Energy storage system is of essence for Solar PV systems?

Energy storage is a vital element in power sector particularly when a country has large volume of Renewable Energy (RE). Cheap, practical and easily available energy storage systems can address some of the critical issues associated with RE sources particularly the Solar PV systems. It will build an atmosphere of confidence in the Utility working, particularly utilities with large scale RE penetration; reduce the investment in transmission systems and development of reserve capacity. 

Widespread deployment of battery storage would mitigate the intermittency phenomenon associated with solar PV systems. In fact, in some of the countries utilities have mandated that all new PV systems should have energy storage systems to smoothen the power variations. Solar PV systems with battery storage could also help to manage unplanned voltage fluctuations, particularly in areas with high penetration of solar PV systems.

Figure 1 shows the main components of a simple Roof top Solar PV system with Battery back up.

Fig.1: Layout of a simple Roof top Solar PV system with Battery back-up

Example of Energy Storage at the Utility end:

Utilities too have promoted Energy Storage systems at their end. Duke Energy has installed a hybrid energy storage system comprising of battery bank and ultra-capacitor at its North Carolina based substation (www.technologyreview.com). In this hybrid storage system, the ultra-capacitor helps the power system during large but short duration power surges, for example when the Solar output dips during a cloudy day. Battery bank, as customary, will help to recover during large duration shortages. 

It is expected that the two storage systems along with smart electronic controls will help the power system in mitigating the power shortage over periods ranging from seconds to several hours. This hybrid storage is supposed to provide a more economical solution as compared to each of the storage systems when used separately. 

According to officials at Maxwell Technologies, the ultra-capacitor storage system can completely eliminate the output fluctuation of less than 30 seconds and provide sufficient smoothening of fluctuations of up to 5 minutes. In recent years the ultra-capacitor energy storage systems have become more accepted as high power shock absorbers for industrial and transportation applications in combination with Lead-acid batteries or advanced chemical batteries. Ultra-capacitor has high power capability due to very low internal resistance, wide operating temperature range of -40^oC to 65^oC, minimum maintenance, high cycling ability and reasonable price.

The latest utility scale battery storage technology emerged in the commercial market is the 8 MWh capacity Vanadium Redox Battery bank installed at Everett Sub-station, Washington state, and which is to be commissioned in January 2017. This battery system,  compact and concealed in container, is non-flammable and can be discharged upto 100%, i.e. the Depth of Discharge (DoD) can be 100%.    

Essence of Energy Storage to End User:

At the end user level the energy storage system will provide the user with the much needed back up during night hours when the grid is out. In countries like India the Time-of-Day (TOD) tariff is not implemented in the residential and commercial sector. I am sure that in the near future Utilities have to come up with the TOD tariff in these sectors also. Then in such a case a solar PV system with energy storage will be very beneficial. 

TOD tariff is an effective tool to reduce the peak hour kWh shortage. Shifting of some portion of the load to the storage system will be of great relief to the utilities reeling under stress. 

Fig 2: A Roof Top Solar PV system with battery back-up.

In some countries, the feed-in-tariff has been rigorously cut down or eliminated at all. In such a condition, the pay back periods of solar PV systems are highly dependent on the percentage of solar energy used for “self-consumption”. 

"Experts are in the opinion that a solar PV system is worth only if the self-consumption is about 50% or more." 

Energy storage systems enable owners of solar PV systems to increase their self-consumption. Although installing energy storage system increases the overall cost of the system and hence the electricity produced, but still it is not as expensive as many would think of. Lithium-ion batteries are considered as the most affordable and dependable energy storage systems. These batteries can discharge bigger burst of power and can eliminate the need of ultra-capacitor but are costly.

Power Conditioning Unit: A key element of Solar PV System

Power Conditioning Unit (PCU) is a very vital piece of equipment in any solar PV system and is also called solar power conditioning unit. The main components of any solar PV system are PV panels, charge controller, battery bank, inverters, cables, switches and the protection.

"Power Conditioning Unit (PCU) is a combined unit consisting of a solar charge controller, an inverter and a grid charger integrated in a single unit." 

Role of Power Conditioning Unit (PCU)

Power Conditioning Unit (PCU) is usually a DSP based PWM technology using IGBT and MOSFET. It facilitates the charging of battery bank through either solar PV panels or the grid/DG system. All the solar PCU has the ability to continuously monitor the state of battery, solar power output and the load. They not only monitors the state of affairs but even displays vital parameters such as PV voltage and current, load percentage, overload percentage, charging current etc.  

The solar energy is intermittent in nature and therefore a balance has to be made between the solar generation and the demand. When excess energy is generated the extra kWh is to be either fed into the grid, which is possible only in case of grid tied system, or to be stored into the battery bank. Due to over usage of power, if the battery voltage goes below a pre-defined level, the PCU will automatically transfer the load to the utility grid and simultaneously charges the batteries through the grid supply. 

Once the batteries attain a given voltage, the PCU cuts off the grid power to the system and returns back the load on to the solar system. The rest of the battery charging is now done by the solar PV system. In this way the PCU gives priority to the solar power over the grid, and uses the grid power only when the solar power and the battery power of required level is not available.

Fig.1: Back view of an off-grid Solar PCU

Significant advantages of PCU

The significant advantages of PCU are pure sine wave output with low Total Harmonic Distortion (a measure of power quality), higher efficiency, data logging monitoring etc. The commercially available PCU have efficiency more than 85% and nearly 5% Total Harmonic Distortion (THD) for linear loads. These units come with the deep discharge protection and thus ensure the health of batteries. PCUs have nearly 300% overload facility for a few milliseconds which helps during the starting of heavy loads. Thus these units have the inbuilt overload and short-circuit protection and therefore no need to worry about overloads and short circuits. Remote monitoring of the unit can be done with the help of RS232, Ethernet, GSM and GPRS.

Ratings of commercially available single phase PCU

The usual ratings of commercially available single phase off-grid PCU in India are 600 VA/24 V, 1kVA/24 V, 2kVA or 3kVA/48 V, 3/4/ 5/6 kVA/96 V, 7.5 kVA/120 V 

Optimize the electrical loads for a successful solar PV system

As per a recent survey, India is second among the countries in terms of growing electrical demand. To meet this ever increasing electrical demand in an economically feasible and environmentally friendly way, Renewable Energy and Energy Conservation are the best options. In this regard, the Indian government is planning at a fast pace and the result is schemes and programs like Jawahar Lal Nehru National Solar Mission (JNNSM) and many more. Many states in the country have implemented solar schemes such as roof top net metering arrangements and so on.

The current scenario in India is that the roof top solar PV system along with major solar PV plants is coming up at a rapid pace. A good population is keen to know and eager to install a roof top solar PV module. Here are some of the vital basics needed as a priori to install a roof top solar PV system for the residential sector in India.

What should be the ideal capacity of my solar PV system? (a FAQ)

I hope, the following paragraphs will answer most of your queries.  

   

First and foremost thing is to know the key elements of any solar PV system. Any solar PV system, I am talking of stand-alone solar PV system, consists of the below given equipments.

1.  Solar PV module,

2.  Charge controller,

3.  Battery,

4.  Inverter,

5. Miscellaneous items such as supporting frame, wires,   switches, change-over etc.

Of the above listed electrical equipments and elements each has its own rated capacity, and to get a proper matching between them is very important. Prior to working out the optimum capacity of the solar PV system, one has to determine the right electrical loads which can be connected to make this solar PV system viable with a low pay back period. The steps in planning for a solar PV system are:

Step 1: Determination of proper electrical load

Make a list of electrical appliances that you are planning to run from the solar PV system. Prior to this one has to find out how much power each piece of equipment draws for its operation. The electrical power needed by the equipment is given on its name plate from where one can note down and prepare a list. The electrical wattage of some of the commonly used appliances is:

Sr no	Equipment	Electrical load (in Watts)
1	Microwave oven	1200- 2000 W
2	Electric Geyser	2000-3000 W
3	Washing Machine	300-500 W
4	Electric Iron	600-800 W
5	Water pump (domestic, 1-phase)	375-750 W
6	Ceiling fan	80 W
7	PC	100-150 W
8	Color TV	150 W
9	Tubelight (including choke)	45 W
10	Freezer	150-250 W
11	Room Cooler	150-250 W
12	AC	1000-2000 W

The electrical load given above is for reference only. Actual rating may be obtained from the equipment name plate or manufacturers specification only, as the rating or electrical load of any equipment may vary according to its capacity, features, etc.

Step 2: Optimize your electrical load

As per the given list one can very well judge that appliances such as microwave ovens, geysers, AC, hotplates etc are equipments capable of drawing heavy current and hence power. To run these equipments or appliances on solar you have to go for a much higher values of installed solar capacity, which is going to increase the capital cost and the pay-back period (period necessary to get back the investment). So it is advised not to connect or run these equipments on solar PV system.

(If you an inverter technology based refrigerator or AC, then the possibility of running them on solar PV system is there.)

 In fact equipments such as Tubelight, CFL, fan, room cooler, TV, laptop etc, and in some emergency situations, electric iron, should be connected and run on solar PV system.

To separate the equipments requiring heavy power, moderate and low power one has to re-wire the distribution system or separate the circuits from the distribution mains (MCB). One can use a change-over switch also.

Step 3: Battery size

Appropriate battery size is the key element in making your PV system a success. The life of a PV module is around 25 years as claimed by the manufacturer, whereas the expected battery life is 3 to 5 years. Also the battery cost is significant. As one goes for a higher Ampere-hour (Ah; the rating of battery is given in Ah) battery, its cost increases. 

For example; recently I have installed a 400 W solar PV system costing around 52 thousand INR in Bhopal, India, the battery of 150 Ah capacity, with 5 years warranty, alone was of 14 thousand INR.     

So the summary is:

Battery is a vital element of your solar PV system whose life is less as compared to other equipments of the system and is comparatively costlier. The equipments which you have selected to get connected to the solar PV system, if works mostly during day time, is the optimum load as per the solar PV system design. This in turn will permit you to have a battery with lower Ah capacity which in turn will reduce the overall system cost.

If your load is ‘switched off’ mostly during the day time, then you have to keep a large battery to store the whole energy produced by the PV module during the day. Thus, to optimize your electrical requirements you have to look into your usage pattern and the criticality of your application.

For a ready reference, the back-up time for a particular capacity battery is given below

Load	Inverter capacity	Battery capacity/ Back-up time
		100 Ah		150 Ah		200 Ah
		Full Load	Half Load	Full Load	Half Load	Full Load	Half Load
2 TL + 2 F+ 1 PC+ 3 CFL	650 VA	1 hr.	2 Hr, 40 m	1 Hr, 50 m	4 Hr, 20 m	2 Hr, 40 m	6 Hr, 10 m
2 TL + 4 F+ 1 PC+ 3 CFL	850 VA	40 m	1 Hr, 50 m	1 Hr, 10 m	3 Hr	1 Hr, 50 m	4 hr, 20 m

TL and F stands for Tube-light and fan respectively.     

The charge controller is an equipment that controls the charging of the battery and thus helps in improving the battery health and life. The capacity of charge controller is in ampere.  The charging current ( CC ) of a battery is given in manufacturer’s specification, but for ready reference the CC  of a 100 Ah battery is 10 A,  150 Ah battery is 15 A and so on. So for a 150 Ah battery a charge controller of 20 or 30 A is sufficient.

Step 4: Inverter size

Inverter is the equipment which converts the DC voltage of the battery into AC 230 V, so that your normal AC appliances can be connected to the solar PV system. 

Selecting a proper inverter size is very important. Use only the inverter which provides a pure sine wave otherwise your equipments are going to suffer. The output capacity of inverter is given in Volt-Ampere (VA), whereas the appliances are rated in watts. We know that VA multiplied by power factor is watts. So you have to know the power factor of commonly used electrical gadgets for a precise calculation. For simplicity you can assume that the power factor is 0.8, which is the value for most commonly used inductive household equipments.

So a 850 VA inverter is of 850 x 0.8 = 680 W only. So the total electrical loads which can be connected to a 850 VA inverter can be about 600 W. This does not mean that for better utilization of resources one has to keep an inverter of higher capacity. The answer is:

The efficiency of an inverter is about 80% to 90% i.e. 10% to 20% of the energy given to an inverter is consumed by the inverter itself. So higher the inverter capacity higher the losses. Also a higher capacity inverter is useless unless the battery is also appropriately sized.

For a better understanding, have a look into my roof top solar PV system (photo below).

I have installed a 400 W solar PV module (4 panels of 100 W each, make Topsun) along with a 40 A charge controller. The battery used is of Luminous make, 150 Ah, 5 year warranty and the inverter is of 850 VA sine wave of Su-Kam make. 

The whole system is working quite satisfactorily since May 2015. On this system I have used a 150 W room cooler and one 80 W fan the whole day during the summer with occasional load of a 150 W  color TV. I have tried to operate a 375 W water pump and a 600 W automatic iron also. Both the equipments worked nicely, but one at a time. The motor during starting draws a higher current (which is natural for motors), thanks to the in-built feature of the inverter which permits a 300% plus over-current for a few ms to cater such loads. 

Recently I have connected the entire Light & Fan load of 2 bedrooms and a hall. The total connected load on the solar inverter system is now 850 W (4 x 40 tubelights, 4 x 80 W ceiling fan, 1 x 150 TV, 2 x 150 room cooler) but the maximum load at a time is restricted to 400 W.