Archive for Category Roof Top Solar Power

1. Selecting Batteries for your Roof Top PV Solar Energy Systems

Stand Alone or Hybrid Roof top PV solar Energy Systems are designed along with a suitably sized battery bank. There seems to be a lot of confusion with regards to the criteria of selection and sizing the battery-banks. We will attempt to answer some of the most frequently asked questions.

Type of Batteries

Though Lithium Ion Batteries are more efficient, but the cost benefit ratio far outweighs in favour of Lead-Acid Batteries. We use Southern Batteries, C-10 rating, 150 AH or 200 AH for our Solar PV installations. Our Hi-Power Invader Series of Tubular Batteries are rated at C10 capacity. These batteries are assembled in PPCP containers with plate holding cradles with tubular positives and lattice grid pasted negatives made with special lead alloy for minimized gas generation during service.

The Cells are provided with micro porous ceramic vent cum sealed float plugs which allow the liberated gases to escape allowing the water droplets to trap within cells, this reduces the topping up frequency. The level indicators provided on cells indicate the requirement of topping up during service.

Compared to flat dimensional battery the frequency of topping up is improved to once a year by providing additional electrolyte height above plate top in tall version of batteries. This is elevated further with proper system charge controller voltage settings.

What is Ampere Hour capacity (AH) of the battery and Coulomb Discharge Rating ?

AH or Ampere Hours are the No of Amperes delivered by the battery ( constant) x No of Hours that it takes to discharge. AH of a battery is specified in relation to a discharge time. An ampere is the unit of measurement of current and is defined as coulombs of charge passing through a conductor in one second. The capacity of a battery is related to the quantity of active materials in it, and the amount of electrolyte and the surface area of the plates. The capacity of a battery is measured by discharging at a constant current until it reaches its terminal voltage (usually about 1.75 volts). This is usually done at a constant temperature, under standard conditions of 25C (77F). The capacity is calculated by multiplying the discharge current value by the time required to reach terminal voltage. A battery might be rated at 100 A·h when discharged at a rate that will fully discharge the battery in 20 hours. In this example, the discharge current would be 5 amperes. If the battery is discharged in a shorter time, with a higher current, the delivered capacity is less.

Sizing a Battery Bank

  • Inverter Size - Should be able to run on peak load or maximum wattage.
  • Daily energy use - This is available in your electricity bills ( expressed as KWH).
  • Back Up time - needed.
  • Depth of discharge.
  • Take into account the temperature losses.
  • Battery bank capacity - will depend on all above.

2. Calculating Battery Backup for Roof Top Solar PV System

Roof Top PV Solar Energy Generating PV Systems produce DC current and this can be stored using a BATTERY BANK for usage after dark ( when sunlight is not available or is insufficient).

In places like rural Uttar Pradesh, where there are frequent power outages, sometimes for days together, it makes good business sense to install a Roof top PV Solar System with ADEQUATELY SIZED Battery Bank to last you for the night. These Solar PV Roof Top Power generating Systems, when not connected to the Grid, are also called Stand-Alone Systems, or Off-Grid Systems. When compared to the running cost of a Diesel Generator, the running cost of a roof top solar is, just a fraction.

Stand Alone Roof Top Solar systems, or Off-grid Systems can be designed to provide you 24 hour electricity back up. In countries like Germany, households have reduced their dependence on grid by as much as 70% during May to August, as is apparent from the figure below.

Basics of sizing and designing a solar power system

  • One needs to know the energy requirements of a setup (in KWH per day), so as to design a suitable Solar Power System. This can be either calculated by checking the KW/Wattages of appliances that need to be run on solar PV. (Watts = Ampere x 240 V). Electricity bills will also give you an idea of consumption.
  • A suitable inverter that can cater to the surge as well as handle the peak loads should be selected.
  • PV panels should be such that the output caters to the load as well as battery bank.
  • Battery Bank should be such that it caters to the night time requirement, without going into a deep discharge.

3. Inverters and BOS in Roof Top PV Systems

BOS, Balance of Systems and Inverters are integral parts of any Solar Power Generating unit. It is imperative that specified BOS and inverters are used for any given configuration of a solar power system for optimal energy efficiency. BOS includes the wires connectors, AJB’s etc. For Energy Efficiency, it is important to use the wires with correct thickness to avoid heat loss and voltage drop. Ohm’s law ( V=IR ) quantifies the voltage drop as a function of current and resistance. The figure below broadly indicates the wire thickness v/s current function to minimize the voltage drop.

The MPPT Charge Controller

MPPT (Maximum Power Point Tracking) Charge Controller tries keeping the panel at its maximum voltage and simultaneously produces the voltage required by the battery. A basic charge controller simply prevents damage of batteries by over-charging, by effectively cutting off the current from the solar panels (or by reducing it to a pulse) when the battery voltage reaches a certain level. On the other hand, a Maximum Power Point Tracker (MPPT) controller performs an extra function to improve your system efficiency.

What does the MPPT Controller Do ?

Besides performing the function of a basic controller, an MPPT controller also includes a DC to DC voltage converter, converting the voltage of the panels to that required by the batteries, with very little loss of power. In other words, it attempts to keep the panel voltage near its Maximum Power Point, while supplying the varying voltage requirements of the battery. Thus, it essentially decouples the panel and battery voltages so that there can be a 24 volt system on one side of the MPPT charge controller and panels wired in series to produce 48 volts on the other. Thus, offering the ability to provide some charging current even in dull conditions when a simple controller would not help much.

Inverter Efficiency

Solar PV system produces DC current, which is converted to AC using an inverter. Although inverters come with wide ranging efficiencies but typically affordable solar inverters are between 80% to 90% efficient.

Battery Efficiency

Batteries are needed for charge storage. Lead acid batteries are most commonly used.

Since lead acid batteries are usually charged at the float voltage of about 13.5 V and the discharge voltage is about 12 V, the voltage efficiency is about 0.88. In average the coulomb efficiency is about 0.92. Hence, the net energy efficiency is around 0.80

A lead-acid battery has an efficiency of only 75-85% (this includes both the charging loss and the discharging loss). From zero State of Charge (SOC) to 85% SOC the average overall battery charging efficiency is 91%- the balance is losses during discharge. The energy lost appears as heat which warms the battery. It can be minimized by keeping the charge and discharge rates low. It helps keep the battery cool and improves its life.

4. Roof Top Solar Power

We will be briefly providing details of some configurations that could be considered for roof top installations in Rural U.P, where power is acutely scarce. Before we describe the various possible configurations, it would be pertinent to mention that judicious utilization of power goes a long way in saving costs and power units. For example, LED Lights consume only a fraction of energy as compared to the conventional lights/bulbs. Likewise, it would be prudent to chose your electrical devices that are energy efficient. Check their rating. One of the biggest power sump in any household is an Air Conditioner. So if you plan to run an AC on solar, it would be in your interest to go in for 5 star AC models. You could also consider hybrid AC models (which can run on DC)

For purpose of calculation, we will take the loads as follows -

  • Light/LED- 10w
  • Bulbs- 40w
  • Fan- 25-30w
  • Refrigerator- may use about 75w, but does not run continuously
  • TV- 60-100w
  • Laptop 15-25 w
  • 1 Ton AC (conventional)- 2000w
  • 1 Ton AC ( 5 Star or Hybrid)- 1000w
  • 1 KW Panels and I KVA Inverter, with 2 Batteries of 150AH- This STAND ALONE OR HYBRID configuration is for a very small household, wishing to run 3 fans, 3 LED lights and one small refrigerator etc. (**Calculated on intermittent running ( need based use) on a period of full day and night - 24 hours).
  • 1.5 KW Panels and 2 KVA Inverter, with 4 Batteries of 150AH- This STAND ALONE OR HYBRID configuration is for a medium size household, wishing to run 6-10 fans, 3-8 LED lights and one small refrigerator, a few hours of lap-top usage etc- (**Calculated on intermittent running ( need based use) on a period of full day and night - 24 hours).
  • 3 KW Panels, 3 KVA Inverter with 4 Batteries- For relatively bigger households

Our Inverters Specifications

  • DSP based control and MPPT Solar Charging
  • Pure Sine wave output and has less transfer time.(suitable for computer applications)
  • Inbuilt DC priority feature. When solar is present battery will not take any current from AC Mains
  • High efficiency & longer back up time
  • Withstanding surge of double the rating or more
  • Intelligent charging ensuring more battery life
  • Noiseless operation of fans, lamps and appliances
  • Overload Protection & auto recovery
  • Short Circuit Protection
  • Over temperature sensing
  • No load sleep: If the load is less than 10W the Inverter will go to automatic sleep and save power
  • Liquid crystal display for status monitoring
  • Battery Charging through both solar and Grid
  • Inbuilt solar priority system
  • High reliability
  • Intelligent solar Charger
  • Overload Protection
  • Short circuit protection
  • Auto Restart
  • High efficiency - green inverter
  • Generator Compatibility