In the previous article we got to know how to provide a house with uninterrupted power, how to do it in the most cost saving way and even how to get a real three-phase network with only one-phase input ("No problems with electricity", Technica molodezhi N1. 966, 2014). Nothing prevents us from making the house completely autonomous.
Advantages of the MPPT controller
Let's not talk about the limited hydrocarbon fuel reserves, an unavoidable environmental harm even with the most advanced combustion technologies, the problem of waste even of the safest nuclear power engineering as well as about the undoubted advantages of renewable energy, its diversity and achievements of the recent years. All this has been discussed in our journal in various aspects from civilizational generalizations to technical details. The fact is that oil, gas and coal reserves are not infinite, the cost of their production and especially the sales prices are steadily increasing. The green energy becomes cheaper. Thus, the cost of a kilowatt-hour of solar electricity has decreased nearly five times since1990. It's not a surprising fact: on the one hand, technology is being improved giving an increasingly efficient element and aggregate base, on the other hand, the production scale effect has its impact.
Representing these two trends as curves, an ascending and a descending one, it is easy to see that they will cross at one point. And this moment is not so far away as the calculations of some experts show. The future is for renewable sources one way or another. We want to mention one interesting feature of these sources which is important for our article. There are directions in green energy that can be implemented only at large, industrial sites. These are, for example, tidal power stations, it is nonsense to build small ones, but wind generators and solar panels are quite another case. A solar station with an area of tens of hectares will provide electricity for a city while an area of ten square meters will provide electricity for one house. That is it! We want to use this solar station in an uninterpretable power system of our house. It has already been mentioned that modern inverters can charge accumulator batteries with the help of solar panels and wind generators. But experts always want to create not just a system but the most efficient system. Then it turns out that direct connection of solar panels to the battery is not the best option at all. The solar panel is a complicated device. It serves as a source of current (see graph): it means that both at zero voltage at the output (short circuit) and at the voltage close to the maximum (about 17V in the graph) the current remains the same. It is not necessary to explain specifically the fact that the amount of energy produced depends on the sun exposure, it is only necessary to note that the voltage on the solar panel terminals depends on the sun exposure. We'll use solar panels to charge accumulator batteries, that's how they are used in most cases.
Firstly, the efficiency of the solar panel is determined by the voltage on the battery which is being charged: when it is less than what a panel can provide at a given level of sun exposure then the possibilities of the panel are not fully used. Secondly, in case of lacking sun exposure the voltage of a solar panel can be lower than the voltage of a battery being charged, this has to be avoided because in that case the current will go in an opposite direction from the battery to the solar panel. This is not only meaningless but also harmful for the panel. But the battery has its own whims too. The voltage should not go below 10V at the terminals of the working battery. The supply of more than 14.5V is harmful for it. Charging current should decrease by the end of the process otherwise the battery will get out of order. We should say here that manufacturers of solar panels develop their products to connect them to standard batteries with the voltages of 12, 24, 36, 48V. In fact, this parameter is much higher if measured at a certain level of sun exposure because the panels should also work in not very sunny weather. Let's go the simplest way - connect a solar panel to a battery directly. Then we'll have to have more than one solar panel. They will be connected in parallel on a bright sunny day, since the voltage of each of them will be enough for a safe and efficient battery charge. In bad weather when the voltage of the solar panel falls below the comfort zone of the battery, they have to be turned on sequentially, otherwise there will be a reverse current. Secondly, solar panels should be disconnected from the battery before nightfall because of the risk of reverse current. Thirdly, there should be few solar panels and a lot of batteries in our kit so that the current at the end of the charge cycle does not exceed the allowable value for the battery. If everything is done correctly, the process of charging the battery from the solar panel is represented by the red line in the graph from 10 to 14.5V. It is troublesome, isn't it? For sure. In order to avoid these manipulations as well as to avoid the risk of damage of the equipment if one of the manipulations fails to be implemented in time we use special devices called controllers. Controllers with pulse width modulation, PWM controllers, are comparatively simple, inexpensive and therefore widely spread. They limit the current transfer from the solar panel to the battery and protect the solar panel from the reverse current. If you have such a controller, you can simply connect two 24V panels sequentially and connect them to a 24-volt battery for example. These 48 V will provide at least a slow charge even in cloudy weather. Although, the voltage on solar panels can become too high in bright sun so human intervention is required, although much less frequent than with direct connection. Alternatively, another controller should be invented. This will be discussed later on and now it is time to say few words about efficiency. Both the direct connection and the PWM controller equalize the voltages of the solar panel and the battery. The red line in our graph shows the process of charging. This means that the power of the solar panel is used only partially. A very simple calculation shows it. Let's suppose that the graph depicts a 12V solar panel with a power of 100 watts (in fact the panel produces much more than 12V, it can produce a voltage reaching and exceeding 14.5V). The current of the solar panel will be about 6.5A starting from the short-circuit point ("0" on the abscissa axis) and up to about 15V. The power will vary if connected directly or through a PWM controller as the battery charges from 65 W to 94 W (6.5A x 10V = 65W, 6.5A x 14.5V = 94W). The maximum power of this panel, marked with the red circle at the top of the descending branch of the solar panel current makes 6A x 17V = 102W. If all this power could be directed to the battery... Supposedly, another controller is needed. It has been invented and its main function is transfer of the maximum power of the solar panel into the battery. This class of devices is called MPPT-controllers. The abbreviation in this case is Maximum Power Point Tracking, "escorting the maximum power point". What is the point of maximum power? This is the maximum voltage on a solar panel at which the current has not yet begun to seriously drop, in our graph it is about 17V as it has been already mentioned.
SEC МРРТ Pro 200/100
The key difference between the hardware of MPPT and PWM is that MPPT manages to regulate not only current but also voltage. A special algorithm defines the voltage at which the maximum power point is reached at each given moment and the controller keeps such voltage at its input. Then it converts the power taken from the solar panel: it reduces the voltage to the value required for the battery and consequently increases the current without going beyond the battery's safety level which is critical in the final charging period. The conversion is made with high efficiency of 97-98%. As a result, the MPRT controller uses solar energy 20-30% more effectively than the PWM. Let's also recall the first factor that made us think about replacing a PWM controller with something more perfect – it is too high voltage of a solar panel on a very sunny day. MPPT-controllers easily cope with this situation as they can control the voltage. There are different MPPT-controllers: some are relatively simple, others are more complex, some are premium-class. Premium-class controllers are usually characterized by higher power, high input voltage (usually up to 150V), automatic selection of the voltages of installed batteries and a number of additional services and functions. One of them is SEC MPPT Pro 200/100 – the controller developed by the MicroART company, the first premium class MPPT controller in Russia. A record-breaking maximum current up to 100A is one of its distinctive features in comparison with competitors. The SEC MPPT Pro can work with a 96V battery and this current gives a record power from one controller up to 11 kW. 200 V input voltage allows to sequentially connect four solar panels of 24V (their maximum voltage is 45V). Built-in relays allow to switch household appliances directly to the solar panels saving the battery life. What is especially important for us, the SEC MPPT Pro developers provided the possibility of its joint work with a hybrid inverter for the industrial network of 220 V without the battery (although a minimum battery is still necessary).
Personal autonomous power supply based on solar panels and an inverter, even with a 220 V network available means independence, energy saving and our future.
There are probably very few people nowadays who do not know about uninterpretable power supplies which save results of our work when a power outage happens. Are power failures harmful for computers only? Power outages lasting half an hour or an hour still happen even in Moscow, they can last much longer in case of broken wires by a storm or by a heavy snowfall. Things are worse in less "protected" places, from suburban settlements of the Moscow region to towns in taiga... A source of uninterpretable power supply for the household is a good choice for those who want to reliably protect themselves from such troubles. The issue of backup power is not a new one of course. The most obvious solution when the voltage is lost is to switch on a generator with an engine which gives the same voltage as the electricity network.The solution is simple but not the best.
Firstly, it requires a rather long procedure. It is necessary to disconnect from the electricity network, switch on a generator, connect it to the home network (for example we will take a common house), watch for the electricity network for the restoration of the voltage in it. When the voltage is back the generator has to be disconnected from the home network and stopped, then we connect the external network to the home network. But your refrigerator will be defrosted during all the switchings...Secondly, the voltage of the generator is far less stable than the one in the usual network, to which all household appliances are used to. Thirdly, a natural unevenness of consumption leads to a fact that the generator itself is operating in a non-optimal mode for a considerable part of a time. Finally, with frequent short-term outages (this also happens) you will constantly switch from the network to the generator and back.Therefore, autonomous power supply systems based on accumulator batteries (AB) have spread widely in recent decades due to the development of powerful semiconductor electronics. They are especially good for the case we are considering - when an emergency power system is set for a house with its relatively low power consumption.
Accumulator instead of power lines
It is clear that the accumulator can not be connected directly to the home network: it gives a direct current of relatively low voltage but we need an alternating current of 220V. Such conversion is the first function of a device which is called inverter. The first but not the only one. Modern inverters are equipped with control devices, which almost immediately turn off contactors of the external network at an input of the home network and connect their own voltage – energy already converted into the required form from an accumulator battery. But when the battery is being used it is discharging. It needs to be recharged and it is possible to charge it from the same external network when it is functioning. There are also inverters which can not do this. They are more compact, lighter and cheaper but it is not possible to set up a functionally closed system with them. We will continue to consider inverters providing three functions mentioned above: conversion of the accumulator battery voltage into a 220V alternating current; automatic connection in case of power failure and return to the original power supply when it is restored; recharging the accumulator battery when the line voltage is on. For example, inverter MAC SINЕ Pro, developed and manufactured by the domestic research and production and innovative enterprise MicroART. It is designed to convert a 12/24/48V direct current from the accumulator battery into a sinusoidal (very clean form) current of 220V and a frequency of 50Hz. The inverter can do everything that is mentioned above but it can do more, but we will look into this later. And here is what should be said right now - the MicroART company managed to overtake the best world manufacturers in terms of maximum power: the top model of its inverters product line has 18 kW at a nominal power of 12 kW, and at peak load for a period of 5 seconds it is 24 kW. 5 seconds is quite enough to attenuate the starting current of a powerful electric motor. The model has a weight of 50.5 kg, and this is a very good indicator for the model with this power capacity. It is interesting to note that the record capacity of the domestic inverter (in quite ordinary weight and dimensions) was achieved to a large extent due to the use of the domestic component, namely a very large toroidal transformer. This toroidal transformer saves weight and dimensions more than 50% in comparison with traditional transformers, it demonstrates excellent efficiency up to 95%, and besides that it does not make much noise as well as its level of electromagnetic interference is very low.
Accumulator battery plus generator
What if electricity can disappear for a long time in the place where the house is located? Accumulator batteries will discharge and what should be done? A generator has to be switched on. As it has already been noted earlier this procedure is troublesome and besides this it is accompanied by considerable without-light-time. To solve the problems of a long absence of electricity you can include automatic generator start-up system into the kit in case of mains voltage failure. MAC will be transmitting its energy into the house simultaneously recharging the accumulator battery because it includes a built-in charger. Generally speaking, the best solution in this situation is to have at least four 12V batteries at 200Ah each in the emergency power system. For an ordinary household, if powerful consumers are not switched on, this capacity will last for 2-4 days of autonomy. Actually, it also fits well where there is no network at all - for example, in the headquarters of a geological exploration expedition. You can't do without a generator there at all but both an accumulator battery and an inverter are needed too. The generator will not work for months without a break. Inevitable stops of the generator are not the only problem. As it was mentioned earlier, consumption is uneven and the generator is either underloaded or it works at the limit of its power and it consumes only slightly less fuel on low-power modes compared to high-loaded ones. If we have an uninterpretable power system then the process looks different. On the one hand, the unclaimed part of the generator's power will be used, the accumulator battery will be charged with it, and the generator will work in a design mode consuming fuel most effectively. On the other hand, you can turn it off after the battery is fully charged which results in no noise, no exhaust, no consumption of fuel and resources. At this time the inverter will provide the house with electricity with a very high efficiency typical for electronic devices.
The generator is switched on for several hours for a whole energy cycle, the duration of which is determined by a ratio of the battery's capacity and an average power consumption – this gives maximum comfort and savings. One thing has to be mentioned at this point. Most inverters available on the market which are able to charge the battery from a third-party source demand high quality of a generator. The critical moment here is the absence of noticeable voltage fluctuations when the load changes. Hence, the generator must either be very powerful then it does not notice a significant change in the load current or it must have a built-in control system. MAC SINЕ Pro is able to work with the most common relatively cheap generators. Moreover, it can provide a high charging current and quickly charge a large capacity battery, it consumes very little energy at idle, it allows the user to adjust all the parameters. These are the key advantages of the model and it is worth to note that the same price competitors do not have them.
When there is not enough power
It happens that there is electricity but the power is not enough. This has become a very common situation recently. This problem can also be encountered when your network is powered by an autonomous AC power source with limited power and this is a usual situation for remote areas, expeditions, construction sites and other places where there is no stationary power supply. What is required in such conditions? It's a shame to turn off a power-line while there is electricity in it. It would be nice to be able to add energy in the home network. Earlier we spoke about the replacement of the energy from the network by the energy from the battery. We do not want to replace, now we want to add. Synchronization is a new problem here. The network gives us a sinusoidal current and the phase of this current (the timing of the max and min of the sinusoid) is quite definite. The inverter in the discussed mode of operation is not “interested” in this phase, it simply does not exist when the inverter is on the case. It simply provides the sinusoid with the phase that it's made. But if there is voltage in the network then the inverter must output its current with exactly the same phase that is present in the network. This is called synchronizing the output of the inverter with the phase of the network. This function is implemented in more complex, more high-end models which are commonly named as hybrid inverters. MicroART company has such a model and it is called MAC SINE HYBRID. These devices are able to provide an automatic power addition not only to the power line but also to a generator, if it is in an autonomous mode. With the help of a hybrid inverter it is also possible to use sources that are commonly called alternative such as solar panels, wind generators and micro-HPPs. What is more, they can be used to add energy not only to the home network but also to the external one. In fact, an owner of a sufficiently powerful wind generator or a solar panel can become a seller of electricity!
Lithium-iron-phosphate batteries (on the left), being the latest development, have a number of unique advantages, both in comparison with the usual types of lithium-ion batteries, and with modern lead-acid batteries. In particular, this is a record number of cycles (3000 at 80% discharge), high efficiency (94%), fast charge (up to 40 min). In addition, they do not spoil when discharged. However, the charge process of such batteries is much more complicated than that of any others. In addition to a special algorithm (it is incorporated into the MAC inverter), a special control system (in the center) is required for their normal charge and operation. It is called the BMS (Battery Manegement System) and it solves the problem of energy distribution between the batteries and monitoring the state of each of them, providing feedback to the charging inverter. Inverters equipped with such a system have the alphabetical designation "Li". Any type of MicroART inverters can be equipped with the BMS system. It is worth noting that today the price of lithium-iron-phosphate batteries is several times higher than that of the most expensive acidic ones. But in fact among the latter there are record-holders for cycles and the general service life (up to 1500 cycles at 80% discharge). Therefore, they still remain a reasonable alternative. This is primarily stationary batteries, or even cheaper versions of them - traction batteries with ironclad electrodes (on the right), especially if instead of the usual plugs they use plugs with the recuperation of the gases released during the charging process. They can have a capacity of 200 to 3000 Ah, but they are made for a voltage of 2 V. Connecting them in series, you can get 12, and 24, and 48 V operating voltage.
What if machine has to be connected?
If a household has a machine, it is highly possible that it is connected not to a single-phase but to a three-phase network. Does one need to install three inverters next to it? It is useless. Three independent inverters are not synchronized in phases at all and a three-phase motor of the machine will simply not start. Moreover, it is impossible to use such a system to swap in: a resulting phase imbalance can burn all the electrical equipment in the house. The true three-phase network must provide a constant and quite specific phase shift in three wires: a wave of each phase must go with a shift of 1/3 of the period in relation to the other phases.
This is implemented in a three-phase system of three MAC HYBRID 3F units. An additional printed circuit board is installed in each of the three HYBRID MAC inverters to synchronize them and obtain a 3F modification. The board has connectors which provide that all MACs are interconnected by cables and the circuitry that organizes the correct phase shift. The inverters will not lose connection with the network or the generator and with each other but they will smoothly adjust their frequency to the required value when working with an unstable frequency fluctuating within 48-52Hz or even more.