PV system design for low-cost hot water production – pv magazine International

Developed by Spanish scientists, the proposed system design is said to be able to achieve water temperatures above 70 degrees Celsius and to cover around 85% of the annual sanitary hot water consumption of a household with six people.
A residential PV system
Image: tereza Baska/Flickr
A group of scientists at the University of Cordoba, in Spain, has developed a photovoltaic system design for hot water production that is claimed to use around 95% of the available energy it can generate.

The system consists of a 300-liter water tank with electric resistance, connected to a 1.6 kW photovoltaic system by means of a low-cost, experimental electronic conversion system. “It is a model that is versatile, sustainable and economical, which only produces a 5% energy loss,” said the research’s main author, Luis Cámara-Díaz.
The system is equipped with maximum power point tracking (MPPT) as it must constantly operate at the maximum power point. “Without using inverters or batteries, which are more common in traditional solar photovoltaic installations, we have developed a low-cost electronic system capable of acting as an MPPT device,” the researchers explained. “Keeping a fixed reference voltage is the MPPT tracking method used.”
A microcontroller board keeps the reference voltage at the required levels and a metal-oxide-semiconductor field-effect transistor (MOSFET) conducts or cuts the current, depending on the voltage. “The reference voltage, determined by the control system, is equivalent to the maximum power point’s voltage resulting from the grouping of photovoltaic modules,” the academics further explained.

The proposed approach was tested on a PV system built with five JinkoSolar JKM330PP modules with a power output of 330 W each. The electrical heater to produce hot water has a nominal power of 3 kW and heating electrical resistance of 37.8 V. The solar panels were connected in series to reach a nominal operating voltage close to the electrical network voltage.
The system was found to reach water temperatures above 70 degrees Celsius and to cover around 85% of the annual sanitary hot water consumption of a household with six people. “The novelty introduced by this low-cost electronic system is that it is possible to convert the available photovoltaic energy into heat, regardless of whether the existing solar radiation level is high or low,” Cámara-Díaz stated.

The tested system also includes a water recirculation system to allow the input of cold water in the DHW tank, and simulated water consumption and water renovation inside the tank. It was presented in the paper A Cost-Effective and Efficient Electronic Design for Photovoltaic Systems for Solar Hot Water Production, published in Sustainability.
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More articles from Emiliano Bellini
Electrical resistance is not measured in volts.
Not really seeing a benefit here over a conventional inverter based system with a traditional hot water heater.
All that said why are we still using resistance heaters in water heaters when heat pumps are ~5x more efficient.
Greenlee, Can you provide any manufactures literature where they claim that their heat pump will be 5x more efficient than direct electric heaters when providing domestic hot water (at say 55C). In my experience heat pumps tend to only claim to be about 150% to 180% efficiency when operating at high temperatures?
The thing that’s “special” about this is that it cuts out the middle man – the DC to AC inverter. The MPPT system simply adjusts the voltage on the resistive element to maximize power output. I have designed such systems myself – known art, nothing magic.
As for heat pumps, yes, they give efficiency over 100%, but as @Ross A stated, for the high output temperature of a water heater the COP will be much less than 5 (basic physics)
COPheat pump = Thot/(Thot-Tcold) (degrees Kelvin)
So in this case it would probably be cheaper and more reliable to add more PC panels, if there’s room on the roof, than to use a heat pump.
The description of using a MOSFET, and of the heater resistance measured in volts, simply signal the technical ignorance of the university’s PR department.
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