Photovoltaic Solar Water Heater without Batteries using PTC Ceramic Heaters
Overview
The continuous decrease in the price of photovoltaic solar panels has made possible what was unthinkable at the end of the 20th century: using these panels to produce heat. But the need for a regulated current remains, hence the use of storage batteries, with their annexes: charge and discharge controller, inverter... which add to the price of the installation.
This article proposes the use of ceramic PTC resistors for the production of hot water, regulated by a small micro-controller, in order to simplify the system and especially to reduce its price.
https://ourworldindata.org/search?q=solar+PV+modules+prices ; for more précision, download the .csv file
The Constraints of the Laws of Electricity
Electro-solar installations are subject to the same rigid laws of electricity as grid electricity installations, so it is prudent, as a first step, to review the constraints involved and their consequences.
When it comes to producing heat from electricity, the most commonly used method is Nickel-Chromium resistors, which are found in many appliances, from toasters to domestic ovens, from convector heaters to hot water production.
There are other devices: microwave ovens, induction hobs, etc., but they are subject to the same constraints, which will be developed below: so let's avoid spreading ourselves too thin.
All these devices work correctly provided that they are supplied with electricity, i.e. with a correct voltage of 230 V, and a sufficient current. The laws of electricity teach us that a small variation in one of these two parameters leads to a much more than proportional variation in the power available.
In the case of a photovoltaic power supply, everyone can imagine the magnitude of the variations between daybreak and solar noon, but moreover, and this is much less intuitive, the production of electrical energy from a solar panel also depends on the load applied to it, which further amplifies the initial variations. This phenomenon is discussed in all books on photovoltaic panels.
It is not possible to connect a consumer device directly to a photovoltaic panel.
The almost universally adopted solution is to interpose a battery between the photovoltaic panels and the usage. The role of the battery is to store and regulate the energy, without which the solar panels are useless.
But a battery also has its own high requirements. It is out of the question to connect it directly to the solar panels; another power electronic component, the charge controller, is inserted, which plays a multiple role, among others to adapt the current to the needs of the battery, to manage the charge of the battery, to monitor the discharge of the battery in small installations,
Downstream of the battery, depending on the type of equipment to be powered, an inverter must also be installed to convert the battery current into 230 V AC.
The battery
- - is expensive, especially compared to photovoltaic panels;
- is fragile ;
- - has a limited lifespan;
- - Finally, it is the most polluting element of the system.
We propose here, as part of the solution, the use of PTC ceramic resistors. These resistors are subject to the same laws of electricity, but their flexibility of operation adapts well to variations in solar energy.
PTC Ceramic Resistors
There is a multitude of ceramic resistors. In order not to disperse, we will only talk about one kind of resistor, it will then be possible to adapt as much as needed.
Here is a 60 * 21 * 5 mm resistor consisting of
- - a ceramic wafer (not visible)
- - two electrodes made of very thin aluminium sheet, not visible
- - two wires soldered to the electrodes; the soldering is the most fragile part: do not handle the resistors carelessly.
- - an orange envelope made of very thin silicone sheet (?), ensuring electrical insulation
- - an aluminium shell, ensuring the mechanical strength of the whole.
One of the main characteristics of ceramic resistors should be noted here[1][2][3]
- Ceramic resistors are components whose resistance varies significantly according to their temperature. As a first approach, we can say that the resistance decreases in the proportion of 3 to 1, when the temperature increases from ambient to 200°C. The thermal power delivered by the resistor is thus multiplied by three (provided there is a sufficient power supply)
- Once the temperature of about 200°C is reached, the resistance increases very sharply, so the heat output stagnates: unlike the usual Nickel-Chrome or other devices, there can be no burn-out.
Ceramics are commonly used in electronics, sometimes presented as "resettable thermostats".
typical characteristics of a usable resistor for our purpose:
- -Dimensions 35 * 21 *5 mm,
- - operating voltage 36 V,
- - power 65 W at 150°C,
But of course, the designer can use any other resistor he likes.
Regulation
The resistors are placed around the body of the water heater in more than sufficient number for the maximum power of the installation.
A more or less important number of resistors is put in operation with the help of a small micro-controller in which an algorithm of the type "Perturbe and Observe" is implemented: using two sensors, the microcontroller knows the voltage and current of the system at a given moment, and can therefore calculate the instantaneous power; then it perturbs the system, for example by implementing an additional resistor, and then observes the result obtained: if there is an improvement in power, then the microcontroller continues in that direction; if not, it goes in the opposite direction. The loop repeats itself indefinitely, several times a minute.
The main elements of the regulation are :
- the micro-controller, in this case an Arduino Nano at 20 € in original version (recommended) or at 8 € in clone version. A micro-controller includes a (very small) micro-processor, a (small) memory allowing to implant a program, but also and especially inputs and outputs, allowing for example to receive information via sensors (Voltage, Intensity), and after having processed them, to control electronic switches implementing the PTC resistors.
- Voltage and current sensors, worth a few euros each.
- electronic switches, called MOSFETS, which now replace the old electro-magnetic relays.
The secondary elements of the installation concern the power supply of the above elements, which require, in order to function correctly, ... a regulated DC power supply of 5 Volt.
Two components are required:
- - a 48-8 Volt:/ 5 Volt DC-DC converter. From a (permanently) variable current between 48 V and 8 V, this component produces a regulated current of 5 V - 1 to 2 Amps.
- - and a battery, of the type "emergency battery for smartphones", with a capacity of 5,000 milliAmpere-Hour at 5 Volt.
The power consumption of the automation is of the order of that of a smartphone.
A major feature of the control system is its safety
- - in case of overfeeding for any reason, the ceramic resistors enter into stagnation, their temperature stabilizes around 200 ° C.
- - in the event of under-feeding for any reason, the thermal insulation allows the previously accumulated heat to be retained.
The computer program is very basic. It is implemented once and for all in the microcontroller, and it repeats itself in a loop indefinitely.
About the Individual Water Heater
We consider here the case of a prototype water heater made from an existing individual electric water heater that is stripped of its thermal insulation as much as necessary.
The ceramic resistors can be arranged in a ring around the heater, firstly strapped with a bungee cord or similar, and then in a more permanent manner. To adapt the heater to the curvature of the tank, a 0.5 mm lead sheet or similar can be used (available on the internet). Form a hardwood template with a file, and grip the template and lead sheet in a vice to deform it to a depth of 0.22 mm
At what level of the water heater should the heating elements be installed? At the top or at the bottom? The question is open to debate, depending on various parameters, but it is beyond our immediate scope. A good answer is probably: one at each level; with a slight complication, the automatic system can then switch from one to the other to optimise the whole.
This hot water device then reaches an incompressible degree of simplicity, at least in the current state of technology.
Skeptics or disbelievers can try a "proof of concept" with a 60-litre or 200-litre drum, to be connected to solar panels kindly made available for a few days (without having to move them of course). It's almost playful.
Playful? See this short video[4]: the thermal transfers are not optimised, nor is the electrical installation, but we can only applaud what seems to be a first.
Other Applications
The storage of domestic hot water promotes the intelligent use of solar energy, to be shared between self-consumption and injection into the network at optimal times. The use of ceramic resistors for hot water production contributes to reducing the cost of the battery park, thus promoting self-consumption.
The use of ceramic heaters is perfectly suited to inter-seasonal heat storage, see the article of the same name on Wikipedia
But all these subjects are beyond our immediate scope.
Small Scale Food Canning and Essential Oil Production : some Preliminary Thoughts
These are some personal thoughts of the writer of these lines.
It would be a question of an installation of the order of 6 to 9 kW
A) Solar thermal energy.
It is technically possible to supply a 6 to 9 kW system with energy by thermal route
But the thermal route is technologically complicated, and a small installation has as many complications as a large one: there is no scale effect in the number of complications, and in a small installation, the management of complications, called "impedimenta" by the Romans, and more bluntly baptized by a famous general of the Empire, becomes prohibitive: feeding the boiler, adapting to variations in sunlight, general surveillance, require a full-time person with no other concerns. It would be necessary to specifically train a person qualified in a technological field that belongs to the past and for which there is no longer any culture. This is completely unacceptable for a small installation of 6 to 9 kW.
B) Solar photovoltaic energy
Whatever the power of the field of mirrors: 3, 6 or 50 kW, it is necessary to amortize it financially in the best conditions, i.e. to make it work permanently, including outside sterilization hours, in self-consumption (among other things, to produce hot water, of which agro-food installations are large consumers), and/or by optimizing the sale of current on the grid.
In the case of a "green" sterilisation installation, it would be prudent to distinguish, if only conceptually, between energy production (the solar panel field) and energy consumption (the sterilisation installation). And given the amounts to be invested, the different amortisation periods, the different operating conditions, energy production and consumption could be two different legal and economic entities, even financed by different sources, while being closely linked to each other by a form of "preferential contract".
In the case of a sterilisation plant, there is no question of using the ceramic resistors, due to the requirements of the process control system, and the need to be able to resort to the grid in case of temporary insufficiency of the solar panel field.
In the case of a plant for the production of essential oils, the conditions are different; with the help of a skilled professional and depending on the configuration of the boiler, it should be possible to make it a "dual-energy" plant capable of operating simultaneously or separately on the one hand with the solar field, and on the other hand with grid electricity or an oil burner.
Further Information
A more complete version of this document is available at
http://cuisson-solaire-photovoltaique.org/Chauffe-eau-photovoltaique-elements-de-conception.pdf
References
- ↑ https://www.tdk-electronics.tdk.com/download/531162/d78540dfe0589d2bd90cabef477c90b9/pdf-general-technical-information.pdf
- ↑ http://en.hn-yongli.com/products.html?gclid=CjwKCAiAwZTuBRAYEiwAcr67OS4ZJPewLTtuiHCKx81-i_g80scmtX0DNfYnP9vD4z_ghZipqP6qbBoCWMwQAvD_BwE
- ↑ http://www.tiancheng-ptc.com/en/company.html
- ↑ https://www.youtube.com/watch?v=jzfVHSCsU44
