Let’s design a roof top solar PV system

Levelised cost of Solar power:

When solar photovoltaic (PV) modules are installed on a building's roof top to generate electricity, it is called roof top solar PV system/ plant. Although, still in the evolving stage, but the feeling in the air is that roof top solar PV system can be a vibrant green technology in India.
    
Roof top solar PV cost about is about 8-9 INR/kWh and is cost effective where the grid tariff is higher than the levelised cost of solar power such as in the case of commercial and industrial consumers. Levelised cost is the cost per unit of power generated by the solar PV system taking into account all the cost incurred over the life span of the PV plant considering the time value of money.

For example the tariff for commercial consumers is around 12-13 INR/kWh in most of the Indian states. 

Roof top solar PV system is also beneficial for residential consumers in areas where power failures are frequent. Similarly residential consumers having higher energy consumption may also opt for roof top solar PV system. It is estimated that by 2016-17, the roof top solar power cost will reach the grid parity; which is a more encouraging factor in favour of roof top solar PV system in India. Currently the cost of per unit of electricity produced by diesel generators comes to about 16 INR as 1 ltr. of diesel produces about 3 to 4 kWh.      

Factors to be considered before installing a solar PV plant on your rooftop:

The factors to be considered before installing a solar power plant on your building’s rooftop include:

 i)   electrical load, 
ii)  working hours, 
iii) roof size, and 
iv) geographic location of the building. 

The subsidy given by the central and state governments, local utilities, and local community regulations and incentives are also some key determinants in the evaluation.

Roof-top solar arrays are best installed on a large and flat-roof where direct sunlight without shadow is available. Currently, commercially available silicon-based solar PV panels are made from solar cells encased in a special type of toughened glass. These are guaranteed for 25 years of field life but the power yield drops about 0.6 per cent a year. 

One can use mono-crystalline or polycrystalline panels. Mono-crystalline panels are a bit more efficient. The electrical energy (DC) generated by the solar PV modules during the sunshine hours is stored in the batteries. The energy stored in the batteries is converted into 230V AC mains using an inverter for further use. This solar energy can be used for captive consumption or exported to the grid. 

Here in this discussion we are calculating or designing for battery less solar PV system.

Steps in designing your roof top solar PV system

These are few of the steps that have to be taken before finalizing your roof top solar PV system:

1.   Estimate the energy required from roof top solar PV system,

2.   Calculate the shade-free roof top available for installation of roof top solar PV system,

3.   Estimate the capacity of the roof top solar PV system that can be installed,

4.  Approach some of the known vendors of  solar PV system and obtain the quotations as per your requirement,

5. Evaluate the quotations received from the price, warranty viewpoint, and

6.   Finalize the vendor and the deal.

Estimating the required capacity of solar PV system

The amount of energy needed is determined based on the load that needs to be fed from the roof top solar PV system. For this one has to collect the exact electrical load of each appliance that has to be connected to the PV system and it’s working hours/day. The table shows how this work is to be executed.

Table 1

Sr. No	Name of the appliance	Electrical load		Working hours per day	Total number	Energy/day (kWh/day)
		W	kW
1	T.V	60	0.06	4	1	0.06 x 4 x 1 = 0.24
2	Ceiling Fan	80	0.08	10	2	0.08 x 10 x 2= 1.6
Total						1.84 kWh/day

So whatever energy requirement you have calculated is to be divided by the insolation level to get the size of solar PV system. Suppose that the energy requirement of particular premise is 2 kWh/day, then the solar PV system size should be

= Energy requirement/ insolation level

The insolation level in India is pretty good; for example in Bhopal (India) it is about 5.6 kWh/m². 

So the capacity of solar PV system required in Bhopal for an energy consumption of 2 kWh/ day

= 2/5.5 = .36 kW or 360 W   

An extra 30% is added as technical margin. Hence finally the solar PV system capacity required is

= (2 x 1.3)/5.5 = 470 W

In this way you can calculate the capacity of the solar PV system. If one has to keep the investment low, he/she has to keep some of the large loads off the solar PV system; either by switching “off” that load/ loads or by feeding it from some other energy source.

Panel Size

A roof top solar PV system using lower efficiency PV panels will require more roof top area and vice-versa. Suppose that a 1 kW solar PV system with 12% efficiency requires 125 sq. feet of roof top area, then a same capacity plant with 14% efficiency will need only 107 sq.feet area. Normally a roof top solar PV system requires about 100 to 130 sq. feet of shade free roof top area per kW of installed capacity.

Now coming back to our calculations for the PV system with proposed capacity of 470 W, in all 4 panels of 130 W_p capacity each is required. Here we have assumed that we are using 130 W_p, 12 V capacity panels. The calculation is 470/130 = 3.6, thus 4 number PV panels are required for PV system of 470 W (In fact 4 panels of 130 W_p capacity means 520 W).

The standard warranty in case of PV panels is 5 years as given by reputed manufacturers. Additionally the panel must be able to produce at-least 90% of its rated power output (at the given solar irradiation) during the first 10 years. Similarly it must produce at-least 80% of its rated power output (at the given solar irradiation) during rest of its lifespan.

Inverter Size

The electricity generated by solar PV panels are DC in nature and needs to be converted into AC using an inverter to run your normal domestic, commercial or industrial appliances. Inverters determine the quality of the AC power delivered by the solar PV system. Different inverter technologies are available in the market which support different levels of starting current requirement and hence affects the kind of equipments that can operate on the solar PV system. 

Now a day’s hybrid inverters are in the scene that automatically switches between 2 or more sources of power. These inverters have in-built automatic data logger, charge controllers, MPPT controller, islanding prevention, and various other kinds of protections much needed to keep your solar installation, equipments and the premise safe.      

The size of the inverter is kept normally 1.5 times the size of the solar PV system i.e. if the panel size is of 520 W then the inverter should be of 780 W. The inverter’s size is kept higher to prevent throttling of power output of the PV panel.

Solar installation companies, often called integrators, can complete a small roof-top project within a few weeks. Of all the components of a solar PV plant, solar module accounts for nearly 55 per cent of the total project cost. The investment primarily depends upon the size of the power plant. 

So, are you ready for the order?