Energy Savings from Supplying Drinking Water at Two Pressure Levels - a Retrofit in Las Delicias, El Salvador, EWB-MAP

Location

Problem Las Delicias has an existing drinking water system that consists of a well, two pumps (submersible and booster), three storage tanks at elevation (one supplied by a spring), and gravity distribution system. The village experiences water shortage due to high electricity cost. Hoarding water leads to disease. Over 80% of children in the village have parasites which leads to school absenteeism. Standing water provides breeding ground for Dengue Fever mosquitoes.

Solution Our chapter (EWB-MAP) came up with a way to supply about 30% more water for the same energy consumed. About 50% of the population lives below the elevation of ~250’ above the elevation of the well while the tanks are located over 400’ above the elevation of the well. The solution is to add another tank at an intermediate elevation and pump to two different pressures.

Hoarding Water

A typical water system

Critical Elevation Diagram, existing system

Idea explored: new tank plus pump Good for grass-roots design. No space available. Electricity only reaches the existing pumps.

Idea explored: another well at higher elevation

Idea explored: pressurized system solution Tanks 1 and 3 can still be filled by using a booster pump in the manner it is used now. During filling the lower elevations would get water at higher pressure. After Tanks 1 and 3 are full the isolation valves on the two supply lines are closed and the pump only pressurizes the distribution system at lower elevations. A sustainable water distribution system uses gravity to drive the system and has been designed to offer a long service life with little to no maintenance.

Idea explored: tank level control using a float valve There is a float valve in the tank that closes when the tank is full (like in a toilet). When the valve closes the pump gests back-pressured (the operating point goes up on the pump curve). There is a pressure switch high (PSH) on the pump’s electrical controls. It shuts down the pump when the discharge pressure exceeds a certain value. Such scheme completely eliminates the need for an electrical, phone, or radio signal from the tank. The water in the pipe itself conveys the information.

Critical Elevation Diagram, proposed system

Proposed booster pump test

Modified booster pump test The ultrasonic flow meter was not charged. We ran the submersible pump (known flow) to obtain the so-called strapping chart of the cistern (liquid level vs. volume stored). The cistern has a baffle in the middle but one does not have to know the exact geometry to use a strapping chart. We drained the cistern using the booster pump varying the discharge pressure to produce the characteristic (head vs. flow).

Idea explored: liquid level indicator

Submersible pump

Booster pump

Submersible pump test

Booster pump test

Pump characteristics curve

Challenge We have to be able to match the submersible pump flow and the booster pump flow (they match now at high booster pump discharge head). If we run the booster at lower head (pumping to the new tank) the flow will increase. The energy consumed will decrease but (a) the cistern will run dry, (b) the efficiency will decrease. Bigger cistern takes care of (a) but not (b). Solution: Variable Frequency Drive (VFD).

Pump characteristics curve - efficiency

Variable Frequency (Speed) Drive

Pump characteristics curve with a VFD

Map of Las Delicias Water System DOWNHILL

260 Homes Served by Tank 3 33 Homes Served by Tank Homes Served by Tank 1 DOWNHILL Map of current Las Delicias Water System from ADESCO with approx house locations indicated

117 Homes Served by Tan 3 33 Homes Served by Tank Homes Served by Tank 1 Proposed Water Distribution System 280 Homes Served by New Tank DOWNHILL

Map of Current Water System (section relevant for EWB-MAP modifications) RED lines – supply lines to tank (uphill by pump) BLUE lines – distributions lines by gravity DOWNHILL New 17,000 gallon tank Exisiting 11,000 gallon tank

Proposed update to Water System (section relevant for EWB-MAP modifications) RED lines – supply lines to tank (uphill by pump) BLUE lines – distributions lines by gravity NEW SUPPLY LINE 1 NEW DISTRIBUTION LINE 2c NEW DISTRIBUTION LINE 2a NEW DISTRIBUTION LINE 2b NEW TANK A B C D E F DOWNHILL 1.Tank Construction 2.Pipe connections 3.Pipeline Plans 4.Distribution of Work

Proposed update to Water System (section relevant for EWB-MAP modifications) RED lines – supply lines to tank (uphill by pump) BLUE lines – distributions lines by gravity NEW SUPPLY LINE 1 NEW DISTRIBUTION LINE 2c NEW DISTRIBUTION LINE 2a NEW DISTRIBUTION LINE 2b NEW TANK A B C D E F DOWNHILL 1.Tank Construction 2.Pipe connections 3.Pipeline Plans 4.Distribution of Work

Proposed update to Water System (section relevant for EWB-MAP modifications) RED lines – supply lines to tank (uphill by pump) BLUE lines – distributions lines by gravity NEW SUPPLY LINE 1 NEW DISTRIBUTION LINE 2c NEW DISTRIBUTION LINE 2b NEW TANK A B C D E F DOWNHILL

Photos along Pipeline Path Area of Connection to Tubo Madre NEW SUPPLY LINE 1 NEW DISTRIBUTION LINE 2b NEW TANK A B C D

Photos along Pipeline Path NEW SUPPLY LINE 1 NEW DISTRIBUTION LINE 2b NEW TANK A B C D

Photos along Pipeline Path NEW SUPPLY LINE 1 NEW DISTRIBUTION LINE 2b NEW TANK A B C D

Photos along Pipeline Path NEW SUPPLY LINE 1 NEW DISTRIBUTION LINE 2b NEW TANK A B C D

Photos along Pipeline Path NEW SUPPLY LINE 1 NEW DISTRIBUTION LINE 2b NEW TANK A B C D

Photos along Pipeline Path NEW SUPPLY LINE 1 NEW DISTRIBUTION LINE 2b NEW TANK A B C D

Photos along Pipeline Path NEW SUPPLY LINE 1 NEW DISTRIBUTION LINE 2b NEW TANK A B C D

Photos along Pipeline Path NEW SUPPLY LINE 1 NEW DISTRIBUTION LINE 2b NEW TANK A B C D

Photos along Pipeline Path NEW SUPPLY LINE 1 NEW DISTRIBUTION LINE 2b NEW TANK A B C D

Photos along Pipeline Path NEW SUPPLY LINE 1 NEW DISTRIBUTION LINE 2b NEW TANK A B C D

New Tank

New tank detail

Possible distribution scheme

Conclusions If different areas served by the same water system are located at different elevations it is possible to save pumping energy by pumping to two (or more) different storage pressures. One can just change the backpressure on the pump. The thing to watch for is the pump’s efficiency. A VFD can fix the problem. Multiple pumps and storage tanks at different elevations can be used in grass-roots design. Additional savings come from time-of-day power cost and power factor (VFD takes care of the PF; otherwise, one can use capacitors).