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Slow Electricity: The Return of DC Power?

Directly coupling DC power sources with DC loads can result in a significantly cheaper and more sustainable solar system.

Picture: Brighton Electric Light Station, 1887. Stationary steam engines drive DC generators by means of leather belts. Source.

Picture: Brighton Electric Light Station, 1887. Stationary steam engines drive DC generators by means of leather belts. Source.

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In today’s solar photovoltaic systems, direct current power coming from solar panels is converted to alternating current power, making it compatible with a building’s electrical distribution. Because many modern devices operate internally on direct current (DC), alternating current (AC) electricity is then converted back to DC electricity by the adapter of each device.

This double energy conversion, which generates up to 30% of energy losses, can be eliminated if the building’s electrical distribution is converted to DC. Directly coupling DC power sources with DC loads can result in a significantly cheaper and more sustainable solar system. However, some important conditions need to be met in order to achieve this goal.

Electricity can be produced and distributed using alternating current or direct current. In the case of AC electricity, the current changes direction periodically, while the voltage reverses along with the current. In the case of DC electricity, the current flows in one direction and voltage remains constant. When electrical power transmission was introduced in the last quarter of the nineteenth century, AC and DC were competing to become the standard power distribution system — a period in history known as the “war of currents”.

AC won, mainly because of its higher efficiency when transported over long distances. Electric power (expressed in watt) equals current (expressed in ampère) multiplied by voltage (expressed in volt). Consequently, a given amount of power can be produced by a low voltage with a higher current or by a high voltage with a lower current. However, power loss due to resistance is proportional to the square of the current. Therefore, high voltages are the key to energy efficient power transmission over longer distances. 1

The invention of the AC transformer in the late 1800s made it possible to easily step up the voltage in order to carry power over long distances, and then step it back down again for local use. DC electricity, on the other hand, couldn’t be converted efficiently to high voltages until the 1960s. Consequently, it was impossible to transmit power effectively over long distances (>1-2 km).

Illustration: Brush Electric Company’s central power plant dynamos powered arc lamps for public lighting in New York. Beginning operation in December 1880 at 133 West Twenty-Fifth Street, it powered a 2-mile (3.2 km) long circuit. Source: Wikipedia Commons

Illustration: Brush Electric Company’s central power plant dynamos powered arc lamps for public lighting in New York. Beginning operation in December 1880 at 133 West Twenty-Fifth Street, it powered a 2-mile (3.2 km) long circuit. Source: Wikipedia Commons

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A DC power network implied the installation of relatively small power plants in every neighbourhood. This was not ideal because the efficiency of the steam engines that powered the dynamos depended on their size — the larger a steam engine, the more efficient it becomes. Furthermore, steam engines were noisy and produced air pollution, while the low transport efficiency of DC power excluded the use of more distant, clean hydro power sources.

More than a hundred years later, AC still constitutes the basis of our power infrastructure. Although high-voltage DC has been gaining ground for long-distance transportation, all electrical distribution in buildings is based on alternating current, either at 110V or 220V. Low voltage DC systems have survived in cars, trucks, motorhomes, caravans and boats, as well as in telecommunication offices, remote scientific stations, and emergency shelters. In most of these examples, devices are powered by batteries that operate on 12V, 24V or 48V DC.

Renewed Interest in DC Power

Recently, two converging factors have renewed interest in DC power distribution. First, we now have better alternatives for decentralized power generation, the most significant of these being solar PV panels. They don’t produce pollution and their efficiency is independent of their size. Because solar panels can be located right where energy demand is, long distance power transmission isn’t a requirement. Furthermore, solar panels “naturally” produce DC power, and so do chemical batteries, which are the most practical storage technology for PV systems.

Solar PV panels naturally produce DC power, and a growing share of our electric appliances operate internally on direct current

Secondly, a growing share of our electrical appliances operate internally on DC power. This is true for computers and all other electronic gadgets, as well as for solid state lighting (LEDs), flat screen televisions, stereo equipment, microwave ovens, and an increasing amount of devices operated on DC motors with variable speed operation (fans, pumps, compressors, and traction systems). Within the next 20 years, we could see as much as 50% of the total loads in households being made up of DC consumption. 2

DC Power plant of the Hippodrome in Paris. A steam engine runs multiple dynamos that power arc lamps. Source unknown.

DC Power plant of the Hippodrome in Paris. A steam engine runs multiple dynamos that power arc lamps. Source unknown.

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In a building that generates solar PV power but distributes it indoors over an AC electrical system, a double energy conversion is required. First, the DC power from the solar panel is converted to AC power using an inverter. Then, AC power is converted back to DC power by the adapters of DC-internal appliances like computers, LEDs and microwaves. These energy conversions imply power losses, which could be avoided if a solar powered building would be equipped with DC distribution. In other words, a DC electrical system could make a solar PV system more energy efficient.

More Solar Power for Less Money

Because the operational energy use and costs of a solar PV system are nil, a higher energy efficiency translates into lower capital costs, as fewer solar panels are needed to generate a given amount of electricity. Furthermore, there is no need to install an inverter, which is a costly device that has to be replaced at least once during the life of a solar PV system. Lower capital costs also imply lower embodied energy: if fewer solar panels and no inverter are required, it takes less energy to produce the solar PV installation, which is crucial to improve the sustainability of the technology.

Fewer solar panels are needed to generate a given amount of electricity

A similar advantage would apply to electrical devices. In a building with DC power distribution, DC-internal electric devices can do away with all the components that are necessary for AC to DC conversion. This would make them simpler, cheaper, more reliable, and less energy-intensive to produce. The AC/DC adapters (which can be housed in an external power supply or in the device itself) are often the life-limiting component of DC-internal devices, and they are quite substantial in size. 2

Image: Power driver for a 35W LED lamp. [^3] All parts that are necessary for AC to DC conversion are marked.

Image: Power driver for a 35W LED lamp. 3 All parts that are necessary for AC to DC conversion are marked.

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For example, for an LED light, approximately 40% of the printed circuit board is occupied by components necessary for AC to DC conversion. 3 AC/DC adapters have more disadvantages. As a result of a dubious commercial strategy, they are usually specific to a device, resulting in a waste of resources, money, and space. Furthermore, an adapter continues to use energy when the device is not operating, and even when the device is not connected to it.

DC power distribution would make devices simpler, cheaper, more reliable, and less energy-intensive to produce

Last but not least, low-voltage DC grids (up to 24V) are considered safe from shock, which allows electricians to install relatively simple wiring, without grounding or metal junction boxes, and without protection against direct contact. 456 This further increases cost savings, and it allows you to install a solar system all by yourself. We demonstrate such a DIY system in the next article, where we also explain how to obtain DC appliances or convert AC devices to DC.

How Much Energy Can Be Saved?

It’s important to note, however, that the energy efficiency advantage of a DC grid is not a given. Energy savings can be significant, but they can also be very small or even turn negative. Whether or not DC is a good choice, depends mainly on five factors: the specific conversion losses in the AC/DC-adapters of all devices, the timing of the “load” (the energy use), the availability of electric storage, the length of the distribution cables, and the power use of the electrical appliances.

Eliminating the inverter results in quite predictable energy savings. It concerns only one device with a rather fixed efficiency (+90% — although efficiency can plummet to about 50% at low load). However, the same cannot be said of AC/DC-adapters. Not only are there as many adapters as there are DC-internal devices, but their conversion efficiencies also vary wildly, from less than 50% for low power devices to more than 90% for high power devices. 678

AC power adapters.

AC power adapters.

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Consequently, the total energy loss of AC/DC-adapters can be very different depending on what kind of appliances are used in a building — and how they are used. Just like inverters, adapters waste relatively more energy when little power is used, for instance in standby or low power modes. 8

The conversion losses in adapters are highest for DVDs/VCRs (31%), home audio (21%), personal computers and related equipment (20%), rechargeable electronics (20%), lighting (18%) and televisions (15%). The electricity losses are lower (10-13%) for more mundane appliances like ceiling fans, coffee makers, dishwashers, electric toasters, space heaters, microwave ovens, refrigerators, and so on. 8.

Lighting and computers (which have high AC/DC-losses) usually make up a great share of total electricity use in offices, shops and institutional buildings. Households have more diverse appliances, including devices with lower AC/DC-losses. Consequently, a DC system brings higher energy savings in offices than in residential buildings.

The largest advantage is in data centers, where computers are the main load. Some data centers have already switched to DC systems, even if they’re not powered by solar energy. Because a large adapter is more efficient than a multitude of small adapters, converting AC to DC at a local level (using a bulk rectifier) rather than at the individual servers, can bring energy savings between 5 and 30%. 691011

The Importance of Energy Storage

If we assume an energy loss of 10% in the inverter and an average loss of 15% for all the AC/DC adapters, we would expect energy savings of about 25% when switching to DC distribution in a solar PV powered building. However, such a significant saving isn’t guaranteed. To start with, most solar powered buildings are grid-connected. They don’t store solar power in on-site batteries, but rely on the grid to deal with surpluses and shortages.

In a net-metered solar powered building, only loads coincident with solar PV output can benefit from a DC grid

This means that excess solar power needs to be converted from DC to AC in order to send it to the electric grid, while power taken from the grid needs to be converted from AC to DC in order to be compatible with the electrical distribution system of the building. Consequently, in a net-metered solar PV powered building, only loads coincident with solar PV output can benefit from a DC grid.

Early DC power stations had a dynamo for every light bulb. Source unknown.

Early DC power stations had a dynamo for every light bulb. Source unknown.

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Once again, this means that the efficiency advantages of a DC system are usually larger in commercial buildings, where most electricity use coincides with the DC output from the solar system. In residential buildings, on the other hand, energy use often peaks in mornings and evenings, when little or no solar power is available.

Consequently, there is only a small advantage to obtain from a DC system in a net-metered residential building, as most electricity will be converted to or from AC anyway. A recent study calculated that a DC system could improve the energy efficiency of a solar-powered, net-metered American home on average by only 5% — the figure is an average for 14 houses across the USA. 1213

Off-Grid Solar Systems

To realize the full potential of a DC grid, especially when it concerns a residential building, we need to store solar energy in on-site batteries. In this way, the system can store and use power in DC form. Energy storage can happen in an off-grid system, which is fully independent of the grid, but adding some battery storage to a net-metered building also improves the advantage of a DC system. However, energy storage adds another type of energy loss: the charging and discharging losses of the batteries. The round-trip efficiency for lead-acid batteries is 70-80%, while for lithium-ion it’s about 90%.

Unfortunately, energy storage adds another type of energy loss — the charging and discharging losses of the batteries — and negates the cost advantages of a DC system

Exactly how much energy can be saved with on-site battery storage again depends on the timing of the load. Electricity used during the day — when the batteries are full — doesn’t involve any battery charging and discharging losses. In that case, the energy savings of a DC system can be 25% (10% for eliminating the inverter and 15% for eliminating the adapters).

However, electricity used after sunset lowers the energy savings to 15% for lithium-ion batteries and between -5% and +5% for lead-acid batteries. In reality, electricity will probably be used both before and after sunset, so that efficiency improvements will be somewhere between those extremes (-5% to 25% for lead-acid, and 15-25% for lithium-ion).

Kensington Court Station: steam engine, dynamo and batteries. Source: Central-Station Electric Lighting, Killingworth Hedges, 1888.

Kensington Court Station: steam engine, dynamo and batteries. Source: Central-Station Electric Lighting, Killingworth Hedges, 1888.

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On the other hand, battery storage brings an additional advantage: there are less or — in a totally independent system — no additional energy losses for the long-distance transmission and distribution of AC electricity. These losses vary a lot depending on the location. For example, average transmission losses are only 4% in Germany and the Netherlands, but 6% in the US and China, and between 15 and 20% in Turkey and India. 1415

If we add another 7% of energy savings due to avoided transmission losses, an off-grid DC system can bring energy savings between 2% and 32% for lead-acid batteries, and between 22% and 32% for lithium-ion batteries, depending on the timing of the load.

In an off-grid DC system, electricity use can be met with a solar system that’s one-fifth to one-third smaller, depending on the type of batteries used.

Assuming 50% energy use during the day and 50% energy use during the night, we arrive at a gain of 17% for an off-grid system using lead-acid batteries, and 27% for lithium-ion storage. This means that electricity use can be met with a solar system that is one-fifth to one-third smaller, respectively. Total cost savings will remain a bit larger, because we still don’t need an inverter, and installation costs are lower or non-existent.

Unfortunately, introducing on-site electricity storage raises capital costs again, because we need to invest in batteries. This will negate the cost advantage we obtained in choosing a DC system. The same goes for the energy invested in the production process: an off-grid DC system requires less energy for the manufacturing of solar panels, but it instigates at least as much energy use for the manufacturing of batteries

However, we should compare apples to apples: a DC off-grid solar system is cheaper and more energy efficient than a AC off-grid system, and that’s what counts. The life cycle analyses of net-metered solar systems do not represent reality, because they ignore an essential component of solar energy systems.

Cable losses

There’s one more important thing to consider, though. As we have seen, power loss due to resistance is proportional to the square of the current. Consequently, low-voltage DC grids have relatively high cable losses within the building. There are two ways in which cable losses can make a choice for a DC system counterproductive. The first is the use of high power devices, and the second is the use of very long cables.

Voltage regulation in early power plant. Source unknown.

Voltage regulation in early power plant. Source unknown.

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The energy loss in the cables equals the square of the current (in ampère), multiplied by the resistance (in ohm). The resistance is determined by the length, the diameter, and the conducting material of the cables. A copper wire with a cross section of 10 mm2, distributing 100 watts of power at 12 V (8.33 A) over a distance of 10 metres yields an acceptable energy loss of 3%. However, with a cable length of 50 metres, energy loss becomes 16%, and at a length of 100 metres, the energy loss adds up to 32% — enough to negate the efficiency advantages of a DC grid even in the most optimistic scenario.

The relatively high energy losses in the cables limit the use of high power appliances

The relatively high cable losses also limit the use of high power appliances. If you want to run a 1,000 watt microwave on a 12V DC grid, the energy losses add up to 16% with a cable length of only 1 metre, and jump to 47% with a cable length of 3 metres.

Obviously, a low-voltage DC grid is not suited to power devices such as washing machines, dish washers, vacuum cleaners, electric cookers, electric ovens, or warm water boilers. Note that power use and not energy use is important in this regard. Energy use equals power use multiplied by time. A refrigerator uses much more energy than a microwave, because it’s on 24 hours per day, but its power use can be small enough to be operated on a DC grid.

Cable losses also limit the combined power use of low power devices. If we assume a 12V cable distribution length of 12 metres, and we want to keep cable losses below 10%, then the combined power use of all appliances is limited to about 150 watts (8.5% cable loss). For example, this allows the simultaneous use of two laptops (20 watts of power each), a DC refrigerator (45 watts), and five 8 watt LED-lamps (40 watts in total), which leaves another 25 watts of power for a couple of smaller devices.

How to Limit Cable Losses

There are several ways to get around the distribution losses of a low-voltage DC system. If it concerns a new building, its spatial layout could significantly limit the distribution cable length. For example, Dutch researchers managed to reduce total cable length in a house down from 40 metres to 12 metres. They did this by moving the kitchen and the living room (where most electricity is used) to the first floor, just below the roof (where the solar panels are), while moving the bedrooms to the ground floor. They also clustered most appliances in the central part of the building, right below the solar panels (see the illustration below). 16

Illustration: Concept for a DC low voltage house. Source: [^16]

Illustration: Concept for a DC low voltage house. Source: 16

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Another way to reduce cable losses is to set up several independent solar systems per one or two rooms. This might be the only way to solve the issue in a larger, existing building that’s designed without a DC system in mind. While this strategy implies the use of extra solar charge controllers, it can greatly reduce the cable losses. This approach also allows the power use of all appliances to surpass 150 watts.

Setting up independent solar systems per one or two rooms is one way to limit cables losses and increase total power use

A third way to limit cable losses is to choose a higher voltage: 24 or 48V instead of 12V. Because the energy losses increase with the square of the current, doubling the voltage from 12 to 24V makes cable losses 4 times smaller, and switching to 48V decreases them by a factor of sixteen. This approach also allows the use of higher power devices and increases the total power that can be used by a DC system. However, higher voltages also have some disadvantages.

First, most low-voltage DC appliances currently on the market operate on 12V, so that the use of a 24 or 48V DC network involves the use of more DC/DC-adapters, which step down the voltage and also have conversion losses. Second, higher voltages (above 24V) eliminate the safety advantages of a DC system. In data centers and offices, as well as in the American residential buildings in the study mentioned earlier, DC electricity is distributed throughout the building at 380V, but this requires just as stringent safety measures as with 110V or 220V AC electricity. 17

Slow Electricity

Shortening cable length or doubling the voltage to 24V still doesn’t allow for the use of high power devices like a microwave or a washing machine. There are two ways to solve this issue. The first is to install a hybrid AC/DC-system. In this case, a DC grid is set up for low power devices, such as LED-lights (10 watt), laptops (20 watt), a television (30-90 watt) and a refrigerator (50 watt), while a separate AC grid is set up for high power devices. This is the approach for homes and small offices that’s promoted by the EMerge Alliance, a consortium of manufacturers of DC products, which devised a standard for a 24V DC / 110-220V AC hybrid system. 18

Late 19th century, the only electric load in households was lighting.

Late 19th century, the only electric load in households was lighting.

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Low power devices are (on average) responsible for 35-50% of total electricity use in a home. Even in the best-case-scenario (50% of the load), a hybrid system halves the energy efficiency gains we calculated above, which leaves us with an energy savings of only 8.5% to 13.5%, depending on the types of batteries used. These figures will be lower still due to cable losses. In short, a hybrid AC/DC system brings rather small energy savings, that could easily be erased by rebound effects.

The second way to solve the problem of high power devices is simply not to use them. This is the approach that’s followed in sailboats, motorhomes and caravans, where a supporting AC distribution system is simply not an option. This is the most sustainable solution to the limits of DC power, because in this case the choice for DC also results in a reduction of energy demand. Total energy savings could thus become much larger than the 17-27% calculated above, and then we finally have a radically better solution that could make a difference.

One way to solve the problem of high power devices is simply not to use them — this is the approach that’s followed in sailboats, motorhomes and caravans

Obviously, this strategy implies a change in our way of life. It would mean that electricity is used only for lighting, electronics and refrigeration, while non-electric alternatives are chosen for all other appliances. Not coincidentally, this is quite similar to how DC grids were operated in the late nineteenth century, when the only electric load was for lighting — first arc lamps and later incandescent bulbs.

Thus, no dishwasher, but doing the dishes by hand. No washing machine, but doing the laundry in a laundromat or with a manually operated machine. No tumble dryer, but a clothes line. No convenient and time-saving kitchen appliances like electric kettles, microwaves and coffee machines, but a traditional cooking stove operated by (bio)gas, a solar cooker, or a rocket stove. No vacuum cleaner, but a broom and a carpet-beater. No freezer, but fresh ingredients. No electric warm water boiler, but a solar boiler and a small wash at the sink if the sun doesn’t shine. No electric car, but a bicycle.

To figure out what’s possible, we’re converting Low-tech Magazine’s headquarters into an off-grid 12V DC system — more about that in the next post.

Comments

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Reactions
Volker

Another idea on how to reduce cable loss: distribute batteries to the high-power peak-load appliances and only distribute a low(er)-current charging current to those.

You probably use the 600W microwave a good dozen times a day for few minutes. Let’s guess one hour of daily operation in total.

In peak you need 600W = 50A for a 12V system. Thus one hour @600W = 600Wh = 50Ah for a 12V system. Both is met by a typical small car battery.

To tricle-load this over a day a current of (a tad more than) 2A is enough - and easy to handle over common cable diameters.

Distribution electr(on)ics for such a decentralized can be quite simplistic, too: a germanium diode towards the decentral battery, a silicium diode for current going back to the “12"V charging grid grid. This way you’ll have a 0.3V hysteresis preference for the local battery before all decentralized ones kick in and redistribute power to the depleted one. But a proper battery management electronics for each local battery probably is an even better solution…

Thom Westergren

That should be “Fewer solar panels . . .” Not “less” solar panels.

You’ll look a lot smarter if you get your grammar right. Especially in your call outs.

Rufus Reno

It is continually refreshing to read thoroughly thoughtful articles. Refreshing to see the comprehensive scope. Nothing is particularly straightforward…

What about using a hydraulic accumulator as a “battery”?

Linda Foss

I’m so glad you wrote this & included the home design. I live in hurricane country. Hurricane Ike left me without power for 11 days. & here, it’s usually very hot afterward. Luckily we got a cool front after a couple days.

Grid tied systems with micro-inverters have to detect power on the line or they shut down to keep from putting power on the lines & hurting workers. So you have no benefit from your solar in an outage. I know there are ways around this, but not approved by our electricity providers/municipalities.

So, since observing how my new flat screen TV is DC & all the audio equipment I want is USB, I’ve had the idea of having a grid-tied array for some AC & another for DC with storage to power some essentials/desirable like some lights, electronics, fans. (The fan is the most important thing!!!) & DC powered kitchen appliances depending on how reasonably they can be obtained. And i figured most of the DC power/wiring would be in some central location ie. the open kitchen which would share a wall with at least one bath.

And speaking of DC appliances & alternatives, I’ve been collecting examples. https://www.pinterest.com/betterways/better-appliances-alternative-ways/ There are solar thermal boosted mini split ACs for example. And DC brushless? motored fans are more efficient than AC which use as much power as the incandescent lightbulbs I no longer use. Cool cupboards can be used to store many foods so you have a smaller more efficient fridge. And the fridge can be kept closed at night. There’s also solar thermal heat & hot water. Waste heat from the fridge or perhaps a dehumidifier.

I’d like to know who “we” is when you say in the next article “we” will…. I always thought this was a one-man operation.

Robin Wilson

My wife and I installed a grid tied solar PV system five years ago and our neighbors have a seven year old off grid PV system (see www.spectrumz.com ). Our neighbors with only 4 panel off grid do as your article suggests using DC power for most of their system. They make little boxes for making sure the DV voltage fits with the device which obviously requires seperate AC DC wiring. We have a micro grid so we can share power during low sun winter days or for power outages.

I disagree with your idea that off grid is cheaper. You didn’t mention the price of the charge controller and batteries. In our grid tied system since our house is happily located in the woods so we had to run a line between our house and our PV panels 230 feet away in a clearing. As you point out high voltage loses less electricity so with the Enphase Microinverters, with a 25 year warentee on each solar panel we could send 220 V AC as opposed to 48 V DC to the house.

Off grid systems have no where to put the extra solar power in the summer so your panels and not fully utilized. Another advantage is grid tied systems qualify for SREC’s which in West Virginia is only $20/MWh (coal lobby!) but with our total production to date is 23 MWh it helps. Enphase also gives online updates of your PV power production which I enjoy.

Off grid systems definitely encourage people to be more frugal in their electric consumption. Our neighbors use 2 to 3 KWh/day and we use 3 to 4 KWh/day. We share our laundry washer. We both like solar dried cloths hung on the line.

KJ

We’ve been using 48V DC in the Telecommunications and Data Center world for decades and we’ve pretty much addressed these issues. That ranges from the milliamps of 48V that power-over-ethernet uses to run your office VoIP phone to supplying high amperage to things like motors. You should check out our world before you let some newcomer vendor lock you into a proprietary voltage technology. There are tens of thousands of vendors in the 48v ecosystem who all basically inter-operate.

Don

Excellent article. I can see ‘possible’ benefits if able to design from scratch, but if retrofitting not economic to replace an AC device with a DC one just for this. MY question is, how could you convert/bypass AC input to an existing device and connect directly to the DC side (safely)?

Nick Laing

This was a great article and I donated because it gave me an idea.

Thanks ‘Low tech Magazine’.

Craig

Going from 12V to 48V would decrease loss by a factor of 16 (e.g. 4 squared), not by a factor of 9.

Sherwood Botsford

While there is some merit with this, the solution is not that clear cut:

My computer uses 5v and 12v internally. I have a digital picture frame that uses 9v. My iphone actually uses 3.3v but can be charged from a 5v USB port by throwing away 1/3 of the energy.

My irrigation system uses 18v. The backup sump pump battery charger uses 14v. My camera battery charger uses 4.5v

14 ga wiring carries 15A safely. The voltage doesn’t matter. But a 12v line at 15A is only 180 watts.

what DC voltage to use? 48v has merit as having enough power to run stoves and air conditioners and microwaves – even electric water heaters.

Lots of electronics could run on 3, 5, 12.

One of the issues with electronics is that they keep changing. Today’s 5v device is tomorrow’s antique.

Power supplies are getting smarter and more efficient.

To me it seems to make more sense to go after the big loads for DC. If you could power the fridge, air conditioner and water heater by DC, and use high efficiency converters for the leftovers it would make more sense.

kris de decker

@ super390

You should read my articles more carefully. My 2010 article was not “an attack on EVs” but an attack on a certain type of EV – a vehicle that is as fast, heavy, and “smart” as today’s gasoline powered cars.

http://www.lowtechmagazine.com/2010/05/the-status-quo-of-electric-cars-better-batteries-same-range.html

See also the follow-up article on electric velomobiles:

http://www.lowtechmagazine.com/2012/10/electric-velomobiles-as-fast-and-comfortable-as-automobiles-but-80-times-more-efficient.html

I’m making the same argument for cars as I do for home electricity use or the internet: we can keep most of the technology if we adjust it to limited energy sources.

Rasmus

I would like to see an experimental ecovillage where all electricity is from a 12V DC microgrid. Cooking and baking is done with biomass (gasification/pyrolysis for clean burn, rather than combustion). Freezing is replaced with canning, fermentation and also food drying using superheated steam (see page I wrote at: http://opensourceecology.org/wiki/Food_Drying_with_Superheated_Steam ). A washing machine could also be biomass-powered for the heating part of it, with the mechanical part from another source.

Matthias

@ Boris Doderer

As you do not need to attend the laundry while in the machine the longer time do not need to be a problem. Run the machine every two days while you are working. For off grid systems low power and low energy consumption are the way to go. As there is less stress for the batteries.

The laundry machine could run during the day and consume the power generated by solar panels without storing in a battery first. In a system with a gas or diesel generator it might be better to use an older and faster washing machine and reduce the working hours of the generator as its efficiency is low in partial load.

With a heat pump system for heating a house, a kilowatt of electricity can get you many kilowatts of heat. And heat pumps can be used for cooling in summer too. There are systems which use the heat of the ground in winter to heat the house when there is not enough solar thermal energy. In summer excess heat from the solar thermal and from cooling the building is stored in the ground. In fact the underground is a seasonal heat storage.

Herman Vanmunster

Hi Boris,

Maybe we are a bit drifting off topic here, but we are both talking about different things. Your post applies to ‘pass-through’ heaters where cold water is heated instantly and immediatly used. In that case it is important to heat as fast as possible and as close as possible to where the heat is needed. You are also right about induction cooking. The faster the water is heated, the less losses there are because the cooker is not thermically isolated. So there is less time to lose heat.

But I am talking about a boiler where water is stored for several hours before being used. The resistor and its surrounding layers are placed in the center of the boiler. They are completely surrounded by water. So any ’loss’ would eventually still leak to the water, where we want it. The heat has no other way to dissipate to than to the water.

If you drive a cavitation pump with electricity, then the losses of the motor are defintily lost because the motor is placed outside the boiler.

The situation is still different is you drive the pump directly from a locally available form of mechanical (kinetic) energy. In that case the cavitation pump will be by far a much more ecological choise than electricity. That is because electricity is still for the biggest part produced by converting heat into motion (in fossil fuel based power plants and in nuclear power plants). In this convertion 50% of the available energy is lost at the moment of production.

Viktor

Hi, Herman,

You might be right. To be honest i’ve only seen youtube videos about these water heaters, never an actual one in real life, but the vids seemed quite impressive. Not an expert on these kind of things either, and i’m planning to cover all my heat-related energy needs from biomass that is readyly available from the land my family owns, and the house would still be on grid, as it makes more sense in my area to sell back solar energy to the company with a simple two-way meter, than spending loads of money on (partially) rewireing the house and spending more on batteries, charge controllers and other whatnots.

Amaru

Speaking of static load storage/ water accumulators, this interesting idea, which is getting a pilot in Nevada, showed up on hackernews yesterday:

http://www.aresnorthamerica.com/

http://revolution-green.com/ares/

Essentially, they are investigating using a Roll-Under-Roll-Off cart system to store/retrieve energy by moving dense weights up and down a hill, with a pilot project in Nevada. When power is generating, the electrified carts pick up dense weights and move them up hill. When power needs to be retrieved, carts pick up weights and move them downhill. They are reporting 80% efficiency, so not quite as efficient as hydro=accumulators, but seems like it has a lot of potential for arid areas near elevation grades,

I also wonder if it would be feasible to combine with the rope transport concept which was discussed earlier in this blog, although perhaps nothing similar to the Roll-Under-Roll-On concept exists for rope transport due to the lowered interest since the 40s.

Paul Holden

Australian ABC program on battery powered homes

http://www.abc.net.au/catalyst/stories/4398364.htm

Tim

“Thus, no dishwasher, but doing the dishes by hand. No washing machine, but doing the laundry in a laundromat or with a manually operated machine. No tumble dryer, but a clothes line. No convenient and time-saving kitchen appliances like electric kettles, microwaves and coffee machines, but a traditional cooking stove operated by (bio)gas, a solar cooker, or a rocket stove. No vacuum cleaner, but a broom and a carpet-beater. No freezer, but fresh ingredients. No electric warm water boiler, but a solar boiler and a small wash at the sink if the sun doesn’t shine. No electric car, but a bicycle.”

You might as well admit solar power is a dismal failure and will never compete with other forms of power. Oh and I’m not living the pre-Industrial Age lifestyle to appease you environmentalist wackjobs.

Tim Martin

Guys

Noble idea, but sounds like a nightmare to execute. Carve up the walls to bring in new wires? Some electronic devices operate on several dc voltages. I have a video projector that requires 5v, 15v, 24v, 85v all plus and minus, and plus 390vdc. One would have to be a very skilled technician to enable his existing ac powered devices to input one or several dc voltages. Warranties would be voided. Repair shops would find modified devices to be difficult or impossible to service.

Good luck

Boris Doderer

@ Herman:

I agree. Nothing to add.

@ Rasmus:

This might still be off-topic, but in slight alteration of your wish:

Have you ever seen the videos about the solar test village in Tamera (combined use of solar radiation for solarthermics and a greenhouse, heat transport and storage by vegetable oil, mechanic and electrical energy produced by low temperature difference stirling engines)? The way they cook there is also very interesting.

Look for “solar village Tamera” and “sunpulse” at Youtube

X Darcan

I’ve been thinking that if neighbors would share their PV power and battery capacities the need to design systems for peak usage would be greatly reduced. The major problem that comes to mind is the cabling.

Even only 250 W PV panel and perhaps a 100 Ah battery per person would be enough, imagine each household had one of these packages for each individual and a thick DC cable or perhaps even a solid piece of metal would go from house to house connecting all the neighbors batteries and pv panels. If cables were not so wasteful, each person in a neighborhood would have access to all the capacities of panels and batteries in the neighborhood.

In a neighborhood of 100 individuals each would have access to 100x250=25 kw PV panels and 10 000 Ah battery capacity. If during the day the peak usage would be greatly supplied by the panels, and since the use of high power devices are mostly random during the day and ON for a short while, the design needs would be lowered. Disregarding devices that are continuously during the day of course.

Another idea I have is to make the PV panel makers to include a small simple voltage limiter that would prevent over charging of the batteries, perhaps adjustable or if the market could decide on a voltage that suits most type of batteries.

This would remove the need for a charge controller I think based on what I know. As said, DC 12/24 systems would also remove the need for an inverter

The charge controllers with high amps and inverters above 3kw are ridiculusly expensive and the prices increase in huge steps after 3kw. The PV panels are almost the cheapest components in the systems it seems and they have very long lifespans. On second glance it is more likely the other components that prevent people from buying them and not PV panels.

You may disregard the 100 Ah example for batteries per person, consider a higher number, yet if shared with neighbors the need to design individual systems for individual peak usage is eliminated.

Bruce MacLean

This is on the market and seems an interesting direction. Not cheap

http://www.hotspotenergy.com/solar-air-conditioner/

DC high efficiency refrigerator/freezers are also available from these folks:

http://www.sunfrost.com/RF19_efficient_ac_dc_refrigerator.html

Dairokkan

I would like to add some informations to this article.

  1. It was possible to use HVDC back then, but efficiency was about 60-70% of conversion. in 1882 there was 2kV HVDC in Germany, In 1903 was local railway Tábor-Bechyně at Austria-Hungary electrified with voltage 1400-1500V DC. Between 1890-1910 many systems emerged with voltage between 6-60kV, usually under 4.5MW. Beside steam there was no powerful source of electricity back then.

  2. Efficiency of power production in DC plant of 430kW was about 95-98%

  3. Microwave oven require few kV of AC that is rectified and doubled. This power is around 1kW! Nothing suitable for low voltage DC.

  4. Almost everything today with motor is “DC Ready” as universal motor that is in almost all 1 phase devices could be operated on AC or DC.

  5. you have wrongly calculated looses in one part, it’s not 10% + 15%, but 1 - (1 - 0.15)*(1 - 0.1), giving us 23.5% (Not much of difference, but more precise, in case of larger looses it could make huge difference.

——–MY OPINION——–

Any low voltage system is nonsense in case of buildings, as well each electronic device require different voltage, sometimes even few voltage levels, symmetric voltage… you will not eliminate all inverters. Sometimes this voltage change is done by dissipating power in regulator (78xx and 79xx integrated circuits), sometimes it require inverter in device.

Despite fact that up to 24V you don’t have to have insulation etc., what you will save on plastics you will spend on copper. And there you go with main problem, choosing voltage, mobile phones tablets etc. - 5.5V. routers, normal things - 9-12V (due low power it could often be reduced by (7909, 7809)), tools - 12-24V. On other hand you have looses that require higher voltage to be eliminated.

With higher voltage you will have problems as DC burns everything it touches with sparks. Reserving DC only to low power devices will not help much as power generated by PVE and not stored or used would be lost, grid connected PVE, under certain conditions, could sell power to grid and reduce production of other power plants.

Fire could be started by any grid if conductors become too hot and are located near flammable things. As heat is generated by current, low voltage with higher current could start fire more easily, but for sparks it could be true. In case of let’s say 24V system you would need 120A or more for peak load.

Your proposal is interesting, but I don’t think it will be useful as you either would have to go for higher DC voltage of 48-100V in one “leg,” or do double wiring (limits are 120W max. for 12V, 240W max. for 24V, 480W for 48V (multiply by 2 for two legs)), in houses of common people, will not happen. And even after that you will have to adjust voltage for many device.

———REPLIES———-

TO: KJ

Yes, going for some standard that is here is good idea, 48V seems as good compromise, if used as - 0 + we could get 960W with 10A. If it is compatible with PoE, it’s even better.

TO: Sherwood Botsford

I agree. Those high power (and where is border? 5W 10W 24W?) devices would cause higher looses and often run independently on owner (fridge, air condition, heating) small devices might not be as important. But it’s question whether use 48V in one leg or 2x24V thus 48V over both. If it would be 48V in one leg then we could go directly to 100V DC. But in light of what was stated by KJ it seems like anything not compatible with PoE

super390

So you support home battery storage. And now Tesla and Mercedes, two automakers, are making them. But you oppose car ownership. The logic of automakers building storage systems is that eventually they will have to replace batteries in their cars because they will be down to 70% original capacity (although no Tesla batteries have reached that point yet), yet those can still be used for many other things. Like home battery storage systems. Your worst-case 2010 attack on EVs was based on purely coal-generated charging, which is rapidly becoming obsolete in parts of the world where EV demand is highest (which you failed to consider), and the embodied energy in producing batteries that you expected would be disposed of. So since you’re already requiring a government powerful enough to outlaw automobiles, why not instead use that government to make people buy home solar and battery storage systems, and let them buy the EVs they want and thereby create the national supply of salvaged batteries that they can charge using their own home systems? It’s not like the cost of new lithium ion batteries was falling over 10% per year. Oh wait…

DavidP

You’d be safe with up to 300 V DC wiring without increasing safety measures, I’d bet. As this engineer demonstrates [1], it’s much harder to shock someone with DC than it is with AC. That’s because of the capacitance of the skin tends to block DC (but not AC).

Great analysis by the way. Really thorough.

[1] https://www.youtube.com/watch?v=snk3C4m44SY

JohnMc

Success in a DC centric power system will not focus on production, but consumption. As one commenter has already observed the Telco industry has solved many of the issues of using 48v as a standard. Therein lies the crux, the mfrs of consuming devices (phones, appliances, computers, etc) do not conform to a standard power input paradigm. Its why wall warts abound even in AC systems, consuming devices requiring different voltage outputs.

Till standards are developed on the consuming side, a DC powered system would still need converters to meet the needs of the myraid of voltage demands of mfrs.

Good article otherwise.

Jim Pawley

First of all thank you for all the work. I really enjoyed this article on Slow Electricity. It is a topic I often include in my lectures on Responding to GW.

However, I was puzzled by this paragraph.

The energy loss in the cables equals the square of the current (in ampère), multiplied by the resistance (in ohm). The resistance is determined by the length, the diameter, and the conducting material of the cables. A copper wire with a cross section of 10 mm2, distributing 100 watts of power at 12 V (8.33 A) over a distance of 10 metres yields an acceptable energy loss of 3%. However, with a cable length of 50 metres, energy loss becomes 16%, and at a length of 100 metres, the energy loss adds up to 32% – enough to negate the efficiency advantages of a DC grid even in the most optimistic scenario.

As I see it, US AWG #7 copper wire is close to 10mm2 in cross section and the resistance as listed at

http://www.daycounter.com/Calculators/AWG.phtml

and the resistance is .00163 ohms/m, (8.33)*2 x 10 x .00163 = about 1.1watt, or a bit more than 1% of than 1%, not the 3% you state. Of course, #7 wire is pretty thick and hard to work with (not to mention that it includes a LOT of copper that is very energy intensive to mine and purify), so perhaps you meant something close to AWG #12 this is 2.05 mm in diameter and 3.31 mm2 in area (and squared this would be about 10?) and the resistance is now .00521 ohms/m. This leads to about a 3.6watt loss or a 3.6% loss on a 100w load.

Personally, I see the the utility of a parallel battery buffered 12v or 24v DC network to run the low-power digital electronics and perhaps lighting and only available in a few rooms. Most of the PV power would still be inverted to AC for net “storage” and running high power equipment through wall sockets (Inverter efficiency continues to improve as does that of DC switching power supplies).

I don’t’ see ladies giving up their 1650w hairdryers, clothes dryers are often 6kw, and microwaves work at high voltages (though this may change as magnetrons are replaced by GHz power transistors.)

Keep up the good work.

Wilbur

Hackaday has featured a few things of this ilk recently too..

http://hackaday.com/2016/12/21/so-wheres-my-low-voltage-dc-wall-socket/

the comments aren’t bad either.

and they link also to this project

https://www.altpwr.net

Kolya Ivankov

Thanks for this really mind-provoking article. Let me add a personal comment.

I am an AI engineer. Since 2016 I’ve been working in rationalizing animal husbandry, and there I’ve learned pretty too well that high-tech is not always better. Just because one uses neural networks, it doesn’t mean that the old technology won’t perform better. Right now I am switching to the AI use in energy, primarily in wind turbines.

My overall opinion is that one has to switch to low-tech as soon as one can. That’s precisely why I am reading and supporting your blog. My goal as a scientist is to figure out how AI may suggest low-tech algorithms and appliances there, where it could be done, not necessarily relying on a too sophisticated machinery. After all, I have a low-tech background as a mathematician, and in my math studies I’ve been doing “downshifting” theories - making theories with less assumptions.

That said, I am an immigrant, and, being quite underpaid in comparison to the people with my education and of my profession (people in agriculture were slow to understand the rules of software development, it was a reason for change), me and my wife had to live as close to off-grid as it gets.

We use public transportation only, despite of the fact that we need about two hours to go in one direction. A good part of the distance is covered by us on foot - if fact, I am walking about 12km every day. We almost never use the fridge, plugging it out altogether at the cooler times of the year and eating fresh. We don’t use dishwasher, having just enough cups and plates for us two and washing them with hands. We don’t have washing machine, instead I go to laundry every 3 weeks or so, after accumulating a large backpack of clothes to wash. In our flat, we’ve been using local warmth appliances to heat ourselves.We don’t have a coffee machine whatsoever. We did use microwave, because…

Well because if you live a semi-nomadic life of an immigrant with residence permit bound to the job and the contracts never exceeding a year, sometimes lasting only several weeks, you can’t allow yourself having too much things. I was living almost off the grid not because I wanted to, but because I’ve had to. The life has shown us that we can live like that. But do we wish to?

In the early to mid 90s Russia, in the time of my childhood, we were to poor to allow ourselves a vacuum cleaner or a wash-machine. I still remember too good the laundry stuck in the bath for days because the parents were too exhausted to do it. I’ve been using the same underpants for weeks sometimes. Same went for the dishes.

Our purchase of a wash-machine in 2000 was like starting a new life. We were finally able to put on fresh clothes, to sleep on fresh blankets, to actually buy new blankets. The machine is still operational in my parents’ flat, by the way, just like the vacuum cleaner we’ve bought about the same time.

So, living as close as it gets to the standards you propagate in this article, I have to say one thing. I am out. As soon as I get a residence permit and hopefully a citizenship, that is, as long as I won’t be forced to go to more and more orwellian Russia, I will buy a wash-machine. I will buy a normal fridge. And - finally - a dishwasher. And I’ll be looking a place with floor heating, for I am tired of my and my wife’s feet being constantly frozen - my current place obviously fell victim of “good energy passport”. And I’ll be seeking to buy a self-driving car for my wife, because she - like my mother, by the way - is simply not apt to drive a car herself, and yet I want her to be safe, healthy and in time. There are good chances I will take part in developing these cars.

Meanwhile, the community in rural Germany I’ve been working an draws 50% of its electricity of to wind turbines.

TS Gordon

I’m currently building my first 2Kw backwoods solar back-up system and addressing these efficiencies is of utmost importance, not to me but to the entire universe of housing and systems engineers and designers.

As was true back in the mid 80’s, as the CEBUS council et. all took on the organizational task of standardizing LV Comm devices, (incl. RS232, 485, Ethernet, Gigabit ‘Enet, USB, I,II,III, etc.,) —WE, the end users and hobbyists, are now responsible for forcing the hand of BIG ENERGY to live up to their promises by showing real world acknowledgement of humanity’s GLOBAL need to conserve, preserve and share access to all the available resources which so directly relate to the very notion of ‘sustainability,’ while creating all of the cultural disparities seen in our friend Kolya’s message above!

Here’s to the 5’s, 7’s, 9’s 12’s and 24’s. May ye soon reign supreme!

Jack

It should be possible to run ‘inverter’ appliances on DC power, but I’m not clear if you could just feed them 240v (or is it 300v) DC, or if you’d need to bypass their AC/DC stages.

For example, it’s much much cheaper to buy a regular inverter aircon, than a specialized DC ‘solar’ aircon, and I suspect the ‘solar’ aircon is basically the same device without the AC/DC stage.

Valery

I’m surprised that arcing is not mentioned. This is a significant disadvantage of DC power.

I think in the nearest future we will have AC power in our homes because:

  • AC/DC converters get more efficient over the years. And cheaper.
  • DC/AC inverters get cheaper and more efficient (97% is already achieved in some applications).
  • AC switching will always be easier and cheaper than switching DC current.
  • New advances in power electronics also relieve the traditional pain of AC power distribution: we will have solid-state transformers soon, HVDC will be cheaper soon.

  1. There is an analogy with hydraulic power: electric voltage corresponds to water pressure, while electric current corresponds to water flow. The invention of the hydraulic accumulator in the 1850s allowed higher water pressure and thus efficient transportation of water power over long distances. ↩︎

  2. Study and simulation of a DC microgrid with focus on efficiency, use of materials and economic constraints (PDF), Simon Willems; Wouter Aerts, 2013-14 ↩︎ ↩︎

  3. Direct Current supply grids for LED lighting, LED professional ↩︎ ↩︎

  4. DC microgrids scoping study: estimate of technical and economic benefits, Scott Backhaus et al., March 2015 ↩︎

  5. DC microgrids and the virtues of local electricity, Rajendra Singh & Krishna Shenai, IEEE Spectrum, 2014 ↩︎

  6. Comparison of cost and efficiency of DC versus AC in office buildings (PDF), Giuseppe Laudani, 2014 ↩︎ ↩︎ ↩︎

  7. Edison’s Revenge, The Economist, 2013 ↩︎

  8. Catalog of DC appliances and power systems, Karina Garbesi, Vagelis Vossos and Hongxia Shen, 2011 ↩︎ ↩︎ ↩︎

  9. DC building network and storage for BIPV integration, J. Hofer et al., CISBAT 2015, 2015 ↩︎

  10. However, DC power in data centers will not bring us a less energy-hungry internet — on the contrary↩︎

  11. Also note that the efficiency of AC/DC adapters could be improved in a significant way, especially for low power devices. Many “wall warts” are needlessly wasteful because manufacturers of electric appliances want to keep costs down. If this would change, for example because of new laws, the advantage of switching to a DC grid would become smaller. ↩︎

  12. Energy savings from direct-DC in US residential buildings, Vagelis Vossos et al, in Energy and Buildings, 2014 ↩︎

  13. In this study, the buildings use a combination of 24V DC for low power loads, and 380V DC for high-power devices and for distributing DC power throughout the house to limit cable losses. ↩︎

  14. Electric power transmission and distribution losses (% of output), World Bank, 2014 ↩︎

  15. Rural areas usually have higher losses than urban areas, and a lone subdivision line that radiates out into the countryside can introduce very high losses. ↩︎

  16. Concept for a DC low voltage house (PDF), Maaike Friedeman et al, Sustainable building 2002 conference ↩︎ ↩︎

  17. A last — and rather desperate — way to lower distribution losses is to use thicker cables. The resistance in electric wires can be decreased not only by shortening the cables, but also by increasing their diameter (diameter here refers to the copper core). For example, if we would use 100 mm2 instead of 10 mm2 cables, we can have cables that are ten times longer for the same energy loss. Distributing 12V DC electricity across 100 metres of cable would yield an energy loss of only 3%. One problem with this approach is that the costs of electric cables increase linearly with the diameter. One metre of 100 mm2 cable will cost you about 50 euro, compared to 5 euro for a 10 mm2 cable. Sustainability also suffers because the higher use of copper has a significant environmental cost. Thick cables are heavy and less manageable, too. Thanks to Herman van Munster en Arie van Ziel for making this clear. ↩︎

  18. Our standards, EMerge Alliance, retrieved April 2016. ↩︎