An autonomous solar power supply system seems very simple. After all, it has only 4 main components – the photovoltaic panels themselves, batteries, a charge controller and an inverter that converts low-voltage direct current to a household standard of ~ 220V. However, this simplicity is deceptive – here, as in any system, all elements must be balanced with each other. The imbalance at best will result in unjustified costs for unused potential, and at worst – the failure of the weakest element and, as a result, the inoperability of the entire system.
First of all, you should find out how much energy is required from the autonomous system. To do this, you will have to determine the peak instantaneous power, as well as calculate two values \u200b\u200bof the expected daily energy consumption – its maximum and average values.
Peak instantaneous power is determined by the total power of all power consumers that can be turned on simultaneously, that is, the worst case in terms of network load.
The expected daily energy consumption is more complicated. It depends on the mode in which it is planned to use the autonomous power supply system being created.
1.Full power supply
Full power supply from solar panels means the complete replacement of mains power supply with an autonomous one without any restriction on the usual lifestyle. To determine the amount of energy you need, just look at the electricity meter or just look at your monthly electricity bills. In order to completely disconnect from the mains but not change your lifestyle in any way, you need a system capable of delivering at least 600 kWh of electricity per month with a continuous power of at least 5 kW, and energy consumption per day can reach 50 kWh with an average value from 10 to 20 kWh per day.
2. Comfortable power supply
Comfortable power supply differs from the complete one only with the exception of the most gluttonous consumers, for example, electric heaters, whose power exceeds 2 kW or the average daily energy consumption exceeds 4 .. 5 kWh. Thus, washing machines, electric irons, bread makers, electric kettles and even electric floor heating in bathrooms, along with electric hot water boilers, continue to remain in the system, but electric stoves, electric ovens, convectors and electric heating of large areas are excluded. Which, of course, does not prevent connecting them to an external network with a separate line.
Typically, comfort mode will require an average of 100 to 250 kWh per month (average daily consumption of 3 to 8 kWh) with a peak consumption of up to 15 kWh per day, and the instantaneous power consumption in continuous mode does not exceed 5 kW.
3. Moderate power supply
This mode offers a noticeable change in lifestyle while maintaining a high level of comfort. However, the list of consumers differs little from the comfort power supply mode, with the exception of such optional elements as electric kettles and electric floor heating. The use of electric hot water heating may also be limited. In addition, the changes also apply to the execution time of not very regular, but energy-intensive work. In order to save on battery capacity, such work should be done not at night and not in cloudy weather, but on sunny clear days, when the flow of solar energy is maximum and partially compensates for the discharge of batteries, and what is discharged will be replenished before dark. These works, for example, include a large wash (especially in an automatic machine with heated water), ironing a large amount of laundry, active work with powerful power tools and garden electrical equipment, etc. If we exclude regular consumers of the second stage (kettle and water heaters), then we should focus on monthly consumption of about 150 kWh with instantaneous power consumption in continuous mode up to 3 .. 3.5 kW and peak power up to 5 kW, and the expected average daily consumption is 4 . .6 kWh with a possible maximum of up to 11 kWh per day.
4.Basic power supply
In this mode, the features of energy consumption have a very significant impact on lifestyle. This influence primarily lies in the constant consideration of the current load on the autonomous power supply and the need to turn on more or less powerful consumers in turn. In addition, in this mode, you should always remember about savings, in particular, turn on the light only there, then and as much, where, when and how much you really need it. The same applies to all other electrical appliances. Nevertheless, despite all the reservations, in this mode it is still possible to maintain a sufficient level of comfort and use almost all home electrical equipment, however, the turn-on time of energy-intensive consumers is largely determined by the weather – all energy-intensive work should be carried out only on sunny days and, preferably, , before lunch, so that by the evening the battery charge is restored to the maximum. In this case, the monthly consumption is approximately 100 kWh with an instantaneous continuous power consumption of about 1 kW with a peak consumption of up to 2.5 kW, and at the time of using a power tool – up to 4 kW, with an expected average daily consumption of 3 .. 4 kWh with a maximum of up to 7 kWh per day.
5. Emergency power supply
Emergency mode implies a severe limitation of demand, but unlike previous cases, it is assumed that autonomous operation in this mode will last no more than a few days in a row, so many energy-intensive electrical appliances can not be used at all until normal power supply is restored. The task of emergency power supply is to ensure minimal convenience and functioning of the most important life support systems at home.
So, in this variant, everything that is not vital is turned off and does not turn on, including the TV is not used, and the refrigerator is not used in winter (in summer, the use of the refrigerator is also assumed to be more careful and rare, which helps to save electricity). In this case, the monthly consumption will be 50 .. 60 kWh with an instantaneous continuous power consumption of approximately 600 W with a peak consumption of up to 1.5 kW (up to 2.5 .. 3 kW at the time of use of the power tool), and the expected average daily consumption is 1.5. 2 kWh and does not exceed 6 kWh, although due to the separation of energy-intensive work on different days, it is quite realistic to limit the daily maximum to 3 .. 4 kWh.
In each case, the data must be considered individually, based on the available technology, your own approaches to its use and established habits. However, the calculation method is the same.
Determination of the possibilities of the Sun
So, we have just determined the energy needs. Now we need to see what can be obtained from the Sun? The basis of such a calculation is data on the power of solar radiation, taking into account weather conditions. It is desirable that the data be for different angles of the panel, at least for vertical and horizontal orientation.
The most important issue is the choice of the angle of the panel. Bearing in mind the possibility of year-round use, an angle of 15 ° more than the geographical latitude should be preferred (in addition, the greater the slope, the less dust and snow will linger on the panel).
The slope is selected. Now we can begin to estimate the potential performance of solar panels, or, equivalently, to estimate the number of solar modules required to operate the system in the desired mode. The assessment should be carried out at least for the worst month (for Moscow this is January), for most of the year (February – November) and for the summer maximum (in Moscow this is July).
Standard insolation is calculated for an area of 1 square meter. However, we do not know the exact area of solar panel elements. But its nominal power is known, which is determined for illumination with a power of 1 kW / m2 at 25 ° C. This is quite enough. Taking the power of solar radiation near the Earth’s surface (maximum insolation) to be the same – which, in general, corresponds to reality – we get that battery output is related to insolation per square meter in the same way that battery power is related to solar radiation power near the earth’s surface on a clear day. weather per 1 square meter, that is, to 1000 watts. By multiplying the monthly insolation from the table by the ratio of battery power and maximum insolation, you can estimate the solar battery output for this month.
Thus, the output of the panel will be calculated according to the following formula
Esb = Eins Psb η / Pins (1),
where Esb – energy generation by the solar battery; Eins – monthly insolation per square meter (from the insolation table); Psb – nominal power of the solar battery; η – inverter efficiency when converting low-voltage direct voltage to standard voltage (if it is supposed to use low-voltage voltage directly, η can be equated to 1, i.e. not taken into account); Rins – the maximum power of insolation per square meter of the earth’s surface (1000 W). Insolation and desired output must be in the same units (either kilowatt hours or joules).
Accordingly, knowing the monthly insolation, it is possible to estimate the nominal power of the solar battery required to provide the required monthly output.
Psb = Pins · Esb / (Eins · η) (2).
It should be noted that, as a rule, the maximum power of the solar battery, which, in fact, is declared by the manufacturer, corresponds to the voltage at its output, which is 15 .. 40% higher than the voltage of the batteries. Most inexpensive charge controllers can either connect the load directly, “draining” the output voltage of the batteries far below the optimum, or simply cut off this “surplus”. Therefore, these losses can also be included in the efficiency, reducing it by 10 .. 25% (power losses are less than voltage losses, since at increased load the voltage “sag” is compensated by a certain increase in current, although not completely; the value can be determined more accurately only knowing dependence of voltage on load current for a particular battery). However, there are controller models that keep these losses within 2. 5%.
Equipment selection
As already mentioned, solar power systems include the following types of devices.
1.Panels with photovoltaic cells.
2. A solar battery controller that regulates the battery output voltage, charges the batteries and (optionally) supplies low-voltage direct current to the load.
3. Electrochemical batteries that store energy during the period of its excess and supply it to the system during the period of shortage with insufficient illumination of photocells or with a temporary increase in consumption.
4. An inverter that converts low-voltage direct current from batteries and photovoltaic cells to a domestic or industrial standard.
The determining selection criteria are two powers – the nominal power of the solar battery and the maximum load power, and in the general case, these values \u200b\u200bare little correlated with each other. Let’s say you can charge batteries from a 200-watt solar battery all summer day, turning it after the Sun and accumulating 2.5 kWh of energy, and spend it on welding in half an hour in the evening using a 5 kW inverter.
But before choosing specific models, you should decide on the low voltage DC voltage that will be used in the system.
System voltage selection
If everything is clear with the choice of the output voltage of the system – in Ukraine it is 220 V AC with a frequency of 50 Hz, then the choice of low-voltage DC voltage, i.e. voltage at the input of the inverter, it is also the nominal voltage of the battery pack and photovoltaic panels – much wider. Standard powerful batteries have a voltage of 12 V, and 6-volt “motorcycle” options are often found. Finally, you can find modules with a voltage of 2 V and assemble a battery from them for any voltage that is a multiple of this step. The nominal output voltage of photovoltaic panels of 50 W and above is usually either 12 or 24 V, but it can also be increased in appropriate steps by connecting batteries in series.
Most inverters are rated for 12V, 24V, 48V, or 96V DC input, depending on power. The fact is that already to provide a power of 1 kW at a voltage of 12 V, a current of more than 83 amperes is needed! If we take into account the losses of the inverter, which can reach 15%, then the current is very close to 100 A. Similar and even 2-3 times higher currents are typical for a car starter, but they rarely flow there and for a short time. Here they must flow in a long, almost constant mode. As a result, the cross section of the wire must be very large – for copper wire at least 25 mm2 (diameter about 6 mm), – and the wires themselves must be as short as possible – no more than a meter, but it is better to try to keep within 20 .. 30 cm. otherwise, they will have too large losses of energy spent on heating them, which is not just useless, but frankly harmful and even dangerous. With a power of 10 kW, the current, respectively, will increase to 1000 A, and the wire cross-section will increase not by 10, but by more than 20 times due to problems with heat removal from the middle of the core – it will be a copper bar with a diameter of almost one and a half centimeters. Even just providing a compact and reliable connection that allows such powerful currents to flow through it for many years is very difficult. For these reasons, inverter manufacturers limit the input current consumed by the inverter in rated power mode to one to two hundred amperes, and when the power increases, they are forced to increase the input voltage.
Unlike photovoltaic panels and batteries, inverters and controllers cannot be cascaded in series, so they must be selected based on the DC voltage according to the required output power of the inverter in the above table.
Within 24 V, this voltage is safe and suitable for the rated output power of the inverter in kW, and even up to kW-h kW is enough for almost all consumers found in a normal household. .If it is required to power several powerful consumers at the same time, then it may be justified to connect them to two or more inverters at the same time – each to its own – despite the fact that the rated power of each inverter does not exceed kW and the input voltage remains within the limits. By the way, this will allow the system to continue working and in the event of a sudden failure of one of the inverters, the remaining one will provide the necessary voltage, although, of course, the load power will need to be monitored more carefully. And only when the power of one consumer exceeds the output power of one inverter, you will have to take a more powerful inverter and therefore switch to a higher DC voltage. It is possible to connect the inverter in this way with some restrictions during installation, otherwise the inverter may fail.
Inverter selection
First of all, the selected inverter must provide the necessary output power. The input (low voltage) voltage is closely related to this power. But besides this, inverters have other characteristics that you should pay attention to.
First, it is the form of the generated current. The simplest models produce an alternating current of a triangular or even rectangular shape (meander). Such a current is successfully “eaten” only by heating devices that do not contain electronic components, and incandescent lamps. All other electrical equipment (any electric motors, transformers, fluorescent and energy-saving lamps, etc.) from a current of this form can either fail, or not start, or work, but very badly – despite the fact that the tester honestly shows 220 V. A little more trapezoidal current is acceptable. Fortunately, inverters that produce these forms of AC output are rare these days. Most often, modern inverters produce the so-called “modified sine”, which is a stepwise approximation to a sinusoidal form. Such
the current form is quite successfully “digested” by almost all modern household appliances and power tools, but the sound of some of them noticeably changes and becomes louder, and power supplies can begin to “ring” noticeably. To eliminate this problem, you can try to use various filters that smooth out the current irregularities. Finally, “pure sine” inverters produce a current that is very close to a perfect sine wave and is usually much better than the current in the public grid. The only drawback of this class of inverters is that they are slightly larger and one and a half to two times more expensive than similar “modified sine” inverters.
Secondly, it is the efficiency of the inverter. The higher it is, the less unproductive energy losses. Most modern inverters are over 90% efficient.
Thirdly, this is the ability of the inverter to work in battery charging mode. In fact, such an inverter complete with batteries is interesting in itself – even without solar panels, it is an uninterruptible power supply (UPS) – approximately the same as used for computers, but with a power of several kilowatts and a capacity of several kilowatt-hours . When working with solar panels, this feature is also very useful – it allows you to reduce the power reserve of solar panels and battery capacity for the most unfavorable situation, since in case of a lack of solar energy, the batteries can be recharged from an external network or from an emergency generator.
Fourth, the more detailed the indication, the better. It is highly desirable to be able to control both the input voltage on the batteries and the output voltage in the outlet. In addition, protection against overload and short circuit in the load is required.
Fifthly, it is very good if the inverter allows a short-term excess of the rated load at least one and a half to two times. This allows the use of electric motors and heaters whose power is equal to the rated power of the inverter. The fact is that when they are turned on, the current for a second or two significantly exceeds the corresponding nominal mode. If the inverter protection is set strictly to its rated power, then at this moment it may work and prevent the use of an electrical appliance, the consumption of which actually fits within the rated power, except for a brief moment of switching on.
Sixth, a useful feature that, when the battery is fully charged, connects an additional load to a separate line, say water heaters. On sunny days, this allows you to usefully automatically utilize excess energy and prevent energy from being wasted on secondary purposes when there is little of it.
And the last. With the exception of some special cases, with a power consumption of up to 10 kW, it is much more convenient to use not three-phase, but single-phase voltage. This simplifies the wiring around the house and eliminates the problems associated with the distribution of phases to consumers. In addition, three-phase inverters are harder to find, more complex and more expensive than single-phase inverters of the same capacity.
Battery selection
The most widely used batteries are 12 V, and it is from them that batteries are usually assembled for any voltage multiple of this value, including 24, 48 and 96 V. The battery pack of the autonomous power supply system is characterized by such basic parameters as operating capacity, charge current and discharge current.
With an operating voltage exceeding 12 V, several batteries are connected in series so that the sum of their nominal voltages corresponds to the required nominal voltage of the unit. If the current strength or energy reserve of one such assembly is not enough, then several assemblies are connected in parallel until their total capabilities reach the required threshold.
Type selection
Currently, there is no economically viable alternative to powerful lead-acid batteries. However, this class of batteries has several varieties.
Which battery to choose gel or AGM? Advantages and disadvantages
Container preselection. Working and buffer energy reserve
First of all, it is necessary to determine the total energy consumption of the battery pack. In most cases, it can be said that the operating energy reserve of such a unit should be chosen approximately equal to the calculated average daily consumption in the minimum acceptable mode. For example, for emergency mode it will be 2 kWh, for basic mode – 4 kWh, for moderate mode – 5 kWh, etc.
Battery capacity calculation
How to choose the capacity of a separate battery? Say, a 24-volt 2 kWh unit can be assembled from eight 12-volt 50 Ah batteries, four 100 Ah batteries, or two 200 Ah batteries. In this case, I prefer 100-amp batteries. The 200-amp ones are very bulky and weigh 65 .. 75 kg, so even moving them alone is not at all easy, especially in tight, inconvenient places. At the same time, 50-amp batteries will require too many connections, and this increases the complexity of installation and reduces reliability. 100 amp batteries weigh less than 40 kg and are not as difficult to lift, place or move by one person, with half the number of switching than using 50 amp batteries, and the total price of the battery pack will be slightly lower.
It should be emphasized that this is only a preliminary selection of capacity, and it must be checked for compliance with the charge and discharge parameters declared by the battery manufacturer. They are the ones that take priority.
Charge and discharge currents. final choice of container
The total charging current, equal to the maximum current of the solar battery, must not exceed the maximum allowable battery charge current specified by the manufacturer, multiplied by the number of parallel assemblies (assemblies, not individual batteries). This condition can be violated if the solar array is powerful and the battery pack is too weak. And then not only a quick failure of the batteries is possible, but even their explosion and fire!
On the other hand, too little charge current will not be able to fully charge the batteries. This happens when the capacity of the battery pack is too high and the solar panel has little power. With a short operation, this will only lead to a reduction in the energy reserve in the batteries, however, constant undercharging reduces the capacity of the batteries and shortens their service life.
Finally, the current consumed by the inverter in maximum power mode should not exceed the maximum allowable discharge current of the batteries, multiplied by the number of their parallel assemblies. To ensure more comfortable working conditions and good energy efficiency of batteries, it is desirable that the discharge current in continuous mode does not exceed half, or better, a fifth of the maximum allowable value.
The exact values of the currents should be viewed in the documentation for a specific battery model, however, for preliminary estimates, the following values \u200b\u200bof these currents in amperes can be taken relative to the capacity in ampere-hours:
- the maximum discharge current is numerically equal to the capacity and is valid only in a short-term mode – less than a minute;
- the optimal discharge current does not exceed 20% of the capacity (for a long-term continuous load, it is better to keep within 5 .. 10%, – say, the load from lighting is less than 10%, and when the refrigerator is turned on, it remains within 20%);
- the optimal charge current is 5 .. 10% of the capacity;
- the maximum charge current does not exceed 20% of the capacity (sometimes up to 30%).
The main criterion for choosing the capacity of batteries is the charge current, since it is it that has the main influence on the durability and safety of their operation. Based on the above figures, the total capacity of battery assemblies in ampere-hours should be 5 .. 10 times higher than the maximum total current of photovoltaic panels assemblies in amperes (not individual batteries and panels, but their assemblies for the nominal low-voltage voltage of the system). And already within these limits, you can focus on the required energy reserve. Some battery models allow you to expand the limits of the allowable range of block capacities up to 3 .. 20 times the maximum generated current of the panels.
Choice of photocell panels
When choosing panels, three factors should be considered – their geometry, rated output voltage and type of photocells.
The geometry is determined by the specific installation conditions, and it is difficult to give general recommendations here, except for one – if there is a choice between one large panel and several small ones, it is better to take a large one – the total area is used more efficiently and there will be fewer external connections, which means higher reliability. The dimensions of the panels are usually not too large and do not exceed one and a half to two square meters with a power of up to 200-250 watts. In order to achieve the desired voltage and power ratings, the panels can be combined into series assemblies, which are then connected in parallel – similarly to how it is shown above for batteries. As with batteries, only panels of the same type should be used in the same assembly.
With voltage, everything is also simple – it is better to choose 24-volt panels, since their operating currents are half that of 12-volt panels of the same power. Panels of the same power of the same manufacturer, designed for different voltages, usually differ only in the internal switching of photocells. Panels with nominal voltages above 24 volts are rare and are usually assembled from lower voltages. 12 volt panels are justified in my opinion only in two cases – for low power systems where 12 volts is the operating voltage of the inverter, and also if for architectural or design reasons it is necessary to use small panels for which 24 volt options do not exist.
When self-assembling panels from individual photocells, one should not forget about the inclusion of protective diodes in the chains, which prevent the flow of reverse current in case of uneven illumination. Otherwise, the power generated by the illuminated sections of the panel, instead of the payload, will be allocated to a temporarily shaded photocell, and this is fraught with its overheating and complete failure (an unlit photocell in this situation will turn out to be an open diode). The permissible current of the protective diodes must be greater than the short-circuit current of the protected photocell string at maximum illumination.
Finally, you need to select the type of photocells. At present, photocells based on single-crystal or polycrystalline silicon are most often offered. Monocrystalline silicon usually has an efficiency in the region of 16-18%, and polycrystalline – 12-14%, but it is somewhat cheaper. However, in finished panels, the price per watt (i.e., in terms of the generated power) is almost the same, and single-crystal silicon can be even more profitable. According to such a parameter as the degree and rate of degradation, there is practically no difference between them. In this regard, the choice in favor of single-crystal silicon is obvious. In addition, often with a decrease in illumination, monocrystalline silicon provides a nominal voltage longer than polycrystalline, and this allows you to get at least some energy even in very cloudy weather and at light twilight. On the other hand, polycrystalline silicon usually has a lower open-circuit voltage (for single-crystal silicon, it can be twice the nominal value). But if you connect the panel to the inverter and the battery not directly, but through the controller, the increased voltage does not matter.
And the last. Usually it makes no sense to choose the total power of photoconverter panels more than the power of the inverter. However, such an excess can be justified in the presence of a powerful constant load and a powerful battery pack, or based on long periods of cloudy weather.
Controller selection
Types of charge controllers
With the right choice of panels, there is no great need to increase the voltage. Much more important is the ability to reduce the relatively high “optimal” voltage of the PV array, corresponding to the maximum power produced, to the lower level required to charge the batteries, converting the excess voltage into additional current and ensuring that the rated battery power is fully utilized. As mentioned above, with direct switching of the output of the photocell panel to batteries, due to a non-optimal load, the voltage can “sag” below the optimum by 15 .. 40%, due to which power losses can reach 25%.
The technology that prevents such losses is called MPPT (Maximum Power Point Tracking) by some controller manufacturers. It consists in constantly measuring the current and voltage generated by the panels and ensuring their optimal ratio, which depends, in particular, on the time of day and on the current situation in the sky (the sun came out or a cloud came running). This allows you to achieve optimal use of battery power in almost all operating modes and reduce losses by up to 3%. However, the cost of such controllers exceeds the cost of the simplest models by several times. Therefore, in low-power systems, it may be more profitable to purchase an extra 100 .. 200 W panel and limit yourself to a simple charge controller, but not overpay for MPPT.
As an additional option, some controllers can turn off the low voltage load when the batteries are too low. However, this function is also not very relevant, since many modern inverters do the same, but for all the power connected to them, and the power of charge controllers is very limited.
Controller power selection
The most common controllers are designed for a current of 10 .. 20 A, sometimes 30 A. More powerful controllers are less common and cost much more. However, it is quite possible to combine several not very powerful controllers in parallel by connecting each of them to its own group of photovoltaic panels. This scheme has some inconveniences, but in most cases it is quite acceptable. However, consultation with the seller (or better, the manufacturer) will not hurt, since specific controller models may have features that do not allow such a connection (this is especially true for controllers with MPPT and smart controllers that change the charge mode as the battery charges).
When connecting panels to the controller, make sure that their total maximum current does not exceed 75% .. 85% of the rated current of the controller. For example, for a 20-amp controller, the total current should be no more than 15 .. 17 A. This margin is necessary so that the controller can withstand excessive generation, for example, on a clear winter day, when white snow, which perfectly reflects light, contributes to overexposure of photocells compared to the calculated one, and moderate frost slightly increases their efficiency. Thus, one 20-amp controller can connect panels at 24 V with a total power of 600 W, and at 12 V – only 300 W.
