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Extravoice
2012-Apr-09, 01:15 PM
Okay, this is pretty far from the board's general topics, but I've learned that people here have a wide breadth of knowledge.

I recently received an offer from my electric utility to install a cycling switch on my central air conditioner. They have two options: The first is to provide me with a new thermostat and a one-time $50 credit on my electric bill. The second is to give me a $4 credit per summer month(June-Sep) and a $1 credit per "cycling event". Each cycling interval is 15 minutes.

Do any forum members have experience with cycling switches? On the surface this sounds reasonable. It reduces peak-load and (allegedly) shouldn't seriously affect my comfort level very much. Plus I save a small amount of money.

According to this page (http://www.pseg.com/home/save/manage_costs/cool_customer.jsp) cycling events are pretty rare. I was leaning toward the $4 per month deal since I don't need a new thermostat, but it looks like it would take about 3 years to achieve the $50 payback.

Since the money involved is so little, maybe the peak-load control is actually a bigger incentive, as it theoretically reduces the need for additional power plants.

Comments?

profloater
2012-Apr-09, 01:21 PM
My first thought is that the system will have to catch up after each short off period so the energy saving is negligible. You would save more just to select a higher air conditioned temperature (or a lower heating cycle temperature) A longer off period at peak load time would make more sense. If the air change is once per hour for example a 15 minute break is not long enough to make a saving. However if they offer cash back why not? no upfront cost?

Extravoice
2012-Apr-09, 01:45 PM
I agree that energy savings will probably be negligible. Any money I "make" from this endeavor would be the credit from the power company.

There's no upfront cost to me, so it is "free money" if there isn't much discomfort when the compressor is cycled off. I discovered a few comments online, and they said their home's temperature went up a few degrees when the event occurred. Of course, the amount depends on several variables, including outdoor temp (probably very hot if the system is activated) and the home's insulation.

ToSeek
2012-Apr-09, 02:28 PM
The point isn't the customer's energy savings, but that the power company can throttle usage at peak periods so they can rely on existing power plants without building new ones, and within existing power plants potentially use the cheaper ones.

My wife and I try to do this ourselves in the summertime - we run the house a bit on the cool side until around 3 pm, then turn off the air conditioning until 6 or 7. But we have a very well insulated house on the north (cool) side of a townhouse block with lots of trees around it.

Extravoice
2012-Apr-09, 03:28 PM
Yeah, this is just the power company attempting to reduce peak load, and offering me small compensation to allow them to do at the expense of my comfort.
So, I'm mostly just trying to figure out how uncomfortable I'm liable to get.

Grey
2012-Apr-09, 04:40 PM
I'd go with the extended repayment choice, assuming that you're living somewhere that you expect to stay for a while. Can you decide to change your mind and opt out later if it's not working out? In that case, you can test it out this summer, see if it causes any serious problems, and then decide what to do. Interestingly, our utility company offers us something similar for our heating and water heater, but they no longer bother to install the actual control switch. Apparently, they need the option so rarely that it's not actually cost-effective to bother to do do that. But the program is popular enough that they kept it, so we get a significant discount on our bill for allowing the utility the option of installing a control switch at some point in the future. I find it perplexing, but I'm not about to point out that it's a pretty silly policy. :)

Extravoice
2012-Apr-09, 05:18 PM
Truth is stranger than fiction!

The ability to opt out is definitely something worth looking into.

publius
2012-Apr-10, 08:38 AM
My first thought is that the system will have to catch up after each short off period so the energy saving is negligible. You would save more just to select a higher air conditioned temperature (or a lower heating cycle temperature) A longer off period at peak load time would make more sense. If the air change is once per hour for example a 15 minute break is not long enough to make a saving. However if they offer cash back why not? no upfront cost?

This is supposed to be a science (which means math) board, so your homework assignment is to model this heat loss and determine the off-time at which it becomes economical to cut the A/C (or heat) off, then re-cool (or re-heat) to set temperature when you return. You will all be graded, of course. :lol: Seriously, think about this and see if you can write out the governing expressions -- you'll find it interesting. And by "you" I mean the board in general, not Profloater specifically.

Hint: heat loss is proportional to temperature difference (and loads of other factors like air exchange from opening doors and how tight the structure against air exchange with all doors and windows closed, how much it leaks air IOW. You do want some air leak, lest you suffocate yourself of course, of course). So roughly, the cost of maintaining a temperature difference is proportional to that difference. Run that for time T, and you must pump dQ/dt * T heat against it.

Now, to lower the temperature, you've got to work against that loss, plus the heat capacity of lowering (or raising) the temperature to the set point.

If you want something to visualize, the situation is analogous to maintaining a leaking glass of water against a water level gradient. For A/C you're keeping the water level in the glass below the outside water level. For heating, you're keeping it above the outside level. Water is leaking out or in trying to equalize, and you must keep pumping water out or in against that leak. The difference in height corresponds to the temp difference, and the volume of water per unit of that height difference is analogous to the heat capacity.

And finally, another reason besides overall capacity that power companies don't like peak load is because the marginal losses increase with load. Basically, losses go as I^2, and since current is proportional to load (apparent power, not just real power, mind you), it goes as the square of the load. The thus the marginal loss, the loss of providing one additional kW of power, increase linearly with load.

Thus, during the nadir, it can be very cheap to supply additional power. At the peak, it can become ridiculously expensive to supply additional power.

profloater
2012-Apr-10, 10:18 AM
The coefficient of performance of AC is not uniform with the set point (or temperature drop required) and of course is majorly affected by the humidity. If you dehumidify separately you reduce the heat extraction required to cool the air and depending on the effficiencies this could make sense as cost reduction. Insulation pays off but over time. If there were any negotiation possible the last point about the cost to the supplier to increase capacity at peak is the most interesting. They really want to manage peaks . To manipulate the system cool a massive heat sink and let them turn off at peak times, for a cash bonus. That actually is the sensible engineering way to go. By storing a cold reservoir you can iron out peaks. The most practical algorithm to answer Publius would IMO be an amp-meter and an experiment with different set points to establish the cost per degree in a given set of ambient conditions.

Grey
2012-Apr-10, 12:49 PM
To manipulate the system cool a massive heat sink and let them turn off at peak times, for a cash bonus. That actually is the sensible engineering way to go. By storing a cold reservoir you can iron out peaks.At the extreme end of this, I know that there have been houses and other buildings carefully designed with a huge thermal mass (usually something like a centrally located block of stone, with appropriate air circulation around it), such that the temperature of the thermal mass lags the driving force (sunlight and ambient air temperature) by about six months. So it warms up during the summer, provides heat during the winter (cooling off in the process), and then cools the house again in the summer. Pretty clever if you can get the math just right.

profloater
2012-Apr-10, 01:42 PM
Yes usually it is a large block of concrete with solar heating under glass and embedded water pipes, another scheme uses an underground water pool (can even be a swimming pool but that restricts the usefulness as a heat store) and these are sized to store nearly a whole winter's worth of heat during summer with heat pumps to manage the heat flows. They can also air-condition and store that heat for winter too.