To put it simply, a unit generated in solar is a unit saved in the grid. Based on this premise, this short article will attempt to highlight a few doorways in which the public can take part in promoting the solar power. Though this will indirectly save power from he grid, but as will be shown in part 2 of this article, may add directly to the grid as well. It will neither require a huge capital investment on the part of the government nor any planning effort, a few policy decisions is all that will be necessary to gently launch itself, saving a substantial grid power.
We are aware of the many apartments blocks in the city, Dhaka and elsewhere in Bangladesh. The utilities provided in these apartment-blocks among others are a connection from the water supply authority. Generally the line pressure in the water mains is so low that it can hardly maintain a flow to the underground water tank. Once this tank gets filled, the water is pumped to the rooftop water tank with the help of a pump. This roof top is generally 6-7 storeyed and the height accordingly and about 80-100 feet vertical, and in the terminology of pumping may present a head between 110 ft or more to the system.
This specific application is identified as an ideal candidate for solar application. The reasons being: The pumping system consumes a reasonable power, as against a lighting system, consequently a reasonable power is saved from the grid , which is therefore a sound proposition from generation/planning/ govt. perspective. There is no need or should not use any battery bank. A slight or substantial deliberate overdesign /selection of the solar panel and the inverter will pave way for income from the system. The pumping of water is so central and important in our lives, the system will not be relegated to the position of stand by junk so often is the case with lighting .
It may not be out of place to mention at this stage that the solar lighting (which is currently practiced) is necessarily a Battery based system. The battery bank itself is a considerable proportion of the total cost of the solar system. The battery will have a life of 3-4 years, expensive imported ones a bit more. As is generally the case, the death of the battery bank is the death of the system, because due to lack of fund of the apartment block administration, the battery bank is not replaced.
Consequently solar panel and accessories becomes a junk, waiting to meet its end-of-life, by rusting. This therefore does not serve the purpose it was intended to, i.e. save the grid power; but merely acts as a superficial set of procedures albeit with all good intentions. Without being overly pessimistic, its safe to assume that the high-end apartments do change the batteries. Now that we have established the need for a pumping system, we may briefly describe the system itself.
A bit of background
Solar panels generate DC voltage, just like in a car battery. If we connect this output voltage to a battery bank ( a series of battery) , the battery gets charged. If we connect the panel output to a DC motor, the motor spins, in other words we can use it for any purpose which require a DC voltage .
However a DC voltage is not suitable for many of the things we do at home; to light bulbs, to run fans or a freezer etc. To do that, we have to convert it first to AC with the help of an inverter, which produces an output ie power, similar to what we get from the main electric lines. Similar in voltage i.e 220 V, similar in frequency 50 Hz , and similar in shape a Sine wave, cheaper inverters however produce modified sine waves.
The pump hardly requires any introduction. We use it in every household, apartment block, shops and offices. The mention of it immediately gives us the image. This is invariably the centrifugal pump we know. Centrifugal pumps have the characteristic of producing its head ie pressure to lift the water to the required height only at the full rpm/speed. This full rpm can only be achieved if the voltage applied is the full ie design voltage, 220V . If the day is cloudy or if there is cloud passing, the solar panel output voltage will drop and so will the inverter output ,and the pump will either stop or run at a speed insufficient to lift the water. The centrifugal pump is therefore unsuitable for this application. The application mentioned above will have to use a different pump altogether. This is often referred to as Screw type or Helical type of pump and falls under the class known as positive displacement pump.
The important characteristics of this pump is that it produces the full pressure and flow on a sunny day , however on a cloudy day or cloud passing over, though there will be a reduced voltage from the inverter, and the pump will also run at a proportionate reduced rpm/speed, and will produce a proportionate reduced flow. However in spite of that, it will produce the required head to lift it to the required height. (this is the characteristic of the positive displacement pump)
In other words in low light condition there will be reduced flow but will not stop unless it is really dark or very low light condition. The matter of screw type pump in this type of application is my personal view on the matter. Somebody may have a different philosophy altogether. He may oversize a centrifugal pump which can do the job in half the time implying double the flow rate and hence bigger solar panel. The machine will remain inactive on cloudy days , passing cloud cover and low light condition.
Sizing the system, some indicative figures:
Overdesigning may be safe, but may not be possible financially and also technically. For an apartment-block having 15 or more apartments, a figure of around 20 gallons per minute flow (IGPM) or slightly more will be required for our solar insolation. There are not too many manufacturers whose pumps will reach that figure for the type of pump mentioned . A more likely figure would be 7 to 11 IGPM, with a Solar panel of 700-1200 W, In order to meet the full days requirement . Having two half capacity pumps would be an alternative solution.
Components of the Pumping System
It is necessary to have a overall idea of the system and its components. Thereafter a description of the components will be given in as much detail as the intent of this article permit. The components are: Solar Panels, inverter/ Inverter cum controller, level control, pump, piping , electrical Cabling Solar Panel: The solar panels are selected to match the power requirements of the pump. A 2 hp pump will require a larger panel than a 1 hp pump and so on.
Inverter: The inverter again is selected to match the power requirement of the pump. As mentioned earlier , its function is to change the DC voltage to AC. It has additional safety feature for the pump, like Low Level Stop to prevent dry running of pump and High Level Stop to stop over flow etc. In some designs , the pump uses DC so no inverter is required, the pump may be connected directly to the solar panel or through some controller which stabilises the voltage before connecting to the pump. It's a safer design philosophy to avoid an inverter, one less potential weak link.
Pump: Screw type, Helical Type stainless steel pump. Selected to match the flow requirements, ie the water requirement for the apartment / building per day. The requirement per day is important because the pump must be so sized/selected that it is able to lift the daily requirement in 1 day which is referred to as the peak sun hour, which is the available sun-time of approximately 4 hrs depending on the season in Bangladesh latitude .
A conservative estimate will be based on July-Aug-Sept sun peak time which is less than other times of the year. Pumps are selected on the required head-flow characteristics. Structurally these pumps are submersible pumps and are protected against dry running.
As already mentioned a DC motor may be preferred, a BLDC (brushless DC ) motor still has some electronics involved as against brushed, which requires the change of carbon brush every two years or so. Nevertheless, BLDc is a mature technology and there is no not to use one.
Piping: Quality water grade PVC pipes are the pipes of choice, there is no point is using 1'' diameter pipes, 1 ½ is the minimum, 2'' should be the normal unless calculations demand greater diameter. Use long radius elbows in the piping run. Using a lower dia piping is long term bad investment, contractors will very eagerly offer to reduce cost. The existing piping for mains driven pump if it has a size as mentioned above may be used, with the facility for switching to this or the other pump. It make sense to select 1 size more than the pump discharge, say select 1 ½'' if pump discharge is 1'' .
Electrical and Control cabling: The cables (solar panel to pump) must be sized to keep the voltage drop within specified limits . Make error on keeping the size higher 2.5-6 sq mm . Try to keep cable as short as possible.
The Economics: No attempt will be made to show the payback period , because it will be necessarily long. The concept is based on saving the grid power . and also to increase the percentage of renewable in the total energy generation.
What Next: To improve the payback time drastically and to make such systems a scheme of investment, I will attempt to show in Part 2 of this article to use the above system to become a source of earning, of course, due change in Govt. policy with be required to attain this.
Summing Up: This article is an attempt to highlight the potential application of Solar Energy to household or commercial pumping. The apartment blocks in the city is a potential customer, as the investment is divided among the apartment owners. These systems normally use reasonable amount of energy in Kwhr, so the potential savings in grid power by the same amount is expected when replaced by solar.
It was also mentioned that there is the possibility of generating solar electricity from these systems and earn a reasonable sum , or decreasing your power bill, in effect increasing the return on investment and increasing the percentage contribution of the renewable to the net power generation . This aspect of the matter will be covered in Part 2 of this article.
There are two sides to its implementation: A policy in this regard that all new commercial and residential apartment blocks must have a solar pumping installation. Existing commercial and residential blocks may be given a grace period of 2-3 years to have one installed. The second is to give appropriate duty free status to the components mentioned above. Since this has a reasonable market, with some components (solar panels etc ) made locally, it could take off without much delay.
We need not look at the return on investment. Energy security, energy from a wider source i.e. broader mix, and on top of that, a renewable source is more than enough reason to give it the due emphasis it deserves.
The writer is a retired senior government engineer
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