Development, planning, implementation and experiences (March - September 2022) with power supply of a house by means of photovoltaics (PV).
This is an "in the nutshell" guide for "Beginners" to the subject of "self-sufficiency with solar power" based on a completed project. It is only assumed that readers are familiar with terms such as photo-voltaic, inverter, etc.
1.)Construction of a small photovoltaic (PV) system for a second and holiday residence (house) in the south-east of Austria with the purpose of ensuring an autonomous, grid-independent electricity supply - if possible all year round. At least to the extent that the most important basic needs in terms of electrical energy are covered even if the public electricity supply - for whatever reason - should fail for several days.
It is a smaller house of older construction of about 50m2 floor space, with a cellar and a habitable loft conversion. One-person household.
2.)Evaluation of the extent to which objective 1.) can be expanded to the extent that total "off-the-grid" operation is feasible within a given cost framework. A fully autonomous operation - i.e. completely independent of grid power - was targeted as the ideal solution.
3.)Cost framework/efficiency: A maximum period of 10 years for the amortisation of costs was taken as a basis. The calculation was initially based on the energy costs in the first quarter of 2022, i.e. initial planning considerations were already made before the outbreak of the Ukraine crisis and its highly inflationary consequences for energy costs. Under these basic assumptions, an "off-the-grid" solution did not initially seem to make economic sense. After the outbreak of the crisis, however, the parameters originally used as a basis had to be "readjusted". In addition, the system planner felt visibly confirmed in his efforts to realise a reliable alternative to grid-based power supply.
Planning steps for the project
Survey of the technical parameters:
First of all, it must be clarified which consumers must be supplied (must haves). This would be, for example, refrigerators and freezers, cooking appliances if electrically operated, lighting, water heating, charging or operating battery-operated devices such as laptops, mobile phones, etc.
In a further iteration step, consumers that are classified as secondary (nice to have) can be included.
In this case, wood is available as an energy source for heating purposes, which simplified the planning considerably. Heating a house with PV panels alone is hardly realistic according to the current state of the art if it is not a low-energy house and/or a heat pump is also available.
The contractually agreed connection of the mains power supply allows a maximum power of 3500VA. This value had always been completely sufficient for all needs in the house, as long as care was taken that large consumers such as ovens, radiant heaters, washing machines, larger machine tools and the like were not operated simultaneously. This already provided a first reference value.
However, this is not enough for cost-optimised dimensioning of all components of a PV system.
The basic components of the system are:
- PV solar panels
- An electronic controller connected downstream of the panels.
- An electrical energy storage device (accumulator) connected downstream of the controller.
- And finally, a so-called inverter which is connected downstream of the accumulator. Its task is to convert the accumulator's DC voltage (in this case typically 12.8V) into 230V / 50Hz AC voltage.
When selecting the components, it makes sense to start from the consumer side. First, measure the power requirements of all consumers. The easiest way to do this is to use a power meter that retails for about 30€. If such a device is not available, you have to orientate yourself on the rating plate or the specification of the respective device.
Important! Many consumers have a considerable reactive power requirement (motors, transformers, etc.). Even the mains network in a larger house without connected consumers can consume quite a few watts (or VA). If such a network is permanently connected to the switched-on inverter, this power loss adds up to a considerable energy drain from the storage battery. To minimise these losses, the inverter should therefore always be switched off unless a load is active. Of course, this is not possible when operating cooling devices or similar.
During the test and commissioning phase of the system in question, it was found that even consumers that one would not expect can consume high apparent power even in standby mode. In the case of an induction cooker plate, approx. 300VA apparent power was measured (strictly speaking, this is almost exclusively so-called reactive power). Or with large switching power supplies (SMPS) in standby mode in the order of 40VA.
What is often overlooked or misunderstood in planning is that apparent power (unit [VA ] VoltAmpere) is the vectorial sum of reactive power (unit [var] volt-ampere-reactance) and active power (unit [W] Watt). The summation of the vectors is determined by the angle phi between the vectors. For many loads, phi is indicated indirectly in the data sheet or on the rating plate in the form of cos phi (cosine of angle phi). The reactive current and thus the reactive power must ultimately also be provided by the energy source. Why is it overlooked? It is probably because most power meters only show active power [W] watts. (Some may display the cos-phi value. Then you can calculate what the apparent power is in [VA]. With a 2-channel oscilloscope and current clamp it is also possible). Another reason is that you never had to worry about this as long as the electricity came from the power company. Only kilowatt hours [kWh] are used for billing electricity consumption. However, the utility uses an average cos-phi for the electricity bill (which can be seen in the detailed breakdown of the electricity bill).
If all this is too complicated for you, you should at least add a factor of 1.2 to the calculated wattage values for reactive power.
The measured values are generally values that are measured in the "steady-state" condition of the loads. For the selection of the appropriate inverter, however, one also needs the peak power that is required. Almost all loads draw a considerably higher current than normal when they are switched on. Not only the amplitude of this resulting power peak is relevant, but also its course and duration. Here again, an oscilloscope (with memory function) would be useful. Often an estimate remains the only option. The author has made the experience that the inverter should be generously dimensioned. This generally results in higher reliability and a longer service life. This has to be weighed against the additional costs.
However, this should not be confused with the peak power that inverter manufacturers usually specify. It is often twice as high as the continuous power, but the time for which this power is made available - before the inverter switches off the output - is sometimes too short. In the system in question, an inverter with 3500W nominal power and 7000W peak power was used. It should also be noted that the self-consumption of different inverters should be compared. The self-consumption can be very significant if an inverter remains switched on 24 hours a day.
Finally, the respective operating time for each consumer must be taken into account. Only the product of power in [W] and operating time in hours [h] provides information about the consumed energy in [Wh] or [kWh]. This results in the battery capacity needed to operate certain devices together or alone for a certain time. This is also the case when there is no sunshine. This depends on the location and season.
Even if all this data cannot be obtained, there is an alternative solution. If a flat or house is converted from grid electricity to PV electricity, the typical energy consumption per quarter or year can be seen from the utility bill. A daily average energy consumption can be calculated from this and serve as a rough guide.
Reserves must be provided for losses in cables, charge controllers, charging and discharging losses in batteries, losses and self-consumption in the inverter. You can calculate this using the manufacturer's data sheets. But with an additional reserve of 30 ... 35% is not very far off the mark. Battery ageing and low ambient temperatures also reduce battery performance. You have to look at this on a case-by-case basis. In the case of the system mentioned, it was not taken into account under the assumption that the batteries are stored at room temperature all year round and that deep discharge cycles are avoided as far as possible through prudent and flexible consumption behaviour.
consumption behaviour can be avoided as far as possible.
The accumulator is the linchpin in the system. First of all, you have to decide on a system voltage. For small systems, this is usually about 12V. For larger systems, multiples of 12V are common as battery voltage. (It should be clear that inverters and charge controllers must be suitable for the system voltage). In the author's opinion, the upper power limit where one can still get by with 12V is about 3000 to 3500W maximum power for less than 30 minutes. The reason why 12V was chosen for this project is that a smaller existing inverter for 12V is intended as a backup. Otherwise 24V would have been the better choice. Why? The lower the
The lower the battery voltage, the higher the currents that the battery has to supply to the inverter. For a certain power, the following applies: current [A] times voltage [V] gives power in [W]. At 12 V, a few hundred amperes quickly accumulate. This requires thicker cables (more copper - more costs) and is detrimental to the efficiency due to higher losses in cables, contact transitions, coils, fuses and electronic switches. Another disadvantage is that batteries do not like it when very high currents are drawn from them. The usable battery capacity is noticeably lower at high currents than at lower currents.
For the battery selection, the following system parameters of the system must be determined:
a) Maximum power required
b) Typical energy demand to be covered
c) Maximum time span that must be bridged when the PV panels supply little or no power.
d) Other criteria would be:
- Working conditions and installation of the system (temperature range, installation in the house, basement or outdoors, ...).
- Expected lifetime and reliability of the system
- Maintenance effort for the storage batteries
- Battery self-discharge (relevant when the system is shut down for a longer period of time)
- And last but not least, acquisition and possible maintenance costs.
For a) Does have the data already been collected for the selection of the inverter.
For b) You can also use data that has already been measured or information from the appliance manufacturers for the respective consumers. It is very useful if you have already recorded statistical values for the typical energy consumption over at least one year. Depending on the usage profile of a house that is to run in stand-alone mode, the worst case is taken as a basis for the battery selection. In the case of year-round occupancy, this is generally the low-sun winter period, which is often accompanied by an increased energy demand (heating, lighting, hot water preparation at low flow temperature, frost protection heating, etc.).
For the system in question, the long-term electrical energy consumption was 1000 kWh per year. However, this could be reduced to just under 700 kWh per year in the last three years solely through energy-saving measures (e.g. replacing all light bulbs with LED lamps, switching off cooling devices in winter and storing food in unheated rooms, replacing audio amplifiers with modern, highly efficient switching amplifiers, using switch-mode power supplies (SMPS) instead of conventional transformers).
The following applies to c): Depending on the usage profile, the worst-case scenario - for a system operated exclusively with PV - will probably always be in the winter months. At the location of the local system, 300 days of sunshine per year can be statistically expected. However, this figure only means that the sun is in the sky at least briefly on 300 days. Documented empirical values on solar energy generation using PV panels were only available for spring and summer. The author accepted the resulting risk factor because a later expansion through either additional storage capacity (connecting more batteries in parallel to the existing ones) and/or adding more PV panels was already considered during the installation.
Since the beginning of March 2022, the system had been running in trial operation, largely independent of the energy supplier's grid. However, the grid had not yet been disconnected, but only disconnected from the meter box within the house.
Initially, only a single LiFePO4 battery with nominal voltage 12.8V / maximum current (charge and discharge) 200A / energy capacity 2560Wh / >= 4000 cycles / Ri <= 25mOhm / G = 21kg / L x W x H = 521 x 238 x 217 mm was purchased.
It was clear that the capacity was not sufficient to cover more than 3 to 4 days without sunlight. The first step was to test how well everything works. Then a second, identical battery was connected in parallel to the first. This doubles the maximum continuous current to almost 400A (due to not quite identical line resistances, the theoretical 400A may not quite be reached). Each individual battery was connected with equally short (approx. 70cm) AWG2 (35mm^2) copper wires to the inverter. Double the current can be drawn for 5 seconds. This harmonises quite well with the 3500W/7000Wmax. inverter.
After initial problems with the optimal alignment of the PV panels and an expansion of the panel area together with the purchase of MMPT (Maximum Power Point Tracking) solar controllers, the energy supply contract with the utility was terminated at the end of June.
(Maximum Power Point Tracking) solar controllers, the energy supply contract with the utility was terminated at the end of June. Since then, the house has been completely energy-autonomous in island mode.
A few more details about the PV panels that are currently installed:
These are monocrystalline folding panels that are actually intended for mobile use (e.g. mobile homes). In the medium term, however, these are to give way to a fixed installation on the roof.
Two panels of 200W each and one 120W panel (manufacturer's nominal power data) are in use. The total area is about 3.8 m^2. The manufacturers' power specifications are not wrong, but they apply under optimised laboratory conditions. In reality, these specifications can be approximately halved for a cloudless day. The manufacturer's specifications are only useful for comparing products from different manufacturers.
The real yield - measured at the output of the panels in the configuration described - is between 600 and a maximum of 1300Wh on sunny days between March and October. Trees and an outbuilding cast shade on the panels in the morning and then in the evening. With a clear view, even more would therefore be possible.
These values can only be achieved with MPPT solar controllers. Cheap PWM controllers only manage less than half. The additional costs of about 100 euros for an MPPT controller are well invested. Caution is advised when MPPT controllers are offered for less than approx. 70 euros. These are usually "cheat packs".
In this configuration, only the PV panel areas are currently the limiting factor. Not in summer, as experience so far shows (the batteries are practically always between >60% and 100% full), but in winter it will probably be tight. Until then, the panel area will be doubled.
It should also be mentioned that the entire system - except for the PV panels, of course - is housed in a fairly compact shelf on castors in the living area. The space required is only about L x W x H = 60 x 30 x 90 cm. The shelf stands directly on the south wall (inside) of the house and the line goes through the wall directly to the PV panels. This is the shortest connection. Further advantages are that the inverter can be conveniently switched ON and OFF from the living room, measuring devices for current, voltage, power, fed-in energy, consumed energy are always in view and the batteries are not exposed to extreme temperature fluctuations because everything is "indoor".
In the past years, the electricity costs amounted to typically 300 euros per year (due to inflation, the costs could not be reduced even in the years after the introduction of energy-saving measures - only the price increase was compensated.
Planning and installation were carried out in-house and are therefore not included in the cost calculation for the system.
The material costs amounted to about 3500 euros. The two lithium-iron-phosphate (LiFePO4) accumulators were the most expensive, with a total of about 5000Wh for about 2000 euros. However, the author is of the opinion that the much cheaper lead-acid accumulators are ultimately not cheaper when one considers that their service life (number of charge/discharge cycles) is much shorter.
As explained at the beginning, a payback and operating time of about 10 years was estimated. Even under pre-crisis energy price levels, this would be feasible with the system as realised. From today's perspective, with rapidly rising energy prices, the calculation looks even better. One can assume that - if the cost trend continues as it has so far - even if the installation is carried out by professionals, it will be possible to amortise the entire production costs in considerably less than 10 years.
The following is also worth mentioning: There are "all-in-one" solutions where the entire electronics (charge controller, inverter and various special functions) plus the storage battery are housed in one casing. Only the solar panels still have to be connected. These solutions are also available in the same power ranges as the system described. The attractive features are the compact design and portability. In terms of price, these solutions are roughly the same as single-component solutions - provided that one provided that the comparison is made with the same battery technologies! However, one must bear in mind that in the event of a defect in the combined unit, the entire system usually fails. In addition, later modifications and upgrades are hardly possible (only external additional batteries). These were the reasons why the described system was built from individual components.
Apart from the cost aspect, there were, in the author's view, other factors and motives for the implementation of this project which cannot be quantified in money. For example, environmental protection, sustainability and, last but not least, the good feeling of independence in a time marked by crises.
(c) 2022 by Robin
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