From power converter to energy manager

For many, the topic of self-consumption is still enshrouded in mystery, as was evident recently at an event on power grids held in Munich. Outraged by the grid operator’s presentation, one visitor in the audience jumped up from his seat. “You didn’t say one word about the distributed use of solar electricity,” the agitated man shouted at the podium. “And anyway, now solar electricity can also be consumed by the producer.” This might not have been so surprising, had the criticism come from someone outside the profession. But as it turned out, he had spent many years fighting for renewable energy. The audience reacted with confusion. “But he’s always been able to do that,” whispered a woman from Munich’s environmental scene, chuckling softly. “Surely he means that electricity for private consumption is now subsidized in Germany.” If you ask around, you will quickly realize that even system operators and installers aren’t always sure how to increase the rate of self-consumption to benefit from the feed-in tariff. One reason for this is that until now, cost-efficient energy-management systems have been hard to find. It remains to be seen whether the systems that a number of inverter manufacturers have recently begun selling or are planning to introduce this year will make up for this dearth. The fact is that the selection of products aimed at optimizing self-consumption is growing.
The question now is who will gain the firmest footing in the energy management systems sector. Companies who have recently gotten into the market with products – or at least have announced such products – include module manufacturers such as SolarWorld and system suppliers such as IBC Solar and MHH Solartechnik, as well as battery manufacturers such as Prosol Invest and companies such as E³ /DC, which have developed battery systems for electric cars. But the inverter manufacturers Voltwerk Electronics, SMA Solar Technology and Kaco New Energy are also already hard at work in this field.

In good hands

Bernd Engel, Professor for Sustainable Energy Systems at the Technological University of Braunschweig, Germany, thinks the inverter manufacturers are in the best position to offer the new product. “The 2012 Renewable Energy Act is practically screaming for energy management solutions,” says Engel, who, in addition to his teaching activities, also acts as Board Representative for Grid Integration at SMA Solar Technology. According to the new feed-in management regulation, operators of photovoltaic systems with outputs up to 30 kilowatts must reduce the maximum feed-in to 70 percent of the array’s installed capacity. Otherwise the system operator must allow the grid operator to down-regulate the system. The first option is suitable for smaller systems. The inverter in such systems can easily increase self-consumption to prevent any loss of kilowatt hours.
Dirk Uwe Sauer, Professor of Electrochemical Energy Conversion and Storage Systems at the Institute for Power Electronics and Electrical Drives at RWTH Aachen University, agrees with Engel. “I believe it makes sense for inverter manufacturers to take on full responsibility. These manufacturers offer the greatest potential when it comes to making use of synergies between the management and the electronics,” he says. “By contrast, battery manufacturers or other system integrators would have to assemble all the products separately, and have only a small share in the value-added chain.” But not every inverter manufacturer thinks it’s their job to get involved in energy management, not by a long shot. This is what Hans-Georg Schweikardt, Head of Product Management at Sputnik Engineering, had to say: “Basically, it’s not up to the inverter to handle the energy management of buildings.” Energy management must also be possible for buildings without photovoltaic power plants, he adds. His company therefore sees itself as part of an energy management system, not as an energy manager per se. Consequently, Sputnik does not currently offer a solution for optimizing self-consumption.

From simple to complex

The easiest way to consume self-produced solar electricity is to install a two-way meter, in addition to the meter for the solar electricity generated. Self-consumption, which is subsidized in several countries, is the difference between the kilowatt hours fed into the grid and those drawn out. A single-family homeowner whose PV system generates approximately as much electricity as the household needs in one year can supply up to 30 percent of its electricity needs with solar power, with no additional equipment needed. This average value was calculated by Franz Breitwieser, who works in system engineering at Fronius.
“The self-consumption rate depends primarily on two parameters,” he says. “It’s consumption behavior on the one hand – that is, when is the most electricity used in the household? And it’s the ratio of the system size to the yearly power consumption on the other hand.” Photovoltaic systems with yields below power consumption achieve higher self-consumption rates; for very large systems, the rates are lower.
The simplest solution for increasing self-consumption is to turn on household appliances at times when the system is producing electricity. The system operator can do this manually or with a timer.

Relay in the inverter

Those who want to increase self-consumption in a more targeted manner can opt for energy management systems, which are now flooding the market. It has already been two years since Kaco New Energy was the first inverter manufacturer to come out with such a product, its “Relay 33.” Danfoss Solar Inverters followed in October of last year with a relay for its TLX Pro+ inverter series. Mastervolt has such a system in its product lineup. Since February of this year, Fronius International is now also offering this feature in three inverter series.
Using Fronius as an example, here’s how a relay works: The relay, a potential-free contact, sits on a plug-in card. A radio transmitter is connected to the relay. It controls the remote-control socket and thus also the household appliances. The installer or system operator programs a threshold value on the inverter, 2,000 watts, for example. When this value is surpassed, the relay is activated and in turn activates the load. If output falls below another defined value, say, 1,800 watts, the load is deactivated. In the Fronius IG TL and Fronius CL series, the so-called “signal card” is integrated as standard. Modest additional costs are incurred only if the card has to be retrofitted in the Fronius IG Plus.

Multiple options

With the energy management feature that Solutronic has been offering since the third quarter of 2011, the system operator must download some software onto the inverter. Customers can download the software free-of-charge from the company’s website. For series 100 and 120 inverters, the “Sol-combox” is also required. Here, too, the inverter is connected to the load via relays, digital outputs and wires. In one household, up to four appliances – a washing machine, a dishwasher, a pool heater and a dryer, for example – can be connected to a Solutronic inverter. The user defines the conditions for switching on the electrical appliances via the software. A specific time can be programmed in, for example. It is also possible to bring the devices into circuit starting at a specific minimum PV system output, and also by a specific time. These options set this energy management feature apart from the Fronius relay, in which only the threshold value is relevant. Neither product includes weather forecasts.
Meteocontrol, an Augsburg-based supplier of monitoring solutions and weather services, launched its Web Log Comfort product back in 2010. The data logger compares the solar electricity produced against household consumption and activates loads on demand. As an additional fee-based service, Meteocontrol provides weather data via the Internet. With this service, for example, a system owner can program the software to turn on the washing machine only when the weather forecast says the sun will be shining for at least two hours.
Diehl Ako has also announced an Internet-based solution for increasing self-consumption, due out this year. According to the manufacturer, any number of loads can be controlled with the Platinum Webmaster Home online control center, and the entire process can be visualized. “The integration of weather data is on the roadmap and will definitely be implemented in 2012,” announced Armin Steck, Head of Product Management for Photovoltaics at Diehl Ako.

Making use of variable tariffs

SMA Solar Technology has also already incorporated weather forecasts. This service even comes standard on the Sunny Home Manager, which the Niestetal-based company will bring to market this quarter.
The Sunny Home Manager is the third component in SMA’s modular energy management system. Its basic component is the Sunny Backup device, which the manufacturer developed to supply electricity during a power outage. Last year, SMA added the Meter Box to the system. The Meter Box records grid consumption, feed-in and self-consumption, and relays these to the backup system. The battery inside stores electricity for the night, thereby increasing self-consumption. The Sunny Home Manager in turn monitors the system, visualizes all power flows in the household and recommends when to turn on loads manually, or activates these automatically via Bluetooth sockets to which loads are connected. It also has a feature unlike anything currently on the market: the device takes variable electricity rates into account. So to control loads, it looks at the following parameters: the generation forecast, the household’s load profile, the load profile of individual household appliances, different electricity rates and the current generation and consumption situation.
According to company reports, with a five-kilowatt system that includes a battery with six kilowatt hours of storage capacity, a “typical four-person household” can increase its self-consumption by “up to 65%” using this combo package. But it is important to note that the self-consumption rate actually achieved will depend on the above-mentioned parameters.
Kaco New Energy, Solutronic and Schüco International, which will also be introducing energy management systems with a built-in inverter and battery this year, are advertising self-consumption rates of a similar order of magnitude.

Integrated solutions

At Kaco, the energy storage system is called Powador-gridsave. This freestanding device optimizes self-consumption, supplies the household if the power fails, enables standalone operation and takes care of load control. Its components include a DC-to-DC converter, a lithium-ion battery with a 4.7 kilowatt hour capacity, a grid cut-off mechanism and an energy management system. The controller controls the operating modes: feed-in, battery charge and standalone operation. The housing can accommodate up to three batteries. “In the future, we also plan to offer devices with external Kaco inverters,” says product manager Anke Richter.
Solutronic developed the Sol-Energy manager in partnership with E³ /DC, an Osnabrück-based manufacturer of storage systems. This power storage and control system includes a Solplus 40 S2 inverter, which Solutronic developed specifically for the energy manager, along with a battery inverter from E³ /DC and a lithium-ion battery from Sanyo. “Right now, the system is designed with 5.4 kilowatt hours of storage. But a second battery can be added,” explains product manager Sibylle Scheuerle-Kraiss. This might be necessary for loads that consume very little of their own power during the day and therefore require a higher storage capacity. The battery could also be used to charge up at night when power is cheaper and then supply this power during the day. The energy manager, which will be coupled to the DC circuit, should be available starting in June.

A question of efficiency

Schüco International is following a similar concept. The Bielefeld-based company has announced an energy manager in two versions, SPE 4000 and SPE 8000, for the end of the second quarter 2012. The difference between the devices is the capacity of the lithium-ion battery. The first version has a capacity of four kilowatt hours, whereas the second has an eight kilowatt hour capacity. The inverter is an integral part of the energy manger. It is connected to the battery on the DC side. “This achieves significantly higher system efficiencies than an AC-coupled system,” says Sascha Beverungen, Head of E³ Product Management and Development at Schüco International.
The photovoltaic array is limited to an output of 5.5 kilowatts. According to Beverungen, the device is efficient, with a yield of 5,000 kilowatt hours per year and an annual consumption of approximately 3,000 kilowatt hours.
But, he admits, the efficiency does depend on consumption. With a storage capacity of four kilowatt hours and a PV array yield of 5,000 kilowatt hours per year, a user who consumes 4,000 kilowatt hours per year could achieve a self-consumption rate of 49 percent. With a consumption of 7,000 kilowatt hours, the rate increases to approximately 65 percent.
He won’t mention the price of the system. Other manufacturers are also reluctant to talk about cost. When asked about price and efficiency, here is what Kaco product manager Anke Richter had to say: “Our aim is to use the amortization period of PV systems as a guideline.” This period is generally assumed to be 10 to 12 years. But views still differ on whether or not a storage system is economically feasible. “Our analyses show that the use of batteries, for example, is not currently viable from a commercial perspective,” says Ralf Dajek, product manager at Danfoss Solar Inverters. And that is why in the short term, the company is not planning to make products that will store energy in batteries and recover it for self-consumption or for the grid.
Franz Breitwieser from Fronius is also skeptical. In a household setting, it is still difficult to recover the investment at the moment, he says. He also expresses his reservation that although short-term storage is possible with the lead gel- and lithium-ion-based battery technologies, long-term storage is not.
Sputnik product management head Hans-Georg Schweikardt sees yet another obstacle: “Without defined interfaces and standards and the implementation of these in-terminal devices, the efforts of the photovoltaics industry are more of a transitional solution than a long-term system solution,” he says. Households can expect to pay between €25 and €30 for a remote-control power socket and radio transmitter, depending on the manufacturer. These additional costs will not go away until there are standardized inputs on the devices.
Energy managers are more worthwhile for larger installations and higher consumption – at least that’s what suppliers of complex energy managers believe. They consider the systems to be cost-effective for single-family houses, small businesses, public institutions, hotels and farmers. When asked, all the companies affirm that the demand is there in any case. But it may first take an increase in the price of electricity to the point where system operators who can’t be classified as “idealists” also begin to think along the lines of self-consumption.