Further scientific news by TU Delft Colophon DO-Archive
Life under the source
Sustainable housing using low-voltage DC power
Astrid van der Graaf
Emergency phone systems, yachts, and caravans increasingly use solar panels to generate the energy they require to operate. The Dutch government offers generous grant schemes for solar panels on houses and offices. Solar energy is the power of the future. There is however a slight drawback because photovoltaic cells produce direct current (DC) at low voltages, usually 24 volts, and this has to be converted into a 230 volts alternating current (AC) for normal European domestic use. But many appliances these days use low-voltage DC power provided by a plug-in adapter that reduces the 230 V AC mains power to a lower DC voltage. Ir. Maaike Friedeman, who graduated from Delft University of Technology several months ago, takes the view that this is putting the cart before the horse. A domestic DC network would obviate the need for these back-and-forth conversions, but the efficient transport of low-voltage power requires a number of changes in building design.
If Friedeman has her say, we will soon all be sleeping
Our houses and offices are becoming increasingly cluttered with adaptors that convert the 230 volts from the mains to 1.5, 6, 9, or 12 volts, and the number of different appliances without a 230 V plug that run on low-voltage DC power is still growing.
An increasing number of houses are being fitted with solar panels to generate electricity. Connecting these photovoltaic (PV) systems to the mains grid introduces conversion losses that might be avoidable if a local DC network were to be used.
on the ground floor, and living and cooking on the top floor, right under the - flat - roof, which will be covered with solar panels. Whether it will save energy still remains to be seen, but it will obviously mean improved ease of use. According to Friedeman, people are getting fed up with the ever increasing jumble of cables, leads, plugs, power points, and boxes with transformers and rectifiers, some of which are larger than the appliances they power.
A domestic DC network would shortcut the last conversion stage so all those power adapters could be consigned to the waste bin. In addition, new
configurations of domestic electrical circuits could drastically reduce the need for power leads. Friedeman quotes as an example the flat power strips used in spacecraft, which could well be suitable for adapting to a domestic environment. The strips could simply be fixed to the walls, and moved to a different position as the interior design of a room is changed. Appliances can be connected to the strips at any point. One of
Friedeman’s three supervisors, Ir. Arjan van Timmeren at the Environmental Technology Design group of the Building Technology section, is also impressed with the system.
“It dovetails nicely with current ideas on flexible, reusable and sustainable building. On top of that, switching the functional areas within a domestic environment is a nice example of how technology that supports aspects of use, behaviour and social interaction is leading building design rather than the other way round.”
Distribution
Whether a domestic 24 volts DC power network, boosted by a limited AC mains system, would be a lot better than the current, exclusively AC mains set-up, is not a simple question to answer. A 24 volts DC circuit cuts out the power losses that occur during the
conversion from sustainable DC power to AC power and vice versa, as well as the losses resulting from the 200 V to 24 V transformation, but does this compensate for the power losses that occur as the electricity is transported at a low voltage? According to her supervisors, Friedeman left no stone unturned in answering this question, as a result of which her graduation research ended up being almost a doctorate project. Her research has grown to cover multiple disciplines and multiple faculties, involving in addition to Architecture, Chemical Technology (Prof.Dr. Joop Schoonman), Industrial Design (Dr.Ir. Sacha Silvester), and Electrical Engineering (Dr.Ir. G. Paap). When she started her research, the chances of the DC power system becoming a success seemed remote, according to Van Timmeren. Research conducted in the Netherlands by the Arnhem based research centre KEMA in 1997 advocated the introduction of a national DC power grid to provide a more efficient way of distributing
electricity. But a study by the Dutch ECN research establishment in 1998 into distribution losses in a DC grid versus an AC grid turned out in favour of the alternating current system. According to ECN, nestled in the North Sea dunes outside of Petten, the DC system
Solar house, to be built at the new “Groene Kreek” development in the commuter town of Zoetermeer. The neutral-energy house, designed by Bear Architects from Gouda, will provide the basis for the design of a house with a DC system.
Operating principle of a traditional silicon solar cell. Sunlight is absorbed, as a result of which pairs of electrons and electron holes are formed. Due to the presence of a space charge layer at the boundary of the N and P type silicons (known as the Schottky barrier), the electrons and holes are separated according to type and concentrated in the P or N layer. Contact strips connected to these P and N layers feed the power to the electric circuit.
The annual demand and consumption of electricity in the design house fitted with 25 m2 of solar panels. This surface area is sufficient to meet a DC power demand of 2400 kWh in the Netherlands, based on the current neutral-energy principles, whereby the summer provides sufficient energy to cover the shortages during the winter. If the DC system is kept completely separate from the normal AC mains system, the storage of energy using current technology would require some five tonnes of batteries.
resulted in energy losses of as much as 11%. However, the study was based on the conventional layout of both houses and distribution networks.
Friedeman: “An alternating current network easily gets the better of a similar direct current network. But on the other hand, if we manage to keep the circuit runs as short as possible by locating the appliances as close as possible to the energy source, the energy losses during distribution can be reduced to 4%, a 70% improvement compared to the conventional layout.”
Energy transport losses can be reduced even further by using flat and wide conductors strips with a larger cross-section than the standard copper wiring (which has a section of 2.5 mm). Yet another option would be to increase the system’s voltage to 32, 48 or a maximum of 60 volts, by changing the way in which the solar panels are interconnected.
“A 48 V network would be great if the excess power could be used to generate hydrogen gas. A 60 V system is just safe to the touch. As it is, too little attention is being paid to the safety aspects of low-voltage DC systems. They are safe for use in wet environments, including bathrooms,” Friedeman says.
Power guzzlers
Analysing the power efficiency of an entire house is easier said than done. Which types of house should one look at, and what kind of appliances do they contain? The final choice was the “Groene Kreek” district of Zoetermeer (a commuter town some 12 kilometres from The Hague), where a number of neutral energy houses are to be built. The selected house - two floors under a large, flat roof - was designed to use solar panels. Friedeman defined the type of appliances to be included in the house, based on published data on the penetration level of electric household appliances in Dutch
households. If a certain type of appliance was used by more than 40% of the average Dutch households, it was included in the reference house.
In 1998, the average annual electrical energy use per house in the Netherlands was 3300 kWh. Extrapolating the value for the type of house and the year 2002, Friedeman arrived at a total power consumption of 3900 kWh. As expected, the rooms where the most energy is consumed are the kitchen and the living room.
Therefore, Friedeman located these areas directly below the sustainable power source, i.e. the roof carrying the solar panels. The kitchen, with its range of power-hungry appliances such as microwave and traditional ovens, washing machine and tumble drier, still required higher levels of electrical power of the mains grid, so an alternating current connection was provided there. “Many appliances can manage with very little power, and only a few require large amounts of energy. In between the two extremes there is a large power range with very few takers. We set the usable power for the low-voltage system at a maximum of 150 watts. The result is that, on a yearly basis, 1500 kWh comes from the AC system, and the DC system provides no less than 2400 kWh,” Friedeman says.
Autonomous
Supply and demand on a summer day (in July).
Supply and consumption on a winter day (in Dec.).
Supply and demand of electricity in the design house fitted with 75 m2 of solar panels, covering practically the entire available roof surface. In this case, a very limited storage capacity suffices.
Although the obvious thing to go for would appear to be a sustainable DC system linked to the standard public mains system, Friedeman opted for a fully autonomous system. This means that the surplus of energy produced in the summer has to be used to compensate for the shortage of energy during the winter, which in turn means that some kind of energy storage system has to be added to the house.
“The inclination towards returning the summer surplus to the national grid is understandable, but there are limits to the extent to which this can be done. If large quantities of locally generated energy were to be pumped back into the national grid in the Netherlands , large winter shortages and summer surpluses would be the result. Technological developments such as large-scale storage and high-power electronics, as well as organisational changes, may help to retain the stability of the grid. Another possibility is to match the supply and demand within built-up areas at house or district level, a local approach that could relieve the grid,” adds her other supervisor at the Building Technology section, Dr. Elisa Boelman, of the Building Services section. Energy storage
The whole set of solar panels, wiring, and batteries is often referred to as the PV system, in which PV stands for photovoltaic, i.e. relating to the process of
converting light into electricity. An autonomous PV system must be capable of supplying the necessary energy during summer as well as winter.
“The problem is that in the Netherlands, the ratio of photovoltaic power production in summer over winter can be as much as five, so an autonomous system will be larger and more complex than it would have to be in a climate offering a more constant level of solar energy supply,” Boelman adds.
For the reference house at the ‘Groene Kreek’, matching the supply and demand of energy throughout the year - the current principle of neutrality - resulted in a system using 24 m2 of polycrystalline silicon solar panels combined with a storage facility comprising 3 m3 of lead/acid batteries weighing some 5 tonnes. A more favourable option - the result of the existing government grant system - proved to be to cover the whole roof with solar panels (75 m2), enabling the system to almost cover the total power demand during the winter, with only a few batteries required to get through the darkest winter months.
According to Friedeman’s third supervisor, Prof. Dr. Joop Schoonman, director of science at the Delft
Institute for Sustainable Energy, hers is a rational choice. “Storing three cubic metres of batteries in a house is not a viable option. I fully expect the lithium-ion battery to be capable of taking on the role of energy storage within the next five years. This type of battery is now common in laptop computers and mobile phones. It is lighter than a lead/acid battery, and delivers twice the voltage, making it eminently suitable for large-scale storage. Thinking of storing energy in the form of hydrogen gas might be a bit premature, but who knows, in ten years we may all be using a fuel cell linked to a hydrogen gas
As soon as a safe and cost-effective way can be found to store hydrogen, the surplus electricity may be used to generate hydrogen. At times when the demand for electricity exceeds the supply limit of the solar panels, a fuel cell connected to the power system can convert the hydrogen back to electricity.
Schematic diagram of the standard alternating current system in an AC-wired house. Two power conversion steps are required for power users running on the same DC voltage as the solar panel. In a house with DC wiring using the same voltage as the PV system, the extra conversion steps can be dispensed with. However, if a higher or lower DC voltage is required, a DC/DC will be required, introducing a single conversion step. In addition, the lower voltage means that currents will be higher, resulting in increased losses as the power is distributed through the house.
Many popular household appliances, including television and stereo equipment, already contain a transformer and rectifier to convert the 230 volts from the AC mains to the low-voltage DC power on which they operate.
generator gently bubbling away in our back garden.” Stepping stone
Whatever the case, after eleven months it was time for Friedeman to take stock. Although some 60% of the total energy demand is supplied by the DC system, for the time being her AC/DC house comes in second best with 5.5% extra energy loss. Having said that, it is still a bit early to draw a conclusion, since the energy losses of the various power adapters (containing transformers and rectifiers) were not included in the calculations. With the help of a number of students, several adapters were measured, revealing losses that ranged from 12% to 60%. Even so, a thorough analysis requires that more household appliances as well as more recent types fitted with different types of transformers be included in the survey.
“What’s more, a number of different houses will have to be surveyed and analysed if we are to draw more general conclusions. The data we used regarding the demand for electricity have been derived from long-term averages and apply to the standard alternating current network,” Boelman adds. That’s why Van Timmeren and Boelman consider the project a good stepping stone to one or more graduate studies. For more information, please contact
Ir. Maaike Friedeman, e-mail mfriedeman@hotmail. com, or
Ir. Arjan van Timmeren, phone +31 15 278 4991, e-mail
a.vantimmeren@bk.tudelft.nl, or Dr. Eng. Elisa Boelman MBA, phone +13 15 278 8789, e-mail e.c. boelman@bk.tudelft.nl, or Prof. Dr. Joop Schoonman, phone +31 15 278 2647, e-mail j.schoonman@tnw. tudelft.nl
The introduction of low-voltage DC wiring reduces the number of power conversions within the system. On the other hand, the lower voltage results in higher losses as the electricity is distributed through the house. These transport losses in a DC house can be minimised by moving the living areas (which use most of the DC power) to the upper floor, as close as possible to the power source.
Wiring designs for both the DC design house and a similar AC house. These drawings were used as the basis to calculate the transport losses for both systems.
A new distribution system using strip conductors could provide the solution for connection problems and may help to further reduce the transport losses.
