Smart Cities - Build a Solar PV system with Battery Backup

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Introduction

  • This lesson will provide instructions for the construction of a solar PV system with battery backup suitable for a tiny house. The components of the system include:
    • 200 Watt solar PV panel
    • 12V Li-Battery (120 Ah, 1.44 kWh storage)
    • 240V inverter

Authors

For more information contact Adam Simankowicz or Edmond Lascaris

Background

  • This system has been designed to demonstrate how a house running low energy electrical appliances can be built and function. The cost of the base system is $2,500. This lesson will also explain
    • how the system functions,
    • limitation of the system,
    • what appliances can be run
    • how to convert to different DC voltages
    • how to convert the battery DC voltage to AC using an inverter
    • how to program and configure different components in the system (MPPT solar charger)

What the system can run

  • The list below provides some examples of what the solar system can run.
    • Engel MT-V45F 40L Portable Fridge/Freezer - 8 Watts average (24/7)
    • Raspberry Pi computer 5 Watts (24/7)
    • Water pump for garden or header tank (Jabsco Pressure Pump "Par-Max" - 11 Litres per Minute - 12 Volt, 5.2 A, 62 Watts) [1]
    • 1000 Watt REDARC inverter to provide AC for running most low energy AC appliances and chargers
      • laptop computer (45 Watt charger, 2 hours)
      • mobile phone (5 Watt charger, 2 hours)
      • electric bike (100 Watt charger, 2 hours)
      • charging portable headlamps (Petzl Actik Core 600 Rechargeable Headlamp)
      • hair dryer (1000 Watts, 2 minutes)

System limitations

Some of the limitations of the solar PV system with battery storage are discussed.

Battery life expectancy

  • The stated battery capacity is 120 Ah, 12V (120 Ah x 12 V) = 1,440 Wh (Watt hours)
  • All this energy is not usable.
  • To preserve the life of the battery (maximise the number of charge/discharge cycles) is it better to only dischange 30% of battery's stated charge capacity. This gives an estimated life cycle of 8000+ charges (8000 / 365 = 21.9 years)
  • If the battery is discharged by a greater amount this will shorten the life of the battery
    • 50% discharge = 5000 cycles (5000 / 365 = 13.7 years)
    • 80% discharge = 2000 cycles (2000 / 365 = 5.5 years)
  • Preserving the life of a battery for as long as possible is better to help minimise environmental impacts and to ensure the resilience of the system. Battery costs and availability (supply chain issues / rationing) are likely to be issues faced in a resource and carbon limited future)

Battery usable energy during discharge cycle

  • If the stated battery capacity is 1,440 Wh and the battery is only discharged by 30% each cycle then the usable energy from the battery is (1,440 Wh x 30/100) = 432 Wh
  • 432 Wh is enough energy to run one 100 W globe for 4.3 hours. Or one 432 Watt globe for 1 hour.
  • Note that this calculation assumes that the battery is not being charged.
  • If a battery is being charged by a solar PV panel during the day then the usable energy will be much greater.
  • In Melbourne a 200 Watt solar PV panel may produce over 600-800 Watts [2]
  • This is enough to recharge the battery and also to supply additional electricity to run electrical appliances.

Battery Discharge Protection

  • All modern batteries now have Battery Management Systems (BMS) that help to protect and preserve batteries.
  • One of the protection measures that are implemented is Low Voltage Cut-off.
  • The iTech120X has a Low Voltage Cut-off of 8.5-9.0V.
  • This is very low (State of Charge 0%) and will still result in permanent damage to the battery.
  • For this reason batteries should always be inspected daily to ensure that the State of Charge does not drop too low.
  • Modern monitoring systems can be configured to protect batteries if the SoC falls below a set point [3]
  • Pylontech US3000C batteries have an integrated battery management system and State of Charge reporting to prevent excessive discharge. This can be configured by the user [4]

12V versus 24V and 48V

  • You don't need to be a licenced electrician when working with 50V or less.
  • Solar PV systems with batteries are typically set to run on 12V, 24V or 48V DC battery supplies.
  • Camping and caravan stores sell electrical appliances that generally run on 12V or 24V DC supplies.
  • If you need to change voltages you will need a DC to DC converter [5]

1000 Watt Inverter power limitations

  • The 1000 W inverter is able to deliver continuous output power of 1000VA (Watts).
  • It can also supply surge power of 1,750VA (Watts) for one second.
  • Any appliances that draw more power that this will either not function or cause the Inverter to shutdown.
  • It is possible to install inverters with greater power outputs (up to 3000 VA).
  • Higher outputs will also put more strain on the battery and larger gauge cables will need to be used to feed the high current from the battery to the inverter.

Inverter Current Drain - No Load Current

  • The conversion of DC current to AC current results in some energy losses within the inverter.
  • Even when a small amount of 240V AC power is used the additional power draw from the inverter (No Load Current) may be as high as 24 Watts (12V x 2A).
  • The maximum efficiency of this inverter is 88%.
  • For this reason it is sometime better to run electrical appliances connected directly to the battery to help minimise energy losses (e.g. Engel fridge connected directly to 12V battery)

Inverter life cycle

  • Inverters typically have a life span of 7-8 years before they require replacement.
  • Minimising the use of the inverter will extend the life span.
  • The RedArc inverter can be set to Power Saving Mode using dip switches to limit power draw to 1.2W (0.1A, 12V). This will also help to extend the life of the inverter.

Bill of Materials

Solar PV panel

  • Top One Solar Photovoltaic module (generic 200 Watt solar PV panel)
  • Rated maximum power 200 Watts
  • Open Circuit Voltage (Voc) 22.3 V
  • Short Circuit Current (Isc) 11.89 A
  • Voltage at Pmax (Vmp) 18 V
  • Current at Pmax (Imp) 11.12 A
  • Module dimensions 1580mm x 808mm x 35mm

Solar Charge Controller

  • Victron BlueSolar MPPT 100/20 (up to 48V) Solar Controller [6]
  • A charge controller is required to optimise the charging of the battery using the solar panel [7].
  • The solar charge controller receives input from the solar panel and feeds this to the attached battery.
  • The solar charge controller needs to be configured using a computer on battery specifications. This helps prevent over charging of the battery.
  • The Victron SmartSolar MPPT 100/20 Solar Charge Controller was used for this installation. It allows for a maximium input voltage of 100 and a maximum current input of 20 Amps. A 200 Watt panel will operate within these specifications.
  • Cost $205.80

Victron VE.Direct to USB interface

  • The VE.Direct to USB interface connects products with a VE.Direct connection to devices with a computer USB port so that the Solar Change Controller can be configured correctly [8].

Battery

  • iTech 120X Waterproof 120 Ah Lithium battery [9]
  • Cost $899
  • Specifications
    • Nominal voltage 12.8 V
    • Recommended Charge Voltage 14.4 V
    • Life cycles 30% discharge 8000+ (21 years)
    • Life cycles 50% discharge 5000+ (13.7 years)
    • Life cycles 80% discharge 2000+ (5.5 years)

Inverter

  • 1000 Watt pure sine wave Inverter (redArc) [10]
  • This inverter produces 240 V AC as an output using the 12 V DC supply from a battery.
  • Cost $1,099
  • Input (Operating) Voltage 10.5 - 16.5VDC
  • Output System Voltage 240VAC
  • Output Power 1000W
  • Warranty 2 Years
  • AC Connection AU/NZ Power Socket
  • Length (mm) 372
  • Height (mm) 83
  • Weight (kg) 3.26
  • Width (mm) 200
  • Weight 3.3 kg
  • When in standby mode (no load current) the Inverter will use 24 Watts (12 V x 2 A) of power.

Shunt and Battery Monitor

  • A shunt with display will provide information on the voltage, amps and state of charge of the battery.
  • Victron Battery Monitor BMV-700 - Grey. Cost $254.40. [11]
  • Battery Monitor with high-precision 100V/500A shunt. Cost $129. [12]

Safe disconnects/isolators

Fuses

Cabling

Terminals

Tools

  • Large crimper for lugs
  • Heat shrink iron
  • spanners / screw driver
  • drill and drill bits

Engel Refrigerator

  • Very energy efficient and low current draw (3 Amps when compressor running)
  • Model MT45F
  • Cost $1,299
  • Energy rating 12V/3A
  • Weight 24 kg
  • Estimated energy consumption per year 72 kWh/year (8.2 W/h)
  • Storage volume 40 L

5 V supply

  • DC-DC Buck-Mode Power Module (8~28V to 5V 3A) [13]
  • The DC-DC buck converter can receive DC inputs between 8 and 28V and convert this to a stable 5V, 3A output.
  • This circuit was used to run a Raspberry Pi computer so that the Victron MPPT Solar Charge Controller and the Victron SmartShunt could be monitored.
  • An inline 10A fuse was placed on the positive supply so that in the event of a short circuit the fuse will blow and isolate the circuit.

Construction

  • Unboxing of solar PV system

  • Initial arrange of components on marine ply backing board.


  • Inverter dip-switch configuration

  • MPPT configuration of bridge

  • Final assembly of system

Configuration of Victron MPPT Charge Controller

  • The MPPT Charge Controller needs to be configured to:
    • Maximise the energy output of the solar PV panel for the battery (Maximum Powerpoint Tracking)
    • Safeguard the battery from over charging (Bulk, Absorption and Float charging stages)

Tools required

  • VictronConnect software [14]
  • Victron VE.Direct to USB interface [15]

Reference materials for configuration of MPPT Charge Controller

The following reference materials and specifications for the iTech120X battery are provided so that the MPPT Charge Controller can be configured correctly.

  • Inverter manual [16]
  • VictronConnect software (MPPT) and shunt communication software (multiple OS)
  • ITECH120X charging parameters for Victron MPPTs [17]

Additional reading material

  • Battery charging - what do bulk absorption and float mean? [18]
  • Maximum Power Point Tracking [19]
  • Victron Solar Controller App Basics (YouTube) [20]
  • AM Solar video - BMV-712 Battery Monitor Programming Guide - The Victron Connect [21]
  • Victron Solar Charge Controller Features Explained [22]

ChatGPT explanation of Float and Absorption Voltages

The float voltage and absorption voltage settings for a Victron MPPT (Maximum Power Point Tracking) charge controller can vary depending on the specific requirements and characteristics of the battery you are using. However, for a LiFePO4 (Lithium Iron Phosphate) battery, you typically set different voltage parameters compared to traditional lead-acid batteries.

Float Voltage (Float Charge): The float voltage is the voltage level at which the charge controller maintains the battery once it has reached its full state of charge. For LiFePO4 batteries, the typical float voltage range is around 13.6V to 13.8V. This voltage level helps maintain the battery's charge without overcharging it. You should consult your LiFePO4 battery manufacturer's recommendations to determine the optimal float voltage for your specific battery.

Absorption Voltage (Bulk Charge): The absorption voltage, also known as the bulk charge voltage, is the voltage level at which the charge controller charges the battery to its full capacity. For LiFePO4 batteries, the absorption voltage is typically set around 14.2V to 14.6V. This voltage level ensures that the battery is charged efficiently and quickly. Again, you should refer to your battery manufacturer's specifications for the recommended absorption voltage.

It's important to note that these voltage settings may vary based on the specific LiFePO4 battery you are using, and it's crucial to follow the manufacturer's recommendations to avoid overcharging or damaging the battery. Additionally, some Victron MPPT charge controllers come with battery presets, including LiFePO4, which simplifies the setup process by using predefined voltage parameters.

For accurate and safe charging of your LiFePO4 battery, it is advisable to consult your battery manufacturer's documentation and follow their recommended voltage settings. Incorrect settings can lead to reduced battery lifespan or safety issues, so it's essential to get it right.

  • Once the MPPT is configured it will appear as a device in the VictronConnect software.

  • Status display for MPPT showing current status of the Charge Controller.

  • Battery settings part A

  • Battery settings part B

Victron SmartShunt Configuration

  • Battery settings

  • More battery settings

  • Victron Battery Monitor settings for iTECH120 Lithium battery

Monitoring System using Raspberry Pi

WiFi Adapter

  • linux compatible wifi adapter (5dBi high gain antennas ($89). Can be plugged in on the pi port directly, or moved to an optimal location for even better reception via the included USB cable. [23]

Estimated Carbon Emissions

Carbon Emissions associated with Battery Manufacture

  • According to the Union of Concerned Scientists, lithium-ion batteries produce about 68kg CO2e per kWh of battery capacity during manufacture [24].
  • The iTech120X 120Ah LiFePO4 Battery has a stated capacity of 120 Ah (12-13.5V) which equates to 1.44 kWh (120 Ah x 12 V).
  • The CO2e emissions are therefore 68 CO2e per kWh x 1.44 kWh = 98 kg, for a 10kg battery.

Carbon Emissions associated with Solar PV panels

  • The average embodied carbon in those references for monocrystalline PV was 2,560 kg CO2e per kWp [25].
  • However some systems may be as low as 615 kgCO2/kWp of installed capacity [26]
  • For a 200 Watt panel the embodied emissions are 512 kg CO2e (2,560 kg CO2 per kWp x 0.2 kW)

Embodied carbon emissions associated with Inverter

  • The embodied GHG emissions of an inverter was estimated at 21.8 kg CO2e for a 1kW system [27]
  • The Victron MPPT Solar Charge Controller will have a similar footprint.

Carbon footprint of Copper cables

  • Copper mining emits 2.3-2.5 tonnes of carbon per tonne of metal, while smelting adds another 1.65 tonnes [28]
  • Up to 1-2kg of Cupper cabling is used in the PV system.
  • The wiring may contribute as additional 8.30 kg CO2e (4.15 kg CO2e from Cu x 2 kg wiring)

Summary of Embodied Emissions in Solar PV Systems

  • 98 kg COe2 12V 120Ah Battery
  • 512 kg CO2e 200 Watt PV panel
  • 21.8 kg CO2e 1000 Watt Inverter
  • 21.8 kg CO2e MPPT Solar Charge Controller assumed to be similar to inverter
  • 8.30 kg CO2e copper cables (2kg of copper)
  • 661.9 kg CO2e TOTAL
  • If annualised over 20 years the embodied emissions are 33 kg/year.
  • Emissions associated with transport and freight not included.

Emissions Intensity for Solar System per kilowatt-hour

  • A 200 Watt panel can produce 600-800 Watts of power per day.
  • In a year this equates to 219 kilowatt-hours/year (600 W x 365 days / 1000)
  • Using an annualised figure for embodied carbon emissions the emissions intensity for this solar system is 150 grams of CO2e per kilowatt-hour (33,000 g / 219 kilowatt-hours).
  • This would be sufficient for one person.
  • This amount of electricity usage is approximately one tenth of the current average Australian daily electricity usage of 2,093 kWh/person/year.

CO2e emissions from Renewable Electricity Generation

  • All forms of renewable energy generate emissions. This may come from all steps of mining, manufacture, transport, maintenance and decomissioning. Currently the estimated emissions from most renewables is estimated to be 50 g/kWh of production throughout the life of the asset. [29].
  • The range for renewables ranges from 12-50 g/kWh depending on the source of renewable energy [30]
  • While some sources of renewable energy can produce electricity at emission levels lower than 50 g/kWh the entire grid needs to consider:
    • upgrading the electrical power distribution network
    • storage of electrical energy. All storage system will create more usable energy but there are energy losses with conversion of energy
    • intermittency of renewable energy sources (solar PV and wind)
    • embodied energy (scope 3) in the renewable energy asset
  • The reality is that all renewable energy is associated with CO2e emissions per kWh over their lifetime, albeit, these emissions are significantly lower when compared to coal (1000 g CO2/kWh) and natural gas (475 g CO2/kWh) [31].

Emissions intensity from electricity generation in Australia

  • In 2021, the emissions intensity from electricity generation in Australia amounted to around 526.9 grams of CO2e per kilowatt-hour [32].
  • The average Australian house uses 5000 kWh per year so each household will produce 2.6 tonnes of CO2e emissions per year [33].
  • The average kWh per year per person is 2,093 kWh based on an estimate of 2.4 persons per household.
  • The emissions intensity is calculated to be 1,097 kg CO2e per person per annum (2,093 kWh x 526.9 grams of CO2e per kilowatt-hour).
  • If the average Australian produces 15 tonnes of CO2e emissions per year and needs to reduce emissions to 1 tonne per year, then permissible emissions per year is 73 kg CO2e / year (1,097 kg CO2e / 15).
  • However, CO2e emissions associated with electricity generation do not include scope 3 emissions associated with the use of electrical devices (all other indirect emissions generally associated with the supply chain and the embodied emissions in goods).

Carbon budgets

  • There is a global cumulative carbon budget of 570 GtCO2 from 2018 onward offers a 66% chance of limiting global warming to 1.5°C.
  • Based on current emissions, we are depleting this budget by 41 GtCO2.
  • We need to start reducing these emissions by 18% per year.
  • If 8 billion people on planet Earth all invested in a Solar PV system like the one discussed above this would result in 5.30 Gt CO2e (8,000,000,000 people x 0.6619 tonnes CO2e)
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