Dynamic power control for HW cylinder heating

[alc_aardvark]

I would like to know if it is possible to dynamically control the available current to Hot Water cylinder that has 3 x 4.8Kw immersion heater elements. The house supply is single phase 63A supply. When something like a jug or oven is turned on, then I wish to the HW cylinder load to be dynamically reduced so that it does not overload the 63A supply.
Reason: There is an electricity provider that gives 3 hours of free power (9pm to midnight) and I wish to use this free power to heat sufficient hot water to heat the house in winter and provide hot water for showering. I need to be able to heat a large volume of water in a fixed short'ish period of time but still be able to use power for other things during this time. I know that EV chargers do this but cant see anything for what I want to do.
Is it possible to feed a 4.8Kw immersion heater element with less power even or would it be more like an ON/OFF switch when power is needed for something else.

Thanks

 

[brianmoooore]

An immersion heater can be controlled by what is effectively a high current version of a light dimmer switch, but since you have three separate elements, it would probably make more sense to switch one, two or three of the elements off to keep the demand within limits.
A CT clamp on an incoming tail can monitor the total load, and contactors can switch the immersions. It's theoretically possible to automate the load shedding, but whether equipment is available to do this is beyond my knowledge.

 

[alc_aardvark]

An immersion heater can be controlled by what is effectively a high current version of a light dimmer switch, but since you have three separate elements, it would probably make more sense to switch one, two or three of the elements off to keep the demand within limits.
A CT clamp on an incoming tail can monitor the total load, and contactors can switch the immersions. It's theoretically possible to automate the load shedding, but whether equipment is available to do this is beyond my knowledge.

Thank you for the above. This is the only product that I think comes close to what I am after.

GroBoost | GroHome | Growatt

GroBoost | GroHome | Growatt

www.ginverter.com

 

[alc_aardvark]

actually, I think this unit that is designed to work with an EV charger may be more like what I am after

https://www.garo.ie/docs/2021/specs/gc8ev40dlm%20spce.pdf

 

[James]

That unit as far as I can tell is just a distribution board with metering built in.
the suggestions above about contactors to switch some elements off when demand goes up are probably the best way to go.
but remember, if you are trying to store 45kwh of energy then you will need a BIG TANK.

 

[James]

4200 Joules of energy per litre per degree c
assuming you are heating a tank from 30degrees to 60 degrees
you will 30 x 4200 Joules of energy (126000) per L of water.
this calculates out to about 0.00722kwh of usefull energy can be stored in 1L of water.
therefore to store 45kwh of energy in water will require a tank size of
45 / 0.00722 = 6250L

over 6 tonnes of water will need a huge tank
p.s. I have assumed that you will not be able to draw much usefull heat out of the tank when the water gets below 30 degrees as the radiators will then be Luke warm to touch.
also heating much above 60 degrees would mean that radiators become hot enough to burn skin.

you maybe better with say a 200L tank for domestic hot water, showers etc.
and a battery storage to store the rest of the energy for later use.

I strongly suggest you do your own calculations regarding heating requirements and your normal usage before investing in any form of storage.

 

[James]

Sorry, there is an error in the calculations above, I haven’t time today to delve into why they are wrong but as I say, check these things out before investing.

 

[brianmoooore]

@James In practice, the water in the thermal store would be heated to a little more than you assumed, say 70 degrees or so, then fed to the heating circuit at a reduced temperature by a blending valve, in a similar way that the boiler flow temp. is reduced for UFH systems.

 

[Lucien Nunes]

Element power can be limited either by controlling the effective voltage using a phase-angle controller (i.e. dimmer) as mentioned above, or by switching elements between full-on and off. In your application, there is a definite advantage to full-on / off method compared to phase-angle control. This is because what you are trying to achieve is a definite maximum currrent, limited by the 60A supply, but within that limit you want to obtain as much power as possible. It follows therefore that you must aim to get as much heating power per amp as possible, which means having the highest possible load power factor, preferably unity.

With a resistive heating element full on, the power factor is unity, but as soon as you start reducing the power by phase-angle control, the power factor starts to drop due to the heavily distorted current waveform. So although you can use this method to keep the current within the 60A limit, you will be unnecessarily curtailing the power input to the thermal store due to the lowered power factor. In short, with phase-angle control, at half the amps, you will have less than half the watts. Whereas if you switch on half the elements, you will have half the amps and half the watts, which is what you want. Industrially, the preferred way to regulate heating power is by burst firing, repeatedly switching the element on and off for a few cycles. The problem when the element load makes up much of your total, is that each switching cycle is likely to cause lights to flicker etc due to fluctuations in the voltage drop of your supply.

Probably the best and simplest method as mentioned above is to switch the heating in steps of one element at a time, based on load thresholds, and the device to do this is a 'load shedding controller' or 'load shedding relay' which is what you need to be googling for. The main circuit for house load passes through its sensing circuit, while its output contacts operate a relay to disconnect the element when the house load exceeds the threshold set on the dial at the front. You could put three single-channel units in a chain with their thresholds set differently, each controlling one element. Here's a very simple example, I think this one only goes to 20A but it clearly shows the configuration: Load shedding relay 2...20A ELAR-20 - https://www.eibabo.uk/eberle/load-shedding-relay-2...20a-elar-20-eb11101084

Obviously if the element load goes through the sensing circuit as well as the house load, the controller would need to have a hysteresis (dead band) greater than the element current so that it doesn't hunt. This is programmable on the more sophisticated units, which can also offer multiple channels.

I haven’t time today to delve into why they are wrong

4.2kJ/kg/K - correct.
Heat input for 30°C rise = 30 * 4.2k = 126kJ - correct
The error is in conversion from kJ to kWh
126/3600 = 0.035 kWh.

I am not sure about the temperature limits. With a thermal store one would definitely want to be using a higher maximum temp, e.g. 90° C. What the lower limit would be depends on the application. If you were considering heating water for domestic use using an amply-sized counter-flow exchanger, you could go as as low as 30°C and therefore the thermal store differential would be 60° storing 0.07kWh/l and requiring only 650 litres to store 45kWh. But 30° is too low as a practical return temperature for normal radiators so not all of this differential could be utilised for space heating. The return to the store from the heating circuit might be nearer 45-50°C.