Module Cleaning – Operations & Maintenance Viewpoints

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A lot has been deliberated about module cleaning but it continues to a key area of interest at Solar Power Plants. The belief is cleaner Modules translate to better generations nos. and thereby higher revenues. However, it needs to be seen in this perspective – “Different Strokes for Different Plants”. Module cleaning using water cannot be a clinically driven process scheduled once or twice a month.


A more pragmatic approach is required to carry out this activity based on the dust levels on the panels, terrain, weather conditions, appreciable drop in performance ratio(PR) under no break down conditions. Many clients still go for a programmed schedule and expect the Operations & Maintenance(O&M) provider to do this activity religiously since it is a contractual obligation; the time has come to have alternate approaches and to use available technology to salvage a precious commodity, water. Yes, water, since Solar Plants may last 25 years but for how long will the water be available for this mundane activity across regions? Also, time should be devoted to inverter performance analysis to arrive at critical areas where performance is affected and to check whether it is attributed to deficiencies in module cleaning.Introduction: - A lot has been deliberated about module cleaning but it continues to a key area of interest at Solar Power Plants. The belief is cleaner Modules translate to better generations nos. and thereby higher revenues. However, it needs to be seen in this perspective – “Different Strokes for Different Plants”. Module cleaning using water cannot be a clinically driven process scheduled once or twice a month. A more pragmatic approach is required to carry out this activity based on the dust levels on the panels, terrain, weather conditions, appreciable drop in performance ratio(PR) under no break down conditions. Many clients still go for a programmed schedule and expect the Operations & Maintenance(O&M) provider to do this activity religiously since it is a contractual obligation; the time has come to have alternate approaches and to use available technology to salvage a precious commodity, water. Yes, water, since Solar Plants may last 25 years but for how long will the water be available for this mundane activity across regions? Also, time should be devoted to inverter performance analysis to arrive at critical areas where performance is affected and to check whether it is attributed to deficiencies in module cleaning.


Economics: A Silicon Polycrystalline module requires a conservative 2.5 litres of water/per module during module cleaning. So, a 10MWp plant typically will use 85,000 litres of water for every cycle. Add to this the cost of water, which is mostly bought in remote & barren sites and the cost of labour. For 9 cycles in dry months it can totally cost upwards of Rs3,00,000/annum. If water is scarce then the costs can be 25- 30% higher.


PROS & CONS of Module wet cleaning:

Cleaning with pressure hose can remove stubborn dirt & grime. However, the dirt can settle at the corners of the module if the water does not flow off properly. Modules at a larger tilt angle are less likely to hold the water due to gravity. Soft water is recommended to be used to avoid scaling on the modules, post evaporation. Availability of soft water cannot be guaranteed in remote areas with scarce resources and that puts a brake on the process at times. Reverse Osmosis(RO) plants installed at many sites also fail quickly because of the hardness of the water available in many parts of the country; this is an added cost to the client.
Wipers can be used to remove any settled dirt but the cloth wipers used should not scratch the surface, which in the long term can be detrimental. The design of the wipers should be fool proof that there is no deposit of lint on the modules while or after cleaning.
A couple of other constraints are there for wet cleaning - it can be done for a short time-period, mostly before 10 AM and after 4 PM when the module temperatures are in the 30-35 degrees C range in the tropics. Many clients shrink the time to a 2 hour window between 5-7 AM and 5-6 PM, which can affect the cleaning cycle and can prolong for more days in a month. It is better to align to the manufacturer guidelines for cleaning and follow a 15day cycle for cleaning at a medium size plant. Cleaning post dusk has its own risks of personal safety to the individuals and the cleaning quality may be erratic too though this is the most preferred time for clients. 

Approaches:
Dust deposition pattern is to be studied in detail to organize the module cleaning activities better. It is usually dependent on weather conditions and the type of soil and vegetation at/near the site. Limited wet cleaning depending on the location can be carried out based on the assessment of the dust and the level of dip in performance ratio. If the O&M provider is meeting contractual obligations on Performance Ratio (PR) then a cycle can be staggered based on mutual consent between the O&M provider & the Plant developer. That can indirectly save the costs of bought out water / power costs of running pumps, if water is available at site.


In India, the dust levels during the pre-monsoon months (Mar- May) are high and the cleaning will have to be more regular and practically daily whereas in the monsoon months it will not be required, and in the other months it can decided on a case to case basis.


Case study:
In a Rajasthan plant, it was noticed that module cleaning, when not done for 2 months in a row did not affect energy generation considerably; the loss of generation of about 3 percent in 2 months could easily amount to at least 70% costs of doing module cleaning itself. If more cycles were planned each month, as some clients may insist, then then it will be reasonable business sense to not clean at all!
Data with respect to module cleaning for a Thin Film plant shown below is for a 6-month period where generation for 10 non-break down days have been considered. No Module cleaning was done in Months Sep & Oct and months Jan & Feb. Only during the Nov & Dec months two cycles of module cleaning was carried out in each month. The results showed up that there was only a 1.5% dip in generation /month on an average. This is at least 50% lower deviation than usually noticed if a plant is not cleaned once in a month. A general thumb rule in the industry is that Soiling losses are at 3% -4% between a plant not cleaned Vs the same plant being cleaned regularly.

 

A point to note is that there is always a possibility of 1.5% -2% difference of generation on daily basis between an inverter having the maximum generation of the day and the median generation across all inverters in a medium / large size plant where module cleaning is regularly happening. This is because the cleaning cycle will be block wise and will cover associated inverters day by day. So, a minimum 0.5% - 1% generation loss may be inherently seen at a plant level with inverters’ performance variations (as an example for a 10Mwp plant) even if module cleaning is regular.A point to note is that there is always a possibility of 1.5% -2% difference of generation on daily basis between an inverter having the maximum generation of the day and the median generation across all inverters in a medium / large size plant where module cleaning is regularly happening. This is because the cleaning cycle will be block wise and will cover associated inverters day by day. So, a minimum 0.5% - 1% generation loss may be inherently seen at a plant level with inverters’ performance variations (as an example for a 10Mwp plant) even if module cleaning is regular.

If plants have seasonal tilt it is better to concentrate more on the module cleaning in the seasons where the tilt is 3-5 degrees with an eye still on the Performance ratios. The trigger points can be decided mutually as to when to do the module cleaning rather than adhere to a schedule which starts on 1st of every month and ends on 15th/20th.Depending on the type of soil at site the cleaning cycle can be altered as some areas have clay like soil where the dust particles can stick more to the glass and it will be imperative to do cleaning regularly whereas in other areas where the soil or dust is hard sand and it may not stick.

If there are unseasonal rains during each month the module cleaning cycles can be abandoned or continued a case to case basis. It is better to review site conditions after unseasonal rains as it leaves behind a lot of unwanted dust on the modules. Selective cleaning can be done on affected modules alone. Inverter wise PR measurements can indicate the blocks that may require cleaning. Miscellaneous Issues:
While doing module cleaning the pyranometers must be cleaned as per a desired frequency. Usually these are cleaned once at the beginning of each cycle. This frequency should not be tampered since this can affect performance ratio measurements at the plant. Cleaning it daily is not advisable as it may lead to disturbing the inclination settings, lead to inaccuracies in measurements and may indicate poor plant performance, which may not be the case.

Additional Module cleaning may be required to be done if the bird population at the plant is high since bird droppings on solar panels will be very common. This is more serious as the acidic nature of the droppings can affect performance significantly by shading and hot spot creation. Water is still the best agent for cleaning bird droppings as solvents are not usually recommended by module manufacturers. Changing bird behaviour is the best approach or some deterrent is to be applied. Use of bird scare mechanism is suggested and has been seen to be an apt deterrent.

Dragon flies are other creatures that can affect Solar plant aesthetics a lot. These flies that thrive near water bodies can lay thousands of eggs on Solar panels, many a time mistakenly considering the panels to be water bodies. Cleaning the egg ridden panel is a tough task though the effect on plant performance has been noticed to be limited.

Way forward for Module cleaning: Use of technology should be stepped up like use of drones for monitoring dust levels at the plants or dry cleaning using robots. The advantages of using robotic cleaning are significant as a consistent 3-3.5% higher output is possible daily over conventionally wet cleaned modules where cleaning schedules are staged over a fortnight /month. The pay back on the investment can be within a decent 5-7 years. Developers can think of working on these lines as it has always been a classic complaint in the industry on the quality and pace of wet cleaning of modules in large size plants. The suspicion mostly is that PRs are low because of inadequacy of Module cleaning.

Other technologies to be looked at here are dust sensors and a self-cleaning mechanism. Different methods – one as a trigger point to clean when dust reaches a threshold can be thought about; those arrays or blocks can be marked for wet cleaning b) to auto clean the panel by creating electro static charge to repel dust. Suitable technology needs to be scouted for and may be commercially available.

Developers can also explore buying more superior modules with good Antireflection and Anti-Soiling properties for any future investments; it can surely cut the recurring costs of wet cleaning.

  Author- Ganesh H, AVP – Analytics.

http://avisolar.com/

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