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Forum Index : Solar : Exploiting smart meter "joule buckets" for excess solar HWS dump loads
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Haxby Guru  Joined: 07/07/2008 Location: AustraliaPosts: 423 |
I've been brooding on the idea of creating a storage hot water heater dump load for my grid tie solar system for some time now. The classic idea is that you redirect any excess grid tie solar inverter energy to a storage HWS. The push-comes-to-shove moment came for me recently when I realised that our 160L HWS was too small for our family of 4 to store enough energy during daylight hours. So I upgraded to a 315L system. That storage capacity will certainly suffice for at least one day, and maybe more. But the BIG problem is how to measure and extract every-single-bloody-joule of excess energy, but not extract one single joule too much and have to pay for the solar shortfall.... This was the problem that I have been brooding on for some time. I had ideas of PWM techniques of controlling the heater element, or even a full blown 3.6KW inverter that I could regulate the power of, down to more crude ideas of transformers with about 3 or 4 centre taps that could load the 3.6kw element at various power levels. The complexity of the above solutions was high, and no solution ticked all the boxes. Then I stumbled on THIS project that exploits the "joule buckets" within smart meters. In summary, many smart meters will only increment their counters after 3600 joules (1 watt hour) pass through them in one direction. (either importing or exporting). BUT during the accumulation of those 3600 joules, energy can be imported and exported from the grid at your discretion. As long as you don't exceed 3600 joules in either direction, the energy meter won't count any energy used. The article linked above explains this in much greater detail. Essentially you can use the grid as a 3600 joule "capacitor". So you can turn on a big 3600W element (or more), deplete the 3600 joule "capacitor", then turn off the element, then wait till the solar system tops up the "capacitor", then turn on the element, deplete the "capacitor", then turn off the element, etc etc. I love the idea of both the simplicity of the circuit and the 100% closed loop aspect that ensures all joules created don't get exported. |
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poida Guru Joined: 02/02/2017 Location: AustraliaPosts: 1418 |
Hell yeah. Let's hack the System! My smart meter makes a LED pulse once per 1W.hr and I have a photodiode stuck on top of it, leading back to a micro that counts the pulses over each 2 minute period. This is accurate and reliable. For your system to always know what and when to put/take power you will need to know when the pulse happened. So you need the photodiode. Water takes a lot of energy to change temperature. 4182 J/C.kg 315L, going from 25C to 80C (or whatever) needs 315 x 55 x 4182J = 72.4 MJ 3600J = 1W.hr so it's 20.1 kWhr to get the water hot this is a lot of energy. Initial conditions for the HWS will vary (eg. near hot, stone cold, Summer input water temp, Winter input water temp, kids away at school camp, etc..) I think the system needs to know the NET power flow through the meter as a continuous number with direction information. That is, a real number as taken from a Wattmeter If I was building this, I would make a program that goes like: initialisation: - disconnect solar dump - wait for smartmeter pulse and measure net flow from Wattmeter - keep checking net flow until 1/2 of the 3600J have passed so we are in the middle between pulses. - now jump to main loop main loop: (trying to keep net energy change from this point = 0) - PWM power into HWS at a rate to balance house load (i.e. net power reading = 0) - prob. need to ramp up PWM width quickly. no idea how fast - if not enough solar, that is net flow goes negative and forces smartmeter pulse then go back to initialisation - check net energy and if it's negative (i.e. drawing from street) increase PWM width else decrease PWM width. This will keep net power (and energy) close to zero and the smartmeter pulse counter close to 1/2 of 3600 Joules This program will attempt to reduce the number of Watt.hr pulses. The result will be reduced import costs and reduced export credits. Alternatively a scheme might be to aim to zero import costs whenever enough solar is available. Again, using the smartmeter LED and this only, plus a micro controller you can program it so that it heats HWS via PWM with PWM increased up to the point where you are taking 1 W.hr from the street. The program will reduce PWM, even to the point of disconnecting it when the smartmeter pulses increase in rate beyond 1 pulse per minute. No need for a wattmeter and the complications it brings. wronger than a phone book full of wrong phone numbers |
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| Haxby Guru Joined: 07/07/2008 Location: AustraliaPosts: 423 |
No need to monitor the flashing LED. If you set the software joule bucket size to a few joules less than the actual joule bucket in the meter, the meter will never increment. That's the beauty of this design. Just one current/voltage sensor on the incoming mains, and one solid state relay to activate the dump load, is all that is required. The circuit doesn't even need to know what size dump load it is switching. |
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