This report deals with:
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: