Environment Standby ● ○ ○ ○ ◑ OPzV ◑ ◑ ◕ ◔ ◑ OPzS ◑ ◑ ◕ ◑ ◑ NiCd ○ ◑ ● ◕ ○ ALTERNATIVE ENERGY SOLUTIONS – OFF GRID -8- AN ELTEK VALERE WHITE PAPER
  9. 9. 2. HOW IT WORKS The previous chapter outlined the pros and cons of the main building blocks when powering off grid telecom sites. This chapter will focus in detail on the discussed building blocks. 2. 1 Diesel generator, Gen- Set The traditional solution for supplying energy to off grid sites is to run diesel powered generators 24h a day – 7days a week. The batteries banks are used as standby energy, which means they will only be active is the generator fails. Autonomy is sized for the time needed to reach the site for service personnel, typically in the range of 6-12h 2.1.1 OPEX intensive Operating at a low efficiency The power rating of the installed diesel generator tends to be oversized compared to the average telecom load it is supplying: Factors that influence the size of the generator are : - Generator is sized for full load plus recharging of standby batteries. - Generator is sized for an air condition start-up current - Historically, Generator reliability was in line with the capacity and physical size. This means that typically a generator will be operating for most of it’s Generator efficiency curve Genset efficiency, kWh/l operation time to supply a load much 3,5 less than it’s rated capacity. Typically 3 as low 10-20%. Output energy [kWh/l] 2,5 2 As seen from Figure 1, when a 1,5 generator is operating against a small 1 load, the operating efficiency is far 0,5 from optimum. 0 0 10 20 30 40 50 60 70 80 90 100 Load [%] High maintenance cost Figure 1 Generator efficiency Diesel generators are maintenance intensive, resulting in frequent site visits. The yearly maintenance cost can easily exceed the initial investment. Summarized the OPEX disadvantages are: • Working life is typically at low efficiency, less kWh pr litre fuel. • Large CO2 emissions • Maintenance and service intensive ALTERNATIVE ENERGY SOLUTIONS – OFF GRID -9- AN ELTEK VALERE WHITE PAPER
  10. 10. 2.1.2 CAPEX favourable When operating a off-grid site mainly on a gen-set however, the site installation and complexity is quite low, as is the CAPEX investment. CAPEX advantages are : • Generators are relatively cheap compared to many other off-grid power sources. • Small battery bank with standard batteries • A standard power controller can be used 2.2 Cycled Diesel Generator To “cycle the diesel generator” means running the generator for a short period of time, then turning it off and operating from the battery bank. The main benefit of this technique is that when running, the gen set is operating close to full load to re-charge the batteries. 1 2 ...n n+1 cha cha discharge discharge discharge charge rge rge 24h ?h Figure 2: Battery voltage, Cycled diesel generator It this configuration, the battery bank changes from being a backup energy source in a site with a continuously running diesel generator, to play a more active role in powering the site. As the role of the batteries has changed, the battery technology used must also be updated. Standard lead-acid batteries are designed to serve as a stand-by backup, and may only survive 200-300 cycles, depending of DOD in each cycle. In this case, a daily cycling period using standard batteries would result in need of replacement of the batteries in less than a year. ALTERNATIVE ENERGY SOLUTIONS – OFF GRID - 10 - AN ELTEK VALERE WHITE PAPER
  11. 11. 2.2.1 CAPEX intensive Comparing only the investment cost for a cycled generator vs. a traditional site works in disfavour of a cycled site: • The need for intelligent start and stop of the generator increases complexity of the controller. • The batteries designed for cyclic application are generally more expensive than standby batteries. • The Ah size of the battery bank needs to be increased to provide discharge time and serve as backup when a normal discharge cycle has ended. 2.2.2 OPEX effects However, when also taking into account the OPEX savings realised during the expected site lifetime, the overall business case tends to strongly favour a cycled Gen-set solution in nearly all situations. • Gen set operates at higher efficiency, more energy pr. litre diesel. • Reduced operating hours means less daily fuel usage and less frequent service related site visits. An example of a successful implementation of a cycled gen set is described in Chapter 4. 2.3. Adding Solar Energy Although solar irradiation can not be predicted on a daily basis, a reliable estimate of the monthly average values can be calculated based on the site location, regional weather data and historical irradiation information. 2.3.1 Solar data Solar energy is available all over the globe, at varying intensity. The radiated energy from the sun is shown in irradiation maps, like the one shown in Figure 3. The maps give a clear indication of where the most suited areas for solar energy are typically found. Figure 3: Global irradiation map ALTERNATIVE ENERGY SOLUTIONS – OFF GRID - 11 - AN ELTEK VALERE WHITE PAPER
  12. 12. The available energy for a specific location can then be analyzed. Figure 4 shows an example of an average monthly energy profile, whilst Figure 5 shows an estimated daily energy graph. Figure 4: Monthly irradiation Figure 5: Daily irradiation Solar data are historical data, typically 10 years average. Predicting the exactly amount of future solar energy on daily basis is not possible, but the historical data gives a good indication of what can be available as an average. ALTERNATIVE ENERGY SOLUTIONS – OFF GRID - 12 - AN ELTEK VALERE WHITE PAPER
  13. 13. 2.4 Sun Path – Fixed Tilt and Tracking Systems The PV panels are mounted into PV stands. There are two major categories of stands, the fixed tilt, and the tracking systems. The best solution will depend on the location of the site, but generally installations close to equator will perform well with fixed tilt, while installations located further north or south will gain from a tracking system. 2.4.1Fixed tilt The fixed tilted stands are simple in design, and demand no maintenance. The optimum tilt angle is normally equal to the latitude, but may vary depending on the application. An installation with fixed tilt will not collect the maximum available solar energy through out a year. AM PM AM PM AM PM 2.4.2 Fixed tilt – seasonal adjustment A variant of the fixed tilt adds the possibility of letting the tilt is manually adjusted. Depending on the regularity of site visits, the tilt can be adjusted to increase the monthly or seasonally collected solar energy. 2.4.3 E-W tracking An ‘east – west’ tracker is a 1-axis automated tracker that follows the suns position from east to west during the day. The tracking will only increase the amount of direct radiated energy and the mix of solar radiated energy (direct – diffuse – reflected) determines how large the total gain is. Generally a 1-axis tracker will be more expensive than a fixed, it will have moving parts that are a single point of failure, and it may also require maintenance. In addition, the tracking systems will use power to operate, thus reducing the overall gain it may provide. AM PM AM PM AM PM ALTERNATIVE ENERGY SOLUTIONS – OFF GRID - 13 - AN ELTEK VALERE WHITE PAPER
  14. 14. 2.4.4 Dual axis tracking, adding N-S tracking In a dual axis tracking system, the daily and seasonal north-south variation of the suns position is also tracked. The gain is stated to be as high as 40% compared with a fixed installation during summer by manufacturers, but the real gain will depend on the mix of the solar energy (direct – diffuse – reflected) available. The higher the share is of diffuse irradiation, the less is the gain from a tracking system. The cost of a dual axis tracker is higher than the 1-axis tracker, and will have more moving parts than the 1-axis tracker. Summer Summer Summer Spring/Autumn Spring/Autumn Spring/Autumn Winter Winter Winter ALTERNATIVE ENERGY SOLUTIONS – OFF GRID - 14 - AN ELTEK VALERE WHITE PAPER
  15. 15. 2.5 PV panel characteristics PV panels are used to transform the solar irradiation into electrical energy. Different technologies are used to do the transformation, in this paper mono-crystalline and poly- crystalline are considered most suitable. The rating of a panel is given at ideal test conditions, so called STC (25ºC panel temperature, irradiation 1000W/m2, and ATM=1.5). As seen in Figure 6 and Figure 7, the available power varies significantly with irradiation and panel temperature. Power vs panel voltage at varying irradiance, Tpanel=25°C 200 180 1000W/m² 160 800W/m² 140 Output power [W] 120 600W/m² 100 80 charge 400W/m² 60 Direct 40 200W/m² 20 0 0 5 10 15 20 25 30 35 40 45 Panel voltage [V] Figure 6: Available power as function of irradiation, 25ºC panel temperature Power vs panel voltage at varying irradiance , Tpanel=45°C 200 180 1000W/m² 160 140 800W/m² Output power [W] 120 600W/m² 100 80 charge 400W/m² 60 Direct 40 200W/m² 20 0 0 5 10 15 20 25 30 35 40 45 Panel voltage [V] Figure 7: Available power as function of irradiation, 45 ºC panel temperature ALTERNATIVE ENERGY SOLUTIONS – OFF GRID - 15 - AN ELTEK VALERE WHITE PAPER
  16. 16. 2.6. Charge controllers As outlined in chapter 1, different technologies are used to charge batteries from PV panels. 2.6.1 1st generation, direct charge control With a direct charge control, the PV panels are combined in series to match the required battery voltage and paralleled in junction boxes to get the desired power. The charger has no active components, other than over/under voltage relay control the contactor. The battery voltage will in all charging modes determine the point of operation for the PV panel. As seen in Figure 7, a battery voltage of typically 24-25V will reduce the output power to ~70% of the maximum available power. So even though the direct charge controller has low losses (only conduction losses and junction box diode losses), the utilization of the PV panel is poor. Immunity to lightning pulses is low, as there is a direct galvanic connection from the PV panels into the telecom system. Adding surge protection devices on this equipment will have limited effect. 2.6.2 2nd generation, MPPT charger The evolution from 1st to 2nd generation charge controller solves the issue of the battery voltage determining the operation point of the PV panels. A dcdc converter enables the possibility to let the output voltage of the charger to be independent from the PV voltage. The charger needs intelligence in form of a microcontroller etc. to perform Maximum Power Point Tracking (MPPT) algorithms. With a proper algorithm the charger should be able to operate at close to 100% PV panel utilization. The topology of dcdc converter is normally ‘one stage non isolated’. This topology will give conversion efficiency in the range of 92-96%. The charger has the same weakness as 1st gen, direct galvanic connection from the PV panels into the telecom system, and limited effect of surge protection devices. 2.6.3. 3rd generation, MPPT charger w/galvanic barrier The 3rd inherits the MPPT functionality of the 2nd generation equipment and solves the issue with galvanic barrier by introducing a 2nd stage in the dcdc converter. The added stage affects the efficiency of the charger; it decreases to typically 86-91%. ALTERNATIVE ENERGY SOLUTIONS – OFF GRID - 16 - AN ELTEK VALERE WHITE PAPER
  17. 17. In combination with a correctly sized surge protection, a 3rd generation charge controller will give good surge protection for the telecom load. 2.6.4 Eltek Valere Solar charger – Flatpack2 HE Solar The Eltek Valere solar charger has all the functionality of the 3rd gen equipment, but due to the patented HE technology the efficiency is raised to typically 96%. The Flatpack2 HE Solar Charger is a product derived from the standard HE technology, and follows the normal telecom standards with respect of safety, EMC, electrical characteristics and transportation. The converter is an extension to the standard telecom range offered by Eltek Valere, that all share the same control interface. 2.7 Batteries As mentioned in chapter 1, a large part of configuring a hybrid site is choosing the optimum battery technology. To find the best fitted technology several parameters must be evaluated, such as CAPEX, OPEX, ambient temperature, size, expected lifetime, weight, and fright. 2.7.1 Standby – AGM Standby lead-acid batteries are designed to work as a backup source, and not in cyclic applications. There they will only survive a limited number of discharge cycles, and if used in cyclic application it will lead to frequent battery replacement. 2.7.2 OPzV The abbreviation OPzV is German, and refers to German standard DIN 40742. O: Ortsfest = Stationary Pz: Panzerplatte = Tubular plate (+) V: Verschlossen = Valve regulated The batteries are sealed gel based. They are slightly better with respect of ambient temperature than regular gel batteries but also more expensive. The lifetime of the battery is a function of DOD, and the number of cycles is stated in the datasheet. Typically the number of cycles can be 1200 cycles at 60% DOD at 20ºC. ALTERNATIVE ENERGY SOLUTIONS – OFF GRID - 17 - AN ELTEK VALERE WHITE PAPER
  18. 18. 2.7.3 OpzS The abbreviation OPzS is German, and refers to German standard DIN 40737. O: Ortsfest = Stationary Pz: Panzerplatte = Tubular plate (+) S: Spezial = Special, Fluid electrolyte with special seperator OPzS is a flooded battery designed for cyclic application. They are better than OpzS with respect of ambient temperature, but as they are flooded they will require refilling at regular basis. The refill interval and lifetime of the battery is a function of DOD and temperature. The number of cycles is stated in the datasheet. Typically the number of cycles can be 1200 cycles at 60% DOD at 20ºC Transport restrictions of the product may exist, and must be considered when planning a project. 2.7.4 NiCd The characteristics of NiCd batteries are specified in DIN 61427. An advantage of nickel cadmium is the battery’s ability to tolerate extremes in heat and cold without a degradation of its useful life. NiCd batteries can be charged at a higher rate than lead-acid batteries. NiCd has more cycles at shallow DOD compared with an OPzV/OPzS battery, but less at deeper DOD. The number of cycles is stated in the datasheet The NiCd requires water topping at an interval given by temperature and DOD. The Cadmium in the battery is a toxic, and special recycling arrangements must be in place in the region of deployment. ALTERNATIVE ENERGY SOLUTIONS – OFF GRID - 18 - AN ELTEK VALERE WHITE PAPER
  19. 19. 3. INTEGRATING PV PANELS IN TO THE POWER SYSTEM Charge control LOAD G AC/DC Figure 8: Symbolic system configuration 3.1 Adding Solar to a Standard System If Solar Power is simply added to a standard 24/7 gen-set site, the PV panel power will be an addition to the available power from the diesel generator. The added power will of course reduce the power drawn from the diesel generator, but it will actually force the diesel generator to operate at an even lower efficiency, as the load on it will be less. In cases when the radiated energy exceeds the instant load and the batteries are fully load, the excess power from the PV panel will be lost. 3.2 Adding Solar to cyclic application Adding PV power to a cyclic application, where the generators are prevented to run when the PV most likely will deliver energy, will most likely increase the PV energy fraction. Still there is a potential to increase the harvested PV energy. 3.3 Optimizing the hybrid cyclic application To optimize the hybrid application several factors needs to be considered. As the batteries will operate most of the time in partly SOC, batteries suitable for this condition must be used. • The batteries must be sized so that they can supply the energy on at least one day without solar energy, and still give the desired backup time if the diesel generator fails to start. • The diesel generator and rectifiers must be sized so that the maximum charge current of the batteries gives an optimum point of operation of the diesel generator. • The control of the diesel generator must be timed so that the potential PV energy will be fully utilized. • If using lead acid batteries, the battery cells must be regularly balanced by a boost charge, as the batteries will work in a partial SOC. The battery manufacturer’s recommendation must be followed. ALTERNATIVE ENERGY SOLUTIONS – OFF GRID - 19 - AN ELTEK VALERE WHITE PAPER
  20. 20. A typical charge/discharge cycle involves the phases shown in Figure 9. • From midnight until sunrise the load is supplied by the batteries, and thus the batteries are discharged. • From after dawn the PV panels delivers energy, and after awhile it is sufficient to charge the batteries. • At sunset the load is again supplied by the batteries, and they are again discharged. • At a given time or DOD the generator starts, and supplies the load and charges the batteries. At a DOD above 0% the generator stops, making sure that the remaining battery capacity can be charged by the PV panel energy the next day. 1 2 ...n n+1 d d i i c c s s h h disc c disc c disc Partly a Partly a Partly harg h harg h harg charge chage r chage r chage e a e a e g g r r e e g g e e 24h ?h Figure 9: Battery voltage, Cycled diesel generator with solar panels ALTERNATIVE ENERGY SOLUTIONS – OFF GRID - 20 - AN ELTEK VALERE WHITE PAPER
  21. 21. 4. CASE STUDY Site Location: In the equatorial zone with a stable and predictable regional weather pattern. Initial site power configuration : Off-grid BTS site powered with 1+1 15KVA diesel generators, running 24 hours a day, 7 days a week. A small lead-acid battery bank connected to the DC power system for emergency back- up only. Stated site load profile: The average load was stated as ~2.5kW and autonomy requirement was 3 days. Upgrade project target: The initial requirement was to upgrade the site to be partly powered by PV panels to reduce the OPEX with a limited CAPEX. Upgrade activities The site was upgraded as follows: - Original gen-sets removed and replaced with a single 20KVA gen-set. - DC power system upgraded to 18kW - 3kW solar solution installed. - Upgraded the battery bank to 3000Ah (with OPzV) - All components integrated and monitored by the DC power system controller. Gen-set cycling was enabled, with an initial DOD value set at 30%. Studying the results from the first months showed that the average load was 40% less than specified, so the actual autonomy was approximately 5 days. After discussion with the battery supplier the restart charging level of the batteries was increased to 50% DOD, resulting in longer intervals between starting of the diesel generator. Conclusions Energy split, August 2009 21 % With the current configuration the energy supply split for a collection of sites in August 2009 is shown in Figure 10. Approximately 21% of the energy used on the sites was delivered by PV panels. The remaining energy was supplied by the diesel generator. 79 % Gen set Solar Figure 10 : Energy split August 2009 ALTERNATIVE ENERGY SOLUTIONS – OFF GRID - 21 - AN ELTEK VALERE WHITE PAPER
  22. 22. Figure 11 shows a comparison of diesel consumption between the original configuration and the Eltek Valere hybrid configuration. The fuel reduction of 75% has been confirmed by the operator, reducing the monthly carbon footprint by 10 ton, and significantly increasing the diesel generator service interval. Fuel usage, August 2009 6000 5000 4000 Diesel [l] 3000 2000 1000 0 Gen set site Eltek Hybrid solution Figure 11: Fuel usage comparison, August 2009 For this project, the payback time for the extra investment will be approx 3 years, including battery replacement after 7 years. ALTERNATIVE ENERGY SOLUTIONS – OFF GRID - 22 - AN ELTEK VALERE WHITE PAPER
  23. 23. REFERENCES Electric Power Monthly, November 2007 Edition (Energy Information Administration) Verizon Corporate Responsibility Report 2006 Verizon Communications – Green House Gas Emission Reduction Initiatives (on Clean Air, web site) Telefonica CR Report 2006 NTT Group Environmental Protection Activity Report 2004 Energy Information Administration Qwest Power Engineering Organization - Central Office Rectifier Replacement Study ALTERNATIVE ENERGY SOLUTIONS – OFF GRID - 23 - AN ELTEK VALERE WHITE PAPER
  24. 24. www.eltekvalere.com

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