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Surge Protection Device for Solar PV installation

Effects of Lightning Strike on Solar PV system: A solar PV installation is vulnerable to the effects of lightning stroke. The potenti...

Friday, 29 May 2015

Know your Solar Batteries

Lead-acid batteries are commonly used as an energy storage device for the stand-alone or off-grid solar PV system. Batteries are rated according to their voltage, Ampere-hour (Ah) storage capacity and their ability to deliver the stored energy over a given period of time called the C-rating. Figure 1 shows a 150 Ah, 12 V, Flooded Lead-Acid, Tall Tubular Solar battery.


Fig. 1: 150 Ah, 12 V, Flooded Lead-Acid, Tall Tubular Solar battery.

When a voltage, higher than the battery, is applied to the battery through an inverter or a charge controller of the solar PV system, a charging current will flow into the battery and will charge it. The flow of charging current is opposite to the flow of load current supplied by the battery. The rate of charge or current that will flow into the battery will depend on the difference between the battery voltage and the applied voltage (the voltage supplied by the solar PV system etc.).

Once the battery is fully charged, it is necessary to stop the charging otherwise it will damage the battery. Its here the charge controller comes into picture. A charge controller, usually used with the solar PV system, is thus essential to ensure that the battery is never overcharged.

Overall efficiency of charging and discharging of LA battery:
The temperature of operation and the rate of charge/discharge affect the performance of the battery. Because of the electrical resistance (internal resistance of the battery and the electrical path), some of the electrical power (I2R) and hence energy supposed to charge the battery is converted into heat. During the charging process, some of the hydrogen ions combine with free electrons and are converted into gaseous hydrogen. When hydrogen is lost during charging, energy is also lost. 

Thus, the charging efficiency is about 95%. 

Similarly, energy is also lost during discharging of battery and hence, 
the overall efficiency of charging and discharging of a lead-acid battery is nearly 90%.

Since the battery loss due to resistance is proportional to the square of the current, therefore 

high current charging or discharging will result in higher internal losses and reduced overall efficiency.

Effect of Temperature on Battery performance:

There is also an increase in the battery temperature during charging and discharging, particularly at high rates, due to higher I2R losses. If the battery remains too warm for a longer duration its life is reduced. Thus, charging and discharging rates should not exceed the specified values given by the manufacturer for a given type of battery. Increase in temperature accelerates the chemical reaction process in the battery. Thus, in high temperature the battery is liable to get self-discharged without any external load and hence has to be kept in a cool place with preferable temperature of 27 oC.

A fully charged 24 V battery will have a voltage of around 25.5 V. As the electrical load, connected to the battery through the inverter, draws the current, the level of charge in the battery is reduced and thus the voltage of the battery. The battery voltage will fall down quickly at first, but as the discharging continues, the rate of voltage drop slows down and will reach 24 V when the battery has drained to half of its rated charge capacity. As the battery approaches the fully discharged state, the voltage starts to fall at a greater pace again. When the electrical load is drawing a heavy current from the battery, the voltage will drop. The larger the battery, smaller is the voltage drop.

What is different in a Deep discharge flooded lead-acid battery?

Deep discharge flooded lead-acid battery, commonly used with solar PV system in India, uses antimony to strengthen the lead and can be used down to 20% of their initial capacity. The plates are thicker, with less area and hence are designed for sustained lower level currents. Flooded batteries need water topping at least twice a year. Although deep discharge batteries are capable of deep discharge, but their life depends on the depth of discharge incurred every time during the operation. Deep discharge i.e. 70 to 80% discharge of the rated charge leads to decrease in battery life.            

Friday, 22 May 2015

Washing of Solar PV Arrays

A solar PV array is a set of solar PV modules mounted together and connected in series, parallel or series-parallel combination to get the desired electrical output i.e. voltage, current and hence the DC power.

Why washing of Solar PV arrays is important?

Solar PV arrays are usually called maintenance free, but they require regular cleaning and washing to maintain the rated power output. The glass covers are cleaned regularly to remove dirt and other deposits such as bird droppings. Cleaning of Solar PV system is regular and a natural process during the rainy season, but in other seasons, regular washing of the same with simple water is needed to enhance the system performance. Figure 1 shows the manual cleaning of a small roof top solar PV system.

Fig.1: Manual cleaning of a small roof top solar PV system.

Wrong cleaning practices, bad quality water and use of in-appropriate cleaning agents may damage the PV modules or other components. It may also reduce the system performance. Certain Solar PV sites require specific cleaning procedures. Procedure also depends on the water quality and the cleaning mechanism used.

What precautions to be taken while cleaning of Solar PV system?

The water used for cleaning of PV panels should be clean, soft and low in mineral content. If the water is comparatively harder, the water must be squeezed off to avoid building up of scale. In any case, the water hardness should not be more than 200 ppm. 

One can use lukewarm water and soft sponge or micro-fibre cloth also if the deposits are thick and stubborn. If necessary, a mild, non-abrasive, non-caustic detergent may be used. Situations, more prone to diesel fumes and deposits, may require a mild de-greaser agent along with the soap.

Cleaning of solar PV arrays should be carried out when they are not excessively hot i.e. the arrays should preferably be cleaned in the morning or evening time. Thus, cleaning of small solar PV arrays is similar to cleaning of any ordinary glass window. 

But in large Solar PV systems, several PV modules are connected in series, producing enormous DC voltage. In PV systems having higher DC voltages, cracks in modules, cables or joints may pose dangerous conditions to cleaning personnel. Hence, it is adviced to inspect the modules and cables thoroughly for any cracks or other damages before the commencement of cleaning. Workers engaged in cleaning should wear proper protective equipments while working with larger and higher voltage PV plants.  

Visual inspection of the PV modules should also be done after washing to check for any cracks, chips or discolouration. If any defect is observed, it should be monitored regularly for any further deterioration. During the inspection, condition of the mounting frame should be noted for any loose nut/bolt or rusting. 

Sunday, 10 May 2015

Pachmarhi a small town in Madhya Pradesh with so many waterfalls

Pachmarhi or Panchmarhi, also known as “Satpura ki Rani (Queen of Satpura)”, is a hill station in Madhya Pradesh state of Central India. It is situated at a height of 1100 m in the Satpura hill range in the Hoshangabad district of Madhya Pradesh. 

Blessed with very rich flora and fauna, lush greenery, waterfalls, rich and rare wildlife, Pachmarhi is well connected from Bhopal, Nagpur, Hoshangabad and Pipariya. The nearest railway station is Pipariya (on the Itarsi-Jabalpur section) which is nearly 55 km away. From Bhopal, one can reach Pachmarhi in 5 hours by road and the distance is 200 km. Pachmarhi is a popular tourist retreat and one can visit the place through-out the year. 

The pleasant time to visit the place is from October to December, but during this period the water in the falls is too cold to get a bathe. Even in the March the water in the falls is a bit colder to some people.

The highest point in Pachmarhi is Dhoopgarh which is 1350 m, an ideal place to see sun rise and sun set, although watching sun-rise is now banned by the forest department and the forest barrier opens up only after 9 am. On way to Dhoopgarh, one can see Bison and Peacock, particularly during rainy season. Pachmarhi once served as the summer capital of the Central Province. Pachmarhi is also listed among the Biosphere reserves in the UNESCO list.

Pachmarhi is a small town dependent mainly on tourism and most of the area is under the administration of Pachmarhi Cantonment Board serving the Indian Army. The only petrol pump is the police petrol pump. There are many budget hotels along with hotels and resorts run by M.P.Tourism Department.

Most interesting places to visit in Pachmarhi are:
  Rajat Prapaat or Silver fall,
  Apsara Vihar and Paanchali kund,
  Bee fall,
  Rajendra Giri,

The places of devotional interest (pilgrimage site) are:
  Jata Shankar,
  Bada Mahadev,
  Gupt Mahadev,
  Old Church which is now closed to visitors.

Near to Pachmarhi are 3 wildlife sanctuary/park namely, Bori sanctuary, Satpura National park and the Pachmarhi Sanctuary. These three are collectively called Satpura tiger reserve. The jungle nearby has tigers, panthers, bear, bison, apart from wide variety of birds and small animals such as monkeys (they are also visible in the town and nearby your hotel).

To see the waterfalls and other similar locations one has to hire a Maruti Gypsy, which is the best or the only vehicle available in Pachmarhi to go through the ghats. A full Gypsy can be hired for 1200 INR; one can also pay per person which goes to about 250 INR per head. Apart from this one has to pay the forest fee and guide hiring fee of 900 INR /vehicle/ day to the forest department. In day one, you can visit Pandav caves, Silver falls, Apsara Vihar, Paanchali kund, Bee fall, Reechgarh and Dhoopgarh.

Silver fall

Sunset on a cloudy day at Dhoopgarh
The next day one can go to Duchess fall, although for Duchess fall there is a trekking path of nearly 2 hours (to and fro), so one has to be mentally prepared to visit the Duchess fall. But it is a really nice waterfall to visit, calm and peaceful as compared to Bee fall.

Above the Duchess fall is the AC fall where one can see water in bulk pouring down through a small gap of some 3 feet width. Unlike some of the falls, all the waterfalls in Pachmarhi has waterfall throughout the year.  Similarly for Chouragarh, one has to walk through the hills for some 2 or 3 hours. Other devotional places can be approached by your own vehicle.

Pachmarhi is perhaps among the highest visited hill station in Central India. It is a very nice place for a 2-3 days break at an affordable price. The people are cooperative and easy and special preparation is needed.