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Posted: **02 Jun 2008, 11:09**

Does anyone have experience using an off the shelf commercial UPS in a domestic environment? These are really designed for computer, telecoms, medial equipment and instantly deliver several kW for several minutes in the case of power failure. Presumably they could be connected to a ?critical? circuit in the house ? consisting of the central heating pumps, timers, thermostats, a subset of the lighting etc. Whilst they are quite expensive I expect cheap 3rd party batteries could be added in parallel.

For example, this for ?270
http://www.apc.com/resource/include/tec ... l_watts=50

And this extra battery pack for ?2100:
http://www.apc.com/resource/include/tec ... sku=UXBP24

Provides over a week of 50W, more than 2 days of 200W. A third party battery pack would also be much cheaper.

Posted: **02 Jun 2008, 11:33**

During my office power-cut last week I reviewed my dinky UPS ... and found that it's a modified sine wave unit.

I would suggest that you get a pure sine wave unit if they exist and if you can afford it, as not everything likes modified sine wave.

For example, the charger for my Dell laptop starts buzzing in an horrific manner when supplied with a MSM.

Also, don't forget that the small lead-acid batteries you find in UPS units get tired after a few years, even if not used.

Finally, don't forget safety: if all the lights go out others might get caught out by a live mains socket.

In my office I am using German mains sockets etc for the UPS output, just to show that something different is happening!

Posted: **02 Jun 2008, 12:42**

Use of a UPS unit to supply critical loads has much to commend it, though one should be aware of several possible problems.

1) losses, most UPS units have standing losses of from 5 to 50 watts, 24/7, this will add to your electricity bill, not that much, but it all adds up.

2) limited run time, usually only 15 minutes at full load. This can be increased by reducing the load or fitting an external battery bank.

3)Very limited surge capacity, not much good for motors unless the UPS has a capacity of TEN times the motor rating.

Be aware that there may be NO ISOLATION between the battery terminals and the mains supply.
There is therefore the risk of a dangerous line voltage shock from touching the terminals of any external battery.
The terminals of any external battery must be regarded as mains connections and protected against being touched.

Also be aware that the inverter is probably only rated for short term use, due to the limited run time of the internal battery.
If the inverter is run for longer via an external battery than it may fail due to overheating.
To avoid this, the continous load should not exceed 50 % of the rating.

Posted: **02 Jun 2008, 13:06**

UPS installed at my Mothers home in the West country.

Made by APC rated at 2,400 VA/1,800W (I think, from memory)

Internal battery (removed) 24 volt 17 A/H
External battery 24 volt 220 A/H

Supplies one low energy lamp in each main room, fridge, freezer, small TV, cellphone charger, base unit for cordless phone.

Run time is about 24 hours normal use.

In case of prolonged power failure I have a large inverter that could be used of the car battery, with the engine running.
Purchase of a generator is being considrered.

Posted: **02 Jun 2008, 17:00**

Vortex wrote:
Also, don't forget that the small lead-acid batteries you find in UPS units get tired after a few years, even if not used.

Expect very poor life from the internal batteries - my Belkin UPS only lasted 18 months - 2 years, and I didn't find out until the power went off and the damn thing didn't do its job!

Posted: **02 Jun 2008, 17:12**

What about using a flywheel to store the energy instead of lead-acid batteries? Does anyone know much about this?

Posted: **02 Jun 2008, 18:53**

clv101 wrote: For example, this for ?270

I wouldn't pay that for one, check out ebay or the skips on your local industrial estate.

Adam1 wrote: What about using a flywheel to store the energy instead of lead-acid batteries? Does anyone know much about this?

Not much point in replacing one heavy chunk of metal with another and probably not very practical unless you're running a Difference Engine.

Posted: **02 Jun 2008, 19:38**

Adam1 wrote:What about using a flywheel to store the energy instead of lead-acid batteries? Does anyone know much about this?

It can certainly be done...
http://www.jet.efda.org/pages/focus/power/index.html

In total, JET's power supply system has an installed capacity approaching 1400 MW (MW = megawatt = one million watts), a significant proportion of the maximum output of a large power station.
...
On big machines, energy may be stored using massive flywheels. JET, a large tokamak with pulses extending 20 seconds and more, is an obvious candidate for the flywheel solution.

Each JET pulse consumes around 10 GJ (GJ = gigajoule = one thousand million joules) of energy, with the peak power requirements exceeding 1000 MW. This amount of power cannot be taken directly from the UK National Grid so two massive flywheel generators (Fig. 9) are used to supply the additional energy needs. The rotating part (rotor) of each generator is 9 metres in diameter and weighs 775 tons (!), much of which is concentrated on the rim to form a large flywheel. For experts - the total moment of inertia is 13.5 million kg.m2 per flywheel!

Before each pulse the flywheel is accelerated by its 8.8 MW electric motor - even high-speed trains like Eurostar or TGV have less powerful motors...

Each flywheel can be spun up to 225 rpm (3.75 Hz) so that the edge of the flywheel rotates at the speed of 380 km/h (236 mph)! That is where the rotor carries the pole windings. Positioned as closely as possible to these rotating windings are stationary pole windings mounted on the stator, which is the fixed cylindrical construction around the flywheel.

Not sure I'd want that installed in my house though, it's pretty big:

Posted: **02 Jun 2008, 20:05**

21st_century_caveman wrote:
clv101 wrote: For example, this for ?270
I wouldn't pay that for one, check out ebay or the skips on your local industrial estate.

Adam1 wrote: What about using a flywheel to store the energy instead of lead-acid batteries? Does anyone know much about this?
Not much point in replacing one heavy chunk of metal with another and probably not very practical unless you're running a Difference Engine.

Another CAT student I met at the last module is doing his thesis on flywheels as an energy storage medium. He seemed to think that they look quite promising. The key advantages seem to be: (1) lower losses than lead-acid equivalents; (2) far fewer maintenance issues; (3) less environmental impact.

Here's one company's offering: http://www.activepower.com/solutions/up ... -ups-60hz/ ... admittedly, they're a bit too big for domestic use.

I think the maintenance issues are particularly important now, when such systems are only used relatively infrequently. There are so many stories of back up systems letting people down when they are called into service. The latest one in the news was during the recent blackout, when a hospital's back up system caught fire because it was overloaded, although a flywheel based system may not have stopped that particular incident from happening.

Posted: **02 Jun 2008, 20:28**

This article compares the two...

http://pepei.pennnet.com/Articles/Artic ... cmp=PENews

Life Cycle Costs
While a flywheel system costs up to twice as much as a similarly sized monitored five-minute VRLA battery bank (the lowest-cost available), it recovers its cost rapidly, usually in two to three years. Thereafter, the flywheel solution becomes an ongoing source of both power security and cost-containment. Over a 20-year design lifespan, the cost savings are in the range of $100,000 per unit deployed ? enough to repay the cost of the flywheel several times over. Costs are lower due to the greatly reduced maintenance and replacement needs.

Recommended hardware service for the lowest-maintenance flywheel model on the market is just a quick, low-cost capacitor replacement once every six years (roughly 52,000 hours), similar to that required by UPS manufacturers. Compare that to battery strings, which would need to be replaced five times based upon an (optimistic) life expectancy of four years. Individual cell replacements (which degrade the entire string), monitoring system costs, temperature control and ventilation services and costs related to battery disposal, space utilization, fire-hazard permitting, hazardous-materials handling, stored replacement cells, acid spill containment, inspections, OSHA compliance and the most costly of all, lack of reliability and resulting downtime all add up very quickly.

Energy efficiency is one area where most flywheels fall short. The most popular steel flywheel provides about 3,500 watts of standby electric usage. With the generational leap made by maglev carbon fiber technology, energy efficient high-speed models use less than one-tenth that amount ? 300 watts ? which is comparable or less than the float charge of a similar-sized battery bank. The energy efficiency of flywheel energy storage is better than that of temperature control- and ventilation-bound batteries.

Throughout the United States and around the world, data centers, broadcasters, hospitals, laboratories, airports, manufacturers, military facilities and other sites are hardening their battery strings or eliminating them altogether by applying flywheel energy storage to their UPS systems. Giving up old habits, especially bad ones, definitely takes intestinal fortitude. However, those who have taken the plunge are glad they did.

Posted: **02 Jun 2008, 21:26**

For experts - the total moment of inertia is 13.5 million kg.m2 per flywheel!

For comparison, the moment of inertia of RenewableCandy (m = 55 kg, mean r =/= 0.15m when not holding her arms out) is approximately 0.5 * 55 * 0.15 * 0.15 = 0.619 kgm^2.

Posted: **03 Jun 2008, 08:15**

Adam1 wrote:What about using a flywheel to store the energy instead of lead-acid batteries? Does anyone know much about this?

AFAIK flywheel energy storage is not available in sizes suitable for domestic use.
Also the run time is very limited, and can not be readily extended by adding extra batteries as is the case with a battery based system.

Posted: **03 Jun 2008, 08:19**

Joules wrote:
Vortex wrote:
Also, don't forget that the small lead-acid batteries you find in UPS units get tired after a few years, even if not used.

Expect very poor life from the internal batteries - my Belkin UPS only lasted 18 months - 2 years, and I didn't find out until the power went off and the damn thing didn't do its job!

True, Belkin are the cheaper end of the market, a better brand UPS with better batteries should give about 4/5 years standby life before new batteries are required.
If fitting an external battery pack, then of course one could use better batteries and achieve a standby life of 10 or even 20 years, could be expensive though.

Posted: **03 Jun 2008, 10:38**

APC generally have a battery life of 4 - 5 years in my experience. We sell them for Servers. If anybody is looking to buy a UPS, pm me as I might be able to do you a good price through our trade suppliers. Mind you ebay is often cheaper....

Posted: **04 Jun 2008, 12:20**

Remember also that sealed lead acid batteries are temperature sensitive and are easily killed by excessive temperatures.
As a ROUGH guide the standby life of such batteries is halved for every 5 degree increase in temperature.
At higher ambient temperatures, the float charge voltage should be reduced slightly, good battery chargers do this automaticly, though I doubt that a cheap UPS does.

(O/T, but lead acid vehicle starter batteries generally require annuall replacement in very hot climates such as Arizona)

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