Burning Issues 4. Solar and Battery 2

Boiler Phaseout: 4. Solar and Battery – Part 2

There are vigorous arguments over whether or not to install micro-inverters or power optimisers which add cost, or to rely on bypass diodes.  One common resolution is to only add microinverters or power optimisers Similarly, debate persis

Details, Details

History: Solar panels are far from being a new invention (see Further Reading).  The production of electricity on exposure of some materials to light was noted in 1839, and the first actual panel was created in 1881.  However these panels weren’t very efficient (<1%).  The first modern solar cell design was patented in 1941, and panels began to be produced commercially in the 1950s.  This was followed by use to power space travel, and further Research and Development (R & D) leading to escalating installation in the 21st century.

The story of batteries dates back even longer, to the ‘Baghdad pot battery’ two thousand years ago (see Further Reading).  A number of scientists experimented with Leyden jars during the 1740s, leading Benjamin Franklin to coin the term ‘electrical battery’ to describe collections of such jars.  This was followed by demonstration of the principle of batteries in the 1780s by Luigi Galvani, and of a working ‘wet’ battery in 1800 by Alessandro Volta.  The first rechargeable battery (lead-acid) was devised in 1859.  Dry batteries followed in the 1860s and 1880s, and then Waldemar Junger’s nickel-cadmium (NiCad) battery in 1899 and Thomas Edison’s nickel-iron battery in 1900.  Alkaline batteries were introduced in the 1950s, and the first nickel-metal hydride (NiMH) batteries in 1989. Meanwhile, after many years of experimentation with lithium, commercial lithium-ion (Li-ion) versions were introduced in the 1970s, and patented in 1991.

Today’s Solar Panels:  Most PV cells incorporate wafers or thin films of silicon, with a surrounding structure for protection., and an anti-reflective coating,  Cells are joined together in series in a string, and further grouped into modules connected by bus bars to make up a single panel.  In modern panels, individual modules with say 20-30 cells in series also contain a bypass diode, that as its name implies can bypass the string in case of a fault developing (see below for more on this). More recent designs are evolving from a single string of cells in a panel, to ‘half panels’, which have the same number of cells laid out in two separate strings. Alternatively, newer, shingled cells make contact through overlapping rather than via bus bars.  Panels are laid in arrays either on available roofs, or on the ground. The efficiency of solar panels has increased dramatically from less than 1% in early prototypes, up to 25% or even 30%.  Some experimental formulations are now achieving close to 50% in the lab.  The average panel is now rated at 450-500 W per panel.  This means that a relatively modest array of 10 panels can produce up to 4.5 – 5.0 kW of electricity, enough to run many electric appliances in a typical home.

Output is affected by several different factors.  Cloudy skies will yield less electricity than full sun.  On the other hand, summer heatwaves can heat rooftop panels to well over 40 deg.  Such heating causes a progressive loss of efficiency, even though the summer sun is much higher in the sky.  One very good piece of news is that prices have dropped substantially, from hundreds of thousands per panel in the 1950s, to £100 or slightly less today.  In addition, better panels have become much more available as R & D and manufacturing capacity have ramped up - from negligible two decades ago to 3,200 GWh per year (1 GWH = 1 million kWh) – with about 80% currently produced in China (also by far the world leader in installed solar, wind and battery capacity).  Panels are also very durable.  Output does degrade over time, typically by about 0.5-1.0% / year.  This means that the current panels are likely still to produce c. 85% of their rated power by the end of the typical warranty period at 25 years.  Older panels are just now starting to be replaced, and initially many of these wound up in landfills. More recently, a number of companies have appeared that provide full recycling capability, or even reconditioning of used panels.

Today’s Solar Batteries:  Intensive R & D has led to several different Li-ion battery formulations, containing various combinations of nickel, manganese, cobalt, aluminium, iron and phosphate.  Other approaches involve sodium-ion, or even solid state batteries without any electrolyte between anode and cathode.  Development is especially concerned with finding a balance between the electricity stored (measured in kWh), and battery weight (measured as maximal energy density in Wh/kg).  Weight is an especially important consideration in moving vehicles, and is a major determinant of their effective range, but is less consequential in solar batteries.  R & D also focusses on such practical issues as the need for cooling, and flammability.  Progressive increases in storage capacity have also been fuelled by a massive increase in use of batteries for storage of electricity.  Reductions in cost have been even more impressive than for solar panels, with battery prices falling by 97% over the past three decades and predicted to fall by 50% further between 2023 and 2024 (See Further Reading).  As with solar, performance of batteries also drops over time, but at 10 years should still achieve >75% of initial levels, even after many thousand charge: discharge cycles.

One disturbing note in the production of batteries is the evidence of labour abuses in the mining of cobalt, leading some battery companies to reduce or halt use of cobalt.  There are also concerns over supplies and environmental costs of mining lithium.  Total remaining reserves of the various metals needed for battery manufacture seem adequate for now, even if mining fairly and sustainably remains an issue.

Installation Considerations: The standard rule has always been to install solar arrays in South-facing locations, either on the ground, or more typically on a roof (Fig. 1).  The increasing efficiency of current panels now effectively allows placement in East- and West-facing locations.  This maximises microgeneration in mornings and evenings, although a combination of East and

Figure 1.  Typical solar Arrays, washed by the Rain

West arrays is desirable to capture midday light, if no South-facing panels are installed.  Those considering installation of solar can use free tools that will model potential solar generation in their geographic location and climate zone (See Further Reading - PVGIS).  In the case of pitched roofs, panels are installed either ‘on roof’ on racks fixed with screws, or ‘in roof’ in trays embedded in the roof.  For flat roof, where the use of screws risks leak in the roof, racks are secured with weights.  One other option is to use solar tiles.  These blend in better with the roof, but are also more expensive, slower to install and of lower efficiency.  As to the angle of installation from the horizontal, the rough rule is that of the local latitude, although there is some flexibility here.  On flat roofs, an angle of 12-14 deg is generally used; this minimises visibility of the panels from ground level, and promotes effective rinsing by rain.

The electricity generated by solar and stored by batteries is direct current (DC), while that used in the home and on the national grid is alternating current (AC). Converting between DC and AC is the job of an inverter.  Historically, DC current from the long string of individual cells running from panel to panel was eventually fed to a single inverter without any electrical modification.  The difficulty with this approach was that dirt, shading or electrical malfunction affecting any parts of this single string could interrupt the downstream circuit going to the inverter, and shut current flow down.  This was especially likely when individual panels are subject to wide differences in light, shading, orientation and separation of panels.

There are now several potential solutions for this.  The first already installed more recently in panels is bypass diodes that reroute current around shaded or damaged cells.  A second solution is to effectively reroute current by installing a micro-inverter on each panel, which also removes the need for the large central inverter otherwise required between the array and the house.  Another option is to install power optimisers on each panel.  These also allow output to be maintained by bypassing any malfunctioning parts of the array. They also have the advantage of providing real time panel-by-panel monitoring and diagnostics, remotely online.  The latter can indicate when one or more panels need to be inspected and/ or replaced.  Power optimisers are also somewhat cheaper than micro-inverters, although they still require a central inverter.  This sounds more complicated than it really is, and a very helpful video with relevant diagrams can be found in Further Reading (Solar Panel Shading (Part1 )).

Regardless of whether or not bypass diodes, micro-inverters, or power optimisers are installed, it is important to keep the panels clean, to ensure maximal microgeneration.  Our rainy clime in the UK does much of this job, especially since panels are rarely installed flat.  However, regular inspection, and manual washing with a mop, or gentle power washing, are easily done.

 

Figure 2. A 10 kWh Solar Battery in a Garage

A typical solar battery of say 10kWh, comprised of one or more modular units stacked outwards or upwards, is about the size of a chest-height bookcase – say 1200 H X 800 W X 250 d in mm.  Many are installed in garages, or sheds, or in utility rooms inside houses.  Some are even featured as objets d’art in plain sight in homes undergoing greening.  Ventilation used to be needed as standard, but more modern batteries may be able to be installed in a closed space, such as a cupboard.  In addition, batteries for use with solar are heavy (100-150 kg for a typical 0kWh battery), and so are most often installed on the floor (Fig. 2).

Inverters: In essence, this piece of kit is the brains of the solar/ battery system.  It converts the DC electricity of the array, to AC for use in the home and export to the grid.  It can also manage the flow of DC electricity in and out of the battery.  There are two basic configurations of inverter.  In so-called DC-coupled systems, a single ‘hybrid’ inverter handles output from the array, and management of the battery.  DC-coupled systems are straightforward to set up, and an example is shown below (Diagram 1).  The inverter can route solar electricity to the home, to or from the battery, and to or from the grid, all in the correct configuration (AC or DC), and all at the same time!

Diagram 1: DC-Coupled Inverter

The alternative is an AC-Coupled set-up, where the battery has a separate inverter coupled to the AC side of the system.  This setup is often used when a battery is added later to en existing solar set-up, and also can easily provide backup for designated circuit in the house, in case of a power cut.  One example is the Tesla Powerwall, is AC-coupled  The downside of this arrangement is the more tortuous route taken by solar-generated electricity passing through the battery.  This involves three DC-AC conversions, each of which involves energy loss, as against the single conversion in a DC-coupled system (Compare Diagram 1 and 2).  There is also the added cost of a second inverter.

Diagram 2: AC-Coupled Inverter

Some households install a solar diverter.  This is a gadget that can send spare solar electricity off to heat a hot water cylinder, although these seem to be falling out of favour.  Another convenient option is to be able to divert spare solar electricity to an EV charger.

It makes sense to match the kw size of the inverter with the theoretical maximum output of the array (number of panels X W/ panel); for example, a 3 kW array would then be matched with a 3kW inverter.  In fact, many solar installations now include oversizing.  This is the deliberate use of more panels - with more total kW output - than the kW rating of the inverter – say the use of a 3kW inverter with a 6 kW solar array.  There are several practical reasons for this.  For example, the kW output of the array will only be close to its theoretical maximum at midday in midsummer, and when all panels are fully functional.  At other times, output may be quite a bit less.  In addition, inverters tend to work best at or near their maximal kW rating, and some are happy at considerable levels of oversizing (100-200%). Power exceeding the maximum capacity of the inverter, and that would otherwise be lost, can also be sent straight to the battery, providing the system is DC-coupled (see Diagram 1).  Naturally, the smaller inverter used with oversizing will also be cheaper.

Another potential benefit of oversizing relates to current regulations that limit grid export from each household with single phase supply to less than 3.68 kW of electricity.  The simplest approach is to limit the array to < 3.68 kW  6kW and the inverter accordingly, and notify the local Distribution Network Operator (DNO – e.g., Scottish Power) with a G98 form.  However, if an array and inverter >3.68 kW are desired, a waiver application must be made to the DNO using a G99 form. Note, this approval may take some months to process, and may involve payments to the DNO for local modifications to the grid.  The DNO may approve or deny the request, or allow the proposed array providing an inverter of lower kW capacity is installed.  Oversizing allows yet another option – a Fasttrack G99 application for an oversized array >3.68 kW throttled with an inverter of 3.5 kW or less.  Note also that the limited number of homes with three phase electricity with may have yet another option.  This is to export 3 X 3.68 on each phase, or 11 kW in total, without needing a DNO waiver.  Regardless of oversizing or if a waiver is necessary, every array should have isolators to shut off electricity on both DC and AC sides of the system.  This prevents electric shock while electrical work in undertaken in the house, or on the grid during upgrades or power cuts.

Planning the size of battery to be installed ideally takes into consideration the average daily pattern of electricity use (kWh) in the house, so that an appropriate amount of energy is available to cover at least the high electricity demand periods in the day (6-10AM, and especially 4-7PM – see below), or power outages lasting up to say 6 hours.  Many households will start with a battery with energy capacity of around 7 – 10 kWh. A related criterion is how quickly power can be delivered from the battery when needed, especially if energy gourmands (e.g. heat pumps, power showers and EVs) are to be covered.  This ideally requires a battery power output of at least 3, and preferably 5 kW.  Happily, many batteries  come in modular form; with prices plummeting, additional modules can always be added later.

 

Variable tariffs:  A combination of solar and batteries sets up a very functional duet.  For example, around midday when solar production is at a maximum, surplus electricity can either be soaked up by the battery for use later in evening, or sold onto the grid.  The addition to this mix of a tariff that varies throughout the day makes an even more flexible menage à trois.  This is because solar batteries can now stock up with power from the grid at times when electricity is at its cheapest; this is often at times when the sun is not out, for example in late evening and at night.  Such stored power can then be used to run household electrical appliances, or even sold back to the grid when prices are high, typically between 4-7PM.  Equally, energy hogs such as EVs or driers, can be charged or run at night, when grid prices are low.  Furthermore, at some points when national supply substantially exceeds demand and available battery storage, energy companies are forced to dump electricity, and drop the price substantially, sometimes even below zero.  When this happens, users of such variable tariffs are actually being paid to use power from the grid!

 

Further Reading