ÒWhatÕs All This Maximum Power Tracking Stuff, Anyhow?Ó Bradley E. O'Mara c.1992 Bradley E. O'Mara Maximum Power Point Tracking (MPPT) will revolutionize the way RE systems are designed. A similar revolution has already happened in high- frequency electronic power conversion (EPC) and MPPT technology, 30 years ago in space systems, 20 years ago in renewable energy grid-intertie systems, and 10 years ago with DC to AC inverters. In the next five years, we will see MPPT technology becoming available for battery-based, stand-alone PV systems. In fact, these changes have already begun. EPC technology is now being used to couple high voltage PV arrays to low voltage batteries. This reduces voltage losses and frees designers to install arrays where their customers want them, not just where traditional wiring methods would otherwise allow. (See the Things That Work!, p 47 in this issue, also "Long Distance Power Transmission" articles in HP28, HP25, HP12,HP6). Charge controllers based on relays, and simple on/off solid-state units are being replaced by EPC controllers. Voltage converter products today use EPC circuitry to efficiently operate 12 Volt appliances from 24 and 48 Volt batteries. What Are EPCs and MPPTs? EPC devices use high frequency switching to efficiently convert power from one DC voltage to another. You can think of EPCs as "DC to DC transformers". On some EPCs you can set the "number of turns" in these "transformers" with the turn of a knob. MPPTs are EPC devices which are "smarts" and constantly adjust the "number of turns" to optimize power production from your power source (PVs, wind generator, etc.) Think of MPPTs as computers with only one mission: to maximize power out of your renewable energy sources. Or here's another analogy: an EPC is like a manual transmission in your car. It changes the form of the power from the engine (your PV panels) to the wheels (your batteries). If an EPC is like a manual transmission, a MPPT is like an automatic transmission. Same Panels, more power Are your PV panels putting out as much power as they could? Probably not. The current a solar panel delivers depends on the voltage across its terminals (see graph 1a). If the positive and negative wires from the panel are touched together (short circuited) the voltage is very close to zero and the panel will put out its maximum current, (Isc standing for short circuit current). If the leads are attached to a battery, the panel's voltage becomes the battery's voltage (Vbattery on graph 1a), and current (less than Isc) will flow into the battery. If the PV panel's wires are hooked to nothing (open circuited), the voltage of the panel rises to its open circuit voltage (Voc = 22 Volts in graph 1a) and no current flows. The panel will produce current at a continuous range of voltages between zero and Voc, shown by the current vs voltage (or "IV") curve in graph 1a. PV module power is equal to the current times the voltage Ð or graphically, the area of a rectangle under the IV curve. See the shaded rectangles on graph 1a. At some combination of voltage and current, the area of the rectangle reaches a maximum (the darkest rectangle on graph 1a). This is called the maximum power point, shown as Pmp on the graph. INSERT GRAPH 1a,b "IVcurvesPICT" Graph 1b plots power as a function of voltage for the same panel. In this example, holding the voltage of the panel to the battery's voltage Vbattery means the panel produces only 73% of the power it could produce. Moving the voltage up to Vmp allows the panel to produce all the power it can. This example is pretty optimized for MPPTs. You probably wouldn't see this much power gain unless your panels were extremely cold. How much more power could I get with a MPPT? The shape of a panel's IV curve depends on temperature, and slightly on sunlight intensity. Graph 2 shows the theoretical percent power gain as a function of temperature for 36 and 33 cell modules using a MPPT, compared to direct connection to the batteries. Of course some of this gained power will be lost in the MPPT itself, since the units are not 100% efficient. The more series cells per module, and the lower the temperature, the more power the panels will produce by operating at their maximum power point. INSERT %GAINPICT Caption: Theoretical power gain using MPPTs, assuming battery charging at 14 VDC. High Voltage PV Arrays MPPTs can further optimize PV systems by allowing panels to be wired in series strings This takes advantage of low line losses for long distance power transmission. Shrinking array power cables to an affordable size will help you take advantage of that 'perfect' sunny spot, even if it is hundreds of feet away. Back near the batteries the MPPT's electronic power conversion circuitry converts the power to low voltage for your batteries to digest. (See p. 47 this issue). All of this pertains equally to wind and hydroelectric generators. Alternators in these machines will produce more power operating at their maximum power points. Higher voltage alternators can be used, reducing losses in power transmission. INSERT BLOCK DIAGRAM "BlockDiagPICT" How MPPTs work Regardless of battery voltage or any other system variable, a MPPT continuously ÔhuntsÕ for the maximum power voltage, every second of every day throughout the entire lifetime of the array. By using sample-and-hold ICÕs to ÔrememberÕ how much power was happening before the MPPT Ôre-adjustedÕ itself, the MPPT compares two power levels. This allows the MPPT to ÔknowÕ which direction on the IV curve of any RE source it must move in order to get closer to the maximum power point. This logic circuit then tells the EPC part of the MPPT where to set the input voltage. You Cannot Buy a MPPT Today... 'High voltageÕ PV array operation and conversion to lower battery voltages is beginning to be done today by LCBs. But this equipment is incapable of automatically extracting maximum power. These EPC devices canÕt compensate for changing solar cell voltages without being manually re- adjusted. There are many reasons why you can't buy a true Maximum Power Tracker today. They are difficult to design and manufacture on the budget required for the Home Power market. Until recently little awareness has existed for the need for them. A Little MPPT History Surprisingly, PV maximum power point tracking has a long history in the space race. In 1958 the U.S. Vanguard I satellite launched PV technology into space and into government funded labs around the world. In satellites, weight and room for solar cells and batteries was at a premium. Cost was of little concern. In the 1960's, necessity forced engineers invent EPCs and MPPTs. They replaced inefficient and failure-prone relay based charge controllers used aboard the early PV powered satellites. These power processors reduced the size and cost of their space arrays by up to 30%. Batteries lived longer because charge/discharge processes were controlled with precision. Individual system loads were freed to receive power at the voltage best suited to their operation. The system was longer tied to a fixed battery voltage. INSERT"CHRONOLOGYTXT" Here on earth PV systems have applied basic MPPT technology to motors, and to large scale grid intertie synchronous inverters, but not to batteries, the component most in need of its benefits. Today, 25 years later, MPPT technology is about to be reborn in PV, wind, and hydro controllers. Recent advances in switchmode electronic power control will make MPPTs affordable on earth. As MPPTs become available, renewable energy systems will be much more flexible. Power will be generated at its maximum power point - in many cases considerably higher than the battery voltage, and we'll be able to mix and match renewable energy sources producing power at different voltages. Wind and hydro machines will be able to use a wider range of higher voltage generators. These higher voltage power sources will be able to be sited farther from batteries, to take advantage of particularly sunny, windy, or good hydro locations. Day in and day out, every second, MPPTs will help our renewable energy systems to most effectively accept the energy nature gives us. This article was adapted from Brad O'Mara's upcoming book Get More Power: How To Instantly Become an Expert On Maximum Power Point Tracking in Photovoltaic Power Systems Access: Author: Brad O'Mara, Outside Power Company, Midwest Lab Facility, 7477 Lakeshore Drive, Spirit Lake, IA 51360 ¥ 712-336-5045 ¥ Fax 712 -336-5046 Pacific Northwest Address, 130 E. Main St. , Suite 325, Medford, OR 97501 CHRONOLOGY OF PV MPPT CONTROLS Year Company Regulator MPPT 1973 Philips First commercial product No 1975 Solarex SRO series & SHM shunt linears No 1978 Solar Power BVR linear shunt No 1979 Ecotronics Master-Slave solid-state switching shunt No 1980 Photocomm SR-12 solid-state switching shunt No 1980 ARCO BP,VCC,UCC, relay based No 1981 SCI Relay-based series No 1982 Solarex ACR relay-based series ÔCharge PumpÕ No 1082 BOSS Solar Sentry, Centrix solid-state switching No 1982 TriSolar MPC Motor Drive Yes 1983 Rho Sigma Solid-state series linear No 1983 Tideland Model 600 solid-state linear shunt No 1983 Heliotrope Hi Eta No 1983 Photowatt PCU relay-based series No 1983 BOSS PCC motor drive, AC/DC PV power mixer Yes 1983 Solapak Solamax Yes 1984 SunAmp PBR solid-state switching shunt No 1986 Bobier Linear Current Booster No 1985 Photron Solar Brain No 1987 Australian Energy Research Lab, Maximizer Yes 1988 Outside Pwr 1.2 V Ni-Cad battery voltage booster No 1989 Outside Pwr Turbo-Cooler auto ventilator Yes 1991 Outside Pwr Micro power motor tracker Yes 1992 Outside Pwr High voltage MPPT Yes