HOME Schematics Discover Forum Hobby Corner Dave's Circuits Electronic Resources Book Corner Contact Info


What the World Needs Now --
Emergency Electrical Power System  (February 22, 2011)
  

Previous Issues
In times of a local or perhaps even a national disaster, utility power may be lost for a prolonged period of time.  How could a family cope without electricity?  What I would like to see is a portable emergency electrical power system, which could support a family for an extended time.  The available power would be minimal but it could be a real life saver.
Many people own a gasoline or propane fueled emergency power generator.  These machines might work fine if the utility power is off for a day or two.  But, after that, when the generator’s fuel runs out, the home owner may find himself without any electrical power.  A trip to the gas station might be frustrating because nearly all of them need electrical power to pump gas and to ring up a credit card sale.  Generators running on propane might find fuel for a while longer but as the days without electricity continue, those supplies too will dry up.
What I imagine is a portable power system on wheels.  It would contain all the things needed to supply AC power for a long time.  At the heart of the system would be one or two quality 250 watt solar panels.  These 60 cell panels are perfect for charging a 24v lead acid battery.  Assuming about 6 hours of daylight each day, a 250 watt panel should be able to pump about 1500 watt-hours of energy into a battery.  This is a small fraction of the typical 25,000 watt-hours a typical house needs but with some frugal use of the available stored energy, it could be a life saver.
One 250 Watt Solar Panel  Two 250 Watt Solar Panels  Emergency Power System on Wheels
The figure below shows the block diagram of the electrical system for this device.  Between the solar panel and the battery would be a charge controller.  This control box would insure that the battery is not overcharged.  In normal charge mode, the solar panel acts as a current limited power source, charging the battery at a reasonable rate.  As the 24v battery voltage approaches about 28v, the charge control box circuit would taper off the charge current, preventing damage to the battery.

For a battery, a pair of 12v sealed lead acid batteries with a capacity of at least 150 Amp-hours should work.  They might be connected to a small AC powered trickle charger during normal non-emergency days, so when the utility power goes out, the batteries would be already fully charged.  To get the most life out of these devices, they should not be discharged below about their half capacity rating.  So, although a 24v 150 Amp-hour battery may hold 3600 watt-hours they shouldn’t be discharged below 1800 watt-hours.   This type of battery is about right for a solar panel, which can only produce about 1500 watt-hours per day anyway.  But, a larger battery could be installed to capture more unused energy generated during each day.
Battery Charge Controller 12v 150 Amp-hour Lead Acid Battery
To supply AC power to carefully chosen loads in the home, a 3000 watt pure sine inverter should work fine.  These units usually have some intelligence built in so they will automatically disconnect from the battery, when the battery voltage drops below a critical level
  3000 Watt Pure Sine Wave Inverter
To increase the available energy and to deal with cloudy days, my imagined emergency system might include a bicycle generator.  The “Pedawatt” device shown below can crank out 300 peak watts. However, the typical human may only produce about 75 watts for more than a couple hours.  Still, by taking turns, a family of four could keep the battery charged day and night.
300w Pedal Power Generator  
Another possible power source would be a small wind generator.  Wind often occurs after dark, so when it does kick up, the family could take advantage of the extra energy and use the added power to wash clothes, wash dishes or work on their computer.  Some wind generators are small enough that they could be quickly hoisted up on a pole and attached to the side of the house.
   

A long extension cord would bring in the power from the outside power generator into the home.  Additional extension cords and power strips would then allow carefully chosen loads to be connected to the generator.

What could you power with this thing?  The system I imagine would not be powerful enough to support all the loads in the home.  The power system would only provide power to essential devices.  With only 1500 watt-hours of available energy per day, those loads would have to be chosen carefully.

A few compact fluorescent lamps (CFLs) would provide efficient lighting but they would have to be positioned carefully, so they provide light only where it is needed.  Four 9 watt CFLs if used for 8 hours each night would chew up 72 watt hours.  I think small rechargeable battery powered flashlights and camping lanterns would be a better choice for short term spot lighting for reading and eating a meal.  An AC powered battery charger could keep the NiMH batteries in those devices charged during the day. 

Cell phone service might still operate in some locations, even without utility power.  Many nearby buildings have cell phone systems and antenna towers powered by solar panels.  So, it might be worth keeping cell phone batteries charged using AC powered adapters.  Landline phones should run for some period of time after a utility power outage but that service might not last.  Cable TV and Internet cable services are supposed to have backup power generators fueled by natural gas but if the utility power is down for a long time, those services might also drop out.

Satellite TV services will continue but the receiver boxes should be connected to the home emergency power system for only short periods of time.  A large screen TV would have to be kept unplugged in favor of a smaller device.

Refrigerators and freezers, even small ones, will also have to be kept unplugged from the emergency power system.  They just draw too much power.  They average about 1000 watt-hours per day.  There are some new technology refrigerators, which are much more efficient, but it is doubtful that the homeowner would have one of them.  RV style refrigerators which are powered by propane, might be used to keep critical material, like medication, cool during the disaster.

Microwave ovens can be used with the emergency power system.  These devices are quite efficient for quick hot meals.  A family of 4 might heat up 8 meals per day.  Each meal might draw 600 watts for just 2 minutes.  That means only 160 watt-hours of energy would be needed per day.

Long term computer use could tax the meager energy source.  A fully loaded desktop computer could easily suck out all the stored energy from the battery in just three hours.  Even a small notebook computer, drawing perhaps 50 watts, if used most of the day, could draw more than 1/3 of the available energy.  But, a good notebook computer could keep the kids occupied by acting as a DVD movie player, game player and even TV receiver.  Maybe the kids could pedal the bicycle generator as they watch a movie or use the computer so they would not draw from the solar energy reserves.

If the battery is nearly fully charged in the morning on a sunny day, it might be possible to run a full load of wash using the family washing machine, but the clothes would have be air dried.  Daily washing will not be possible.

Home heating and air-conditioning will not be helped by the emergency power system.  A wood burning stove might be kept going in the winter for heat.  Opening some windows during the night might help cool off the house. Electric blankets, space heaters and hair dryers would all have to be kept unplugged.  Electric shavers and toothbrushes would be OK.

I would add a good battery or hand crank powered all band radio to the list of important devices to keep running in an emergency.  Communications from the outside world would be as important to the family as food.

City water services might continue for a while but if the power outage is widespread and continues for a long period of time, the homeowner will need to find other sources of water.  A home in the country will most likely have its own well.  A well water pump can be turned on for a few minutes each day.  A pump might typically draw 1000 watts and pump 10 gallons of water per minute.  A 5 minute pump operation would require about 100 watt-hours and should be enough to fill a 50 gallon water heater, which could serve as a water reservoir.  Gallon jugs could also be filled during pump operation for cooking, drinking and flushing of toilets.  If some other questionable water is brought to the home, it might be wise to pump the water through a filtration system.  Reverse osmosis water systems do a great job in turning dirty water into clean drinking water.  But, the system should only be used for drinking, bathing and cooking water.
Here is how the available 1500 watt-hours of available energy might be spread out.
 

6 hours Note book computer operation = 300wh

5 minutes of well water pumping = 100wh

General lighting = 100wh

Battery charging = 20wh

Microwave oven cooking = 160wh

Misc Loads = 100wh

Total = 780wh

 

 

Note that this total is about half of the available 1500 watt-hours of energy.  So, it would be possible to use the power system for other loads.  Maybe some power could be shared with a neighbor.  A small TV might be run to capture local and national news.  A water treatment system might be powered to produce potable water from pond water.  Additional water could be pumped onto a vegetable garden, to grow some food.  In summation, in times of emergency, a portable solar energy power system could keep a family going for many months.


March 2011     Issue 15

Page 1 Back
Envelope
Circuit
Diagnosis
Experimenter's
Corner
Good Idea
gone Badly
New Products What the World
needs Now
Wily Widget


HOME Schematics Index Hobby Corner Dave's Circuits Electronic Resources Contact Info
Imagineering Ezine    Discover Solar Energy Dave Johnson & Associates Faraday Touch Switches


 About Us   |  Advertise on DiscoverCircuits.com   |   Report Broken Links  |    Link to DiscoverCircuits.com  |    Privacy Policy

Copyright  2002 - 2015 David A. Johnson & Associates.  All Rights reserved. 
 Linking is ALLOWED but COPYING any content or graphics to your web site is EXPRESSLY PROHIBITED.