PV excess Energy data used for hot water production: A Research Article

Hello community, I just want to share with you my research article that I’ve recently published using excess PV energy data of a grid-tied system. The main focus of the study was to assess the feasibility of using excess PV energy to heat up a hot water tank by means of a Diverter. And, whether or not this system can replace conventional water heaters such as Electrical water heaters or Solar thermal water heaters. The energy data has been monitored using a Shelly 3EM for the electrical phases as well as another Shelly EM to monitor a single phase Inverter. The setup is looking something like this:

The results reveals that excess energy can contribute up to 47% of the household’s hot water demand. Moreover, the system was able to maintain an adequate hot water temperature even during the cloudy days since the photovoltaic cells can also produce electricity using diffuse beam radiation.

Finally, throughout the simulated year, the system was able to improve the self-consumption rate of the studied building by an average of 52% as can be seen in this figure.

Enjoy the reading!

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Hi Mohssin,
We’ve been installing Catchpower diverters on solar systems for about 5 years now.
In the main, they provide 100% of the energy required for a ‘typical’ family using a 300 litre HWS and a 6kW system, unless the weather is particularly foul.
Here you can see the Catchpower tracking production and diverting energy to the HWS in the mornings, until the thermostat switches off.
You can also see the Catchpower tracking production all day on a foul day. The thermostat doesn’t switch off due to insufficiient energy. The Catchpower boosts at 11pm to ensure a hot tank for the morning.

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Hey @hippiesparx, indeed the diverter converts such a good amount of excess energy in your case. No wonder why it is covering almost the full household DHW demand. Just want to go back to the setpoint, haven’t you guys tried tanks with higher setpoints like those with 80°C ?

Very interesting!

But Marrakesh is quite a bit sunnier than Belgium :sweat_smile: The title, and highlights do not explicitly call out that the findings only apply to similar geographies/irradiation levels.

Most importantly: zero mentions of a heatpump. A small heatpump would be able to yield a much higher water temperature for the same amount of electricity. Especially in Morocco (with an average winter temperature of 19ºC :open_mouth:). Heatpumps cannot react as quickly as a resistor (would be bad for the compressor to quickly turn it on/off), but given sustained periods (say, 30 mins) of divertible excess energy, this would not be an issue at all.

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Hey @wimleers, Yes Marrakesh is sunnier than most of EU cities. yet, thanks to the photovoltaic effect even during cloudy and fogy days you can still generate a considerable amount of energy (depending on the array size) thing that typical solar thermal collectors lacks (what the paper tried to deliver :sweat_smile:).

As for the HP, in that article we wanted to conduct a techno-economic assessment and by ‘technical’ we meant not only the temperature but also the self-consumption (SC) rate. For instance, a temperature of 55-60°C to prevent the growth of Legionella bacteria’s should not be a hard task for a diverter to reach. However, when using a variable speed heat-pump (to properly use excess energy) the system will quickly reach the setpoint temperature (thanks to the HP’s COP) but the building’s SC rate will not be improved considerably. In Morocco no feed-in tariffs are implemented for residential rooftop systems, therefore the storage rate of excess energy matters. Also, when only Hot water needs are considered (i.e. no space heating), HP’s capital expenditure is just not worth compared to cheaper diverters.

That makes sense for the current Moroccan context, where there’s no feed-in tariff at all. A simple resistor-based water heater then is sufficient indeed!

But that is of course bound to change. Then again, a cheap resistor-based water heater may earn itself back in the time it takes for feed-in tariffs to become a reality there.

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I am a bit late here but my Paladin diverter also covers 100% of our needs. Its a bit smarter than the Catchpower. The standard thermostat in .AU has a setppoint of 65 deg C. Replacing this with an 80 deg C thermostat moves the setpoint above our operating range. We inserted a DSB1820 temperature probe under the HW insulation at the thermostat, the Paladin can control the temperature. 80deg is a bit too high for safe use if the tank is not rated for it so the Paladin has a setpoint of 73 deg C that it controls internally from the probe measurement. This maximises stored energy to get through those rainy days. Legionella is controlledby an internal timer so if the temperature does not hit the required temperature within the required timeframe, mains power will be used to boost the temperature if its ever required. Mains power is also used to maintain the minimum water temperature to 40 deg C. A second DSB1820 probe monitors the hot water temperature via a Shelly UNI which sends temperature data back to Home Assistant.