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Residential Grid Connected solar rooftop PV system (A case study)

It is easier and cheaper than ever before to install solar panels on the roof ! Falling PV module prices and very conducive policy environment has made installation of rooftop PV panels a very lucrative investment for individuals and businesses.

Government of Karnataka launched the solar rooftop PV (SRTPV) policy in November 2014. Net metering was introduced for the first time in Karnataka under this policy.  One of the early projects under net metering policy was designed and installed by Hinren Technologies . The project was of a 2 KWp grid connected solar PV system. This blog post is about the technical and financial aspects of the project.

While this post is specific to the project executed, the technical details can be easily extended to project of any scale.

INTRODUCTION TO SRTPV SYSTEM AND BESCOM POLICY:

What is a Grid Tied System? 

While there are many configurations for solar rooftop power systems, Grid connected systems have gained momentum and popularity mainly in urban areas.

The system is interfaced with the power utility grid. Here, Solar PV system takes the first priority to run the loads. The excess power, if generated will be exported to the power grid. In case of deficit of solar power, the differential power is drawn from the power grid. In this system, a bi-directional meter is installed to measure the net energy (Net metering).

Grid-connected PV systems are the most popular solar electric system in the market todayIt is the least expensive and lowest-maintenance option. Even though the technology behind grid connected solar energy seems complex, in reality it is not so. It is quite simple . Let us explain

  • The sun’s radiation on the solar panels generates DC electricityWorking
  • The DC electricity is fed into a solar inverter that converts it to the 230V 50Hz AC electricity, equivalent and in sync with the power grid.
  • The 230VAC electricity generated will be used to power appliances at the consumer loads (Homes, businesses etc)
  • If the panels generate excess energy than required by the consumer loads, the excess energy is exported to the grid.
  • On the other hand, if the energy generated by the panels falls short of the demand by the consumer loads, the differential (deficient) energy is drawn by the power utility grid.

The BESCOM (Bangalore Electricity Supply Company Limited, Bangalore) and other ESCOMS in Karnataka have initiated a very aggressive solar rooftop initiative and promoting it in a very big way. They have made it very easy for every consumer to be a producer of energy. They are constantly reducing the paper work and other formalities to make it simple for citizens to take advantage of the policy.

Under the current policy, the consumer has three benefits.

  1. He can be the producer of his own electrical energy.
  2. He can sell the excess energy produced to the power utility grid at 9.56 rupees a unit.
  3. Businesses can claim upto 80% depreciation of the asset in the first year itself (This greatly reduces income tax burden).

Our Project: 2 KWp SRTPV plant at Residence of Mr Ramkumar Radhakrishnan

One of our old clients, Mr Ramkumar Radhakrisnan was always interested in Solar energy for his house. He is a very environmentally conscious man. We had earlier designed and implemented Rainwater harvesting at his house. He always wanted to go solar . The launch of the SRTPV policy by BESCOM gave a fillip to his passion and he was one of the first applicants to BESCOM to install SRTPV system.

The exchange of mails and visits started in last week of December 2014. Ramkumar wanted to start the work as soon as possible. But there were some confusion with regard to empannelment of inverter. Subsequently BESCOM came up with the clear list of empanneled inverter manufacturers and we finally kicked off the work from the second week of March 2015. The second operational issue was with regard to the bi-directional meter. We had to wait for almost 25 days for the meter to arrive after all testing and certifications. There were some minor confusion here and there but by and large all the work was completed by 24 April 2015. Two engineers from BESCOM did repeated testing of the system and finally signed off on the 2 of May 2015.

With all the hitches and glitches, we could finish the installation to the satisfaction of our Client and as per the guidelines of the BESCOM. The results obtained from the net meter after a week of the installation shows that daily about 8 Units is being generated

TECHNICAL DETAILS:

After a brief introduction about our project, let us now get into the details of various components and processes involved in the project. Broadly the installation has the following technical processes.

  • Undertaking of comprehensive site assessment;
  • Determining what the installed capacity (i.e. Watts or kilo-Watts) of the system needs to be;
  • Selection and Matching of system components;
  • Analysing the expected average yearly output and losses;
  • Determining the physical size of the PV system;
  • Determining the best location for the installation of system components.

Undertaking a Site Assessment:

Below is the list of objectives of the site assessment to determine –

  1. Energy efficiency: In the first site visit we discussed with the client about the energy-efficient measures that one needs to adopt like using of LEDs in place of CFLs and how one can reduce the consumption of power by using energy-efficient equipments.
  2. Occupational health & safety issues: Following list of safety issues/concerns checked for and incorporated during design and installation.

 

ACTIVITY PERSON AT RISK SIGNIFICANT HAZARDS RISK CONTROL MEASURES  
 
Demarcation of the work area Staff, others Debris falling onto those below ·      The work area, loading and unloading areas were marked out using barriers or cones to prevent unauthorised access.

 

·      Where necessary signage was used to indicate such areas.

 
Working on roofs in general Staff Falls from height ·        Complete roof was checked to ensure that they are in good condition before accessing the roof.

 

·        Checks were made from both the outside area and in the roof space.

 
Access to roof level and installation of solar panels Staff Falls from height ·        Due to it is a flat roof, access to the roof is through steps, which was safe.  
Lifting tools and equipment on to the roof Staff, others Falling equipment ·      As the equipments were not very heavy, they were easily carried to the place of installation.

 

·      Staffs were using tool bags to carry hand tools when working on the roof.

 
Protection of staff Staff Falls from height ·      Staff was wearing appropriate attire when installing solar panels on roofs.

 

·      Staff was wearing head protection, safety shoes.

 
Installation of solar panels Staff, others Falling equipment ·      Solar panels were installed on roofs in accordance with the manufacturer’s instructions.

 

·      Solar panels was installed by trained competent staff only.

 
Electrical installation work Staff Electrical shock ·      Trained and competent staff carried out electrical installation work.

 

·      All work were carried out to the standards stipulated in the Wiring Regulations.

 
Using power tools on roof spaces Staff Falls from height, slips trips and falls, and electric shock ·      Where trailing cables, e.g. extension leads, are used, they were kept clear of walkways.

 

·      All extension leads obtained their electrical supply via an RCCB (residual current circuit breaker) in order to prevent electric shocks.

 
Handling equipment into and out of roof spaces Staff Falling equipment ·      Entrance to work area was well protected to prevent access by others.

 

·      Work area was adequately signed indicating no access for other people.

 

3. Solar Access: During the site visit, we analysed the roof for shadow free area say for at least 5 hours. We also analysed that solar access should not be affected by any of the following –

  • Trees or other vegetation nearby;
  • Other buildings;
  • Parts of the actual building where the system is to be located.

Keeping this in mind, we made sure that the overhead tank on the roof and solar water heater do not shadow the panels during peak sun hours.

After analysing the above mentioned points, we finalized the shadow free area to install the solar panels on the roof.

Calculating the required Capacity of the system:

Analysing the monthly electricity bills of three months, preferably during summer (as the electricity requirement is generally high), one can arrive at the average energy consumption.

In this case, the requirement is approximately 5 Units per day. Keeping this, we chose a system capacity of 2kW.  This produces 8 Units per day during peak sun hours.(So that 3 units can be exported to the grid). Please note that in grid connected system, one can oversize or under-size the system depending on the budget. Energy consumption need not be the parameter for arriving at the configuration.

Selection of PV Modules and Inverter:

  • PV Module: EMMVEE Crystal 250Wp

Specifications: Number of cells in each module is 60 polycrystalline cells. By Matching the output required with the inverter, on can arrive at the minimum modules required.

  • Inverter: Delta RPI H3 (3 KW)

Reason: The requirement of the proposed site is 2KW. we arrive at the consideration that the above mentioned inverter can be selected. The said inverter is transformer-less, with 97% peak efficiency. These inverters are compact in size with durable quality to ensure smooth PV system operation. IP65 enclosure provides higher level of protection and enhances its durability in a harsh outdoor environment.

P.S.: Even though the present requirement is 2KW, a 3 KW inverter was selected. It is because 2 KW inverter of delta make is not available. Also it is usually cheaper to purchase an inverter with the estimated higher capacity than buying two small inverters, i.e. in case of increasing capacity of the system when additional need arises (expansion to 3KW, equivalent to the sanctioned load).

Matching the Array and the inverter for maximum output:

After matching the PV array and the inverter we obtain that 1 string of 8 modules is required to generate the required power.

Site Plan of the project:

Slide1

List of Balance of system components and their IP Ratings:

Sl. No. List of Components for Balance of System IP rating
1 The inter-array cabling and cabling to the array junction box IP56/65/66/67
2 The array junction box (sometimes called DC combiner box) if required IP65/66/67
3 The main cable from the array junction box (or array if no junction box) to the inverter IP65/66/67
4 Protection and disconnect switches  
Fuses housed in thermoplastic IP65
DC isolators and/or circuit breakers (Non-polarised) IP56
AC isolators and/or circuit breakers IP42 (for covered external applications) and more
5 The AC cabling from the inverter IP42 (for covered external applications) and more

Earthing requirements for the modules and/or mounting systems and inverters (particularly transformer-less inverter):

A PV array system connected with a non-isolated (transformer-less) inverter requires the metallic frames and conductiveEarthing
structural supports of the PV arrays to be earthed because of the high frequency switching which may occur within the inverter and can cause a small AC-like fluctuation in the array cables. The array cables are effectively capacitively coupled to the module frames and over time this produces a voltage on the module frames.

Hence, any array connected to the transformer-less inverter needs to have equipotential bonding on all array frames and mounting rails. This is for the reason that equipotential bonding or protective earthing is intended to minimise the risks associated with the occurrence of voltage differences between exposed conductive parts of electrical equipment and Earth Resistivity Test EB Curdt's 61.8% Methodextraneous conductive parts.

This shall be achieved by connecting the earthing conductor with a minimum cross-sectional area of 4mm2 directly or via the
inverter to the installations earthing system.

All the modules frames, the inverter housing, surge protection devices , the AC and DC distribution boxes were earthed using a 08 Sq mm GI flat.

Two separate earth pits were dug to a depth of 2.0 M . 40mm GI electrodes were driven into these earth pits, interconnected with GI flat and backfilled with conductive material. The GI flat was connected to these electrodes.

Earth resistivity test was conducted and the earth resistivity was found to be 4.81 Ω (Should be less than 5 Ω)

Electrical Schematic of the wiring:

Slide1

Design of mounting structure:

The mounting structure specifications are as follows:

  • The pitch (15º) of the mounting structure has been fixed close to the latitude of Bangalore (12ºN), because if the modules are tilted at approximately latitude and facing South, they receive more sunlight on an average throughout the year;
  • Powder coated 3 mm thick Mild Steel (MS) Plate has been used as mounting brackets.
  • Burnt Brick Ballast has been used as a counterweight. Therefore drilling of the roof is not required to fix the bracket.
  • Wind speed analysis using IS875: Part 3 has been carried out. Designed structure can withhold the Wind velocity upto 19 m/s;
  • The mechanical drawing of the structure has been given herein below:

Slide3

 Expected average yearly output and Efficiency Analysis: 

Considering the values of irradiation data with respect to Bangalore; rated power of the PV Modules and losses (as efficiencies) such as shading, dirt, manufacturer’s power tolerance, inverter efficiency, voltage drop across the cable, temperature de-rating factor; we have obtained 3454 kWh/Year as expected average yearly output.

Following pictures show the self-consumption, efficiency analysis and return of investment (ROI) for the project:

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Grid Tied Rooftop Solar PV System Policies in Stat...
Solar Project at Chandpur Village
 

Comments 1

deepak on Monday, 09 January 2017 05:51

What is a Grid Tied System?

What is a Grid Tied System?

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