Energy Storage Systems by Positronic Manufacturing Pty Ltd www.positronicsolar.com 1300067786
Introduction When the sun is shining, the photovoltaic panels capture sunlight and convert it into electricity. Solar energy is renewable; as long as the sun is shining there is a consistent power source. It is also environmentally friendly and using solar power actively reduces your greenhouse gas emissions. Solar cells are silent, unlike other renewable energy sources such as wind, which can be noisy. The electricity generated by the PV panels then goes to a standard grid connect inverter, which is AC- coupled to the Xantrex XW Hybrid inverter/ charger. An important component in any off-grid system is the battery bank and the known demands that the customer will place upon it. The entire system is designed with the objective to maximize the life of the batteries and ensure depth of discharge is managed, a safe working figure is no more than 80% of the battery capacity removed in a discharge cycle. Systems are designed with a daily depth of discharge of 55%. AC coupling of the panels to the system has a number of advantages. Daytime loads are supplied directly from the panels, reducing conversion losses through the batteries. Grid connect inverters are much more efficient than DC regulators. Grid connect inverters are cheap and ubiquitous, easily installed and maintained by local non-specialist electricians. Using grid connect inverters simplifies the installation process by using smaller cable and easy connection to the system. positronic Manufacturing Pty Ltd 1300067786 2
For off-grid systems, it is always best to use a quality, well constructed power inverter that uses a solid transformer design for maximum surge capacity and less component stress. The Xantrex XW is a true sine wave inverter/charger, with surge capacity to start difficult loads like pumps, refrigerators or air conditioners. In order to protect the inverter/charger from over heating the XW has a temperature-controlled, variable-speed internal cooling fan. The housing design of the unit also promotes vertical airflow through the inverter. This natural chimney effect provides convection cooling at lower power levels and reduces fan run time. The inverter will either send the electricity straight to your loads or if there is more energy being produce than demand then the energy is then stored in batteries. Positronic use lithium ferrous phosphate batteries (LiFePo) which put them above the competition. LiFePo batteries store much more energy than lead weight for weight. This energy density is the main factor allowing packaged energy storage systems. They are also one of the safest batteries on the market. They will not burn if overcharged or pierced and are fully sealed. The energy from the batteries is used when the demand from your house is higher than what is being produced by the solar panels. This means that even when the sun isn t shining you have power. In case of really bad weather or load demand being greater than PV generation the Positronic ESS will automatically start an generator. The generator will deliver power to the loads and charge the batteries. The generator will turn off when the batteries are full or the load on the generator drops too low. Positronic can supply 8kVA silenced generators powered by Perkins, perfectly matched to the Xantrex XW hybrid inverter/charger. positronic Manufacturing Pty Ltd 1300067786 3
Design Basics The most important component of an energy storage system (ESS) is the battery. Undersizing the battery leads to excessive discharge and oversizing makes the system unnecessarily expensive. In the old days batteries were sized to provide five days autonomy in case of bad weather. This was done to ensure normal depth of discharge was not too deep and because panels used to cost ~$10/ watt. These days, panels cost ~$1/watt and battery management systems provide better protection against deep discharges. Ten year battery life is acceptable for energy storage systems. This means the battery should be designed for 4000 discharge cycles. A LiFePo battery can do 4000 cycles to 55% depth of discharge, which means a 100Ah battery can provide 55Ah/day for ten years. These numbers provide enough buffer to allow for the occasional deep discharge. Let s look at an installation using 10kWh/day using a 48V battery: 10 kwh / 48V = 208 Ah 208 / 0.55 = 378 Ah So the battery required is at least 48V 400Ah or 19.2kWh Economically it makes more sense to provide 1-2 days autonomy and enough panels to provide the daily required energy in all weather. In all but the most overcast weather a solar array will generate about 25% of the energy it will on an average day. It follows that an array designed for off grid should be four times the size indicated by the daily energy requirements of the installation. If the solar array is sized properly it will provide enough power for daytime use plus enough to charge the battery for overnight use. In the example, the battery is only required for overnight use so a 48V400Ah battery will provide about two days storage, depending on the load profile. An average solar array can be expected to generate about 4kWh/day/kwp installed. If the installation requires 10kWh/day the array would need to be a 10kW array to provide the energy required in all weather. Some simple rules of thumb pop out of these numbers: Battery size in kwh = 2 x daily kwh use Array size in kwp = 1 x daily kwh use The size of the inverter/charger is determined by the peak demand of the installation. Anything less than 5kW on a normal house will result in unhappiness. The inverter has to be able to deal with demand plus the starting currents of items like fridges and pumps positronic Manufacturing Pty Ltd 1300067786 4
To assist in system sizing we have a Stand Alone Power Calculator spreadsheet, available at www.positronicsolar.com Simply enter the load data to calculate kwh requirements for the PV array and ESS. See#Notes#below. Stand#Alone#Power#usage#Calculator Hours&of&use&in&each&time&frame Total&kwh&during& sunlight Total&kwh&to& be&stored Power&consumers Wattage 00.01am&3&3am 3.01&3&6.00 6.01&3&9.00& 9.01&3&12.00 12.01&3&15.00 15.01&3&18.00 18.01&3&21.00 21.01&3&24.00 Fridge&(Change&example&to&actual) 0 0 0 0 0 0 0 0 Light&1&(Change&example&to&actual) 284 3 3 0 3 3 0 3.408 3.408 Light&2 0 0 0 Light&3 0 0 0 Light&4 0 0 0 Light&5 0 0 0 Light&6 0 0 0 Light&7 0 0 0 Light&8 0 0 0 Stereo 0 0 0 TV 0 0 0 DVD 0 0 0 Kettle 0 0 0 Microwave 0 0 0 Freezer 0 0 0 Battery&charger 0 0 0 Air&Conditioner 0 0 0 Fan 0 0 0 Hair&dryer 0 0 0 Welder 0 0 0 Load 0 0 0 0 0 0 0 0 0 0 0 0 Totals 284 3 3 0 0 0 0 3 3 0 3.408 3.408 Total&kw&drain&per&timeframe 284 284 0 0 0 0 284 284 Total& kwh/day Notes: Important Power#consumers Hours#of#use Wattage Fridges/freezers Total#kw#drain#per#timeframe Help The&more&accurately&you&complete&this&form,&the&more&reliable&and&appropriate&the&system&we&will"e&you&on.&&Big&errors&can&cause&systems&to&fail&and&their&life&span&to&be& significantly&deminished. Use&the&examples&and/or&insert&in&here&every&power&using&item&you&have&that&will&be&connected&at&any&time&to&the&system 1.&&Please&put&the&total&time&the&device&is&on&in&each&3&hour&period&in&a&decimal&format,&so&5&minutes&will&be&0.08&hours. 2.&&If&an&item&is&only&used&once&a&month&or&week,&please&still&add&it,&but&change&the&hours&usage&to&a&typical&average&per&day. This&should&be&the&rated&wattage&on&the&device&compliance&plate&or&globe. These&items&and&others&have&a&duty&cycle,&please&average.&&A&25%&duty&cyle&for&example&will&result&in&6&hour&a&day&total&operation No&need&to&do&anything&here,&I&will&calculate&it&based&on&the&information&you&have&provided Please&call&me&on&the&phone&number&below&if&you&have&questions. The ESS can be supplied with a 4.5kW, 5kW or 6kW XW hybrid inverter. Two or three ESS can be paralleled for installations requiring a larger supply. Three phase supply is available with three properly configured XW inverters. It is possible to connect six XWs together to supply a 36kW three phase load. Positronic ESS are designed to AC couple with standard grid tie inverters. The size of the grid tie inverter can be no larger than the hybrid inverter. For example, in the case of the ESS in the photo on page 2 the XW is 6kW and the two inverters are 2.8kW each. positronic Manufacturing Pty Ltd 1300067786 5
Good quality inverters are able to deal with at least 200% array oversizing, so having a 10kWp array on a 6kW grid tie inverter is not an issue. The grid tie inverters push the panels off their maximum power point when the input exceeds the AC output. Here is an example of 13.5kWp array on a 4.6kW inverter. On a sunny day it generated 41kWh On a rainy day the overclocked system produced 27kWh, compared to a nearby 5kWp/5kWACnom system which produced 11kWh The excess power on a sunny day needn t go to waste. On a sunny day the battery will be fully recharged by about 11:00am. Loads such as water pumps, hot water systems and air conditioning can be controlled by the ESS relative to battery SOC and charging current. If the site is suitable, gravity fed water tanks are a perfect dump load which also provides constant pressure in the taps and removes pressure pumps from night-time load. An electric hot water system is another form of energy storage that is economically competitive compared to bottled gas or the expense and maintenance of a solar hot water system. In hot climates even air conditioners can be turned on during the day to take the edge off a hot house before nightfall. One of our customers has only run his generator for five hours in the last six months, during cyclonic weather. The rest of the time he s enjoying air conditioning, cold beer and the wonders of electricity without a 22kVA diesel banging away in the background. positronic Manufacturing Pty Ltd 1300067786 6
Installation Positronic ESS have all their settings preprogrammed in the factory during manufacture. The systems are comprehensively tested and run through a number of charge/discharge cycles using grid tie inverters, generator/grid input and 12VDC or 240VAC load banks. They are delivered on site ready to install. The Positronic ESS weighs between 500kg and 1000kg and the dimensions are generally 2100Hx400Dx700-2100W, depending on storage capacity. They are bottom heavy. There are aluminium rails under and behind the enclosure which makes slinging and trolley work a bit easier. There are three AC connections; LOADS, GRID TIE INVERTER and GRID/GENERATOR. Once they are made and the installation checked the system is ready to store and produce energy. Autostart or alarm functions for generator start as well as data logging, MPPT regulators etc can be installed to suit customer requirements. positronic Manufacturing Pty Ltd 1300067786 7
Positronic ESS are inverter agnostic. We recommend using Trannergy inverters. They can be derated to suit the limit imposed by the hybrid inverter size and they perform really well with oversized arrays. SMA inverters also work well with the Positronic ESS. Installing a Positronic ESS is as easy as installing a normal grid tie system (apart from physically getting the unit into place). Instead of wiring the grid tie to the meter box it connects to the ESS. The customer s AC load submains also connect to the ESS. Installers will have to allow for a couple of follow up visits as the customer gets used to life with batteries. Maintenance of the ESS is minimal due to the battery management system and the LiFePo battery. Once the customer is settled in an annual system check should be organised to check the battery data, check connections, check for vermin etc. If a data logging system is installed, keeping an eye on the ESS is a lot easier for the customer, installer and Positronic. positronic Manufacturing Pty Ltd 1300067786 8
Specifications Hybrid Inverter Charger 9
Battery 10