Residential Solar Systems: Part I: System Types

The sun radiates a huge amount of energy that can be tapped as an alternative source of free power for your residential solar system (see the Solar^And^Wind^Energy^Basics page for more information). Given the dual nature of solar energy (light + radiant heat), the two technologies that make residential solar systems possible are solar thermal and photovoltaic (PV). Each of these technologies offers systems that are appropriate to specific home energy applications. However, they are distinctly different technologies and the systems based on them should not be confused. To establish a clear foundation for further discussion, we'll first examine residential solar systems that are solar thermal in nature and then touch on solar photovoltaic (PV) based systems.

Solar Thermal Systems

Solar thermal systems collect, trap and transfer solar thermal energy (STE) from the sun and store it for subsequent use. The stored thermal energy, usually in the form of hot water or steam (or even hot air), can then be used to generate electricity, heat the home or other heating purposes. Note that this is the fundamental difference between photovoltaic (PV) systems and solar thermal systems. More specifically, where PV systems produce electricity directly from sunlight, solar thermal systems collect and store heat energy. They produce electricity only when used in conjunction with some type of electricity generating apparatus (i.e. generators. alternators, turbines, etc.).

Relatively speaking, solar thermal is an older technology. As we'll see, its use has continued into the present for a number of reasons. It has also substantially benefited from advances in materials technology. These advances have improved the efficiency of several primary systems components. These components, for residential solar systems, have recently become reasonably inexpensive and can be easily purchased. Some can even be made by the do-it-yourself homeowner.

The four primary components needed for residential solar systems are an STE collector unit, heat transfer medium (typically water), a pump (for active systems, passive systems don't use pumps) and a storage unit. Each is described below.

Solar Thermal Energy Collectors - there are several types of STE collectors, but all use either a reflective redirecting form of energy collection or a passive greenhouse method of energy collection to do their work. The reflector type collector (i.e. parabolic/hyperbolic trough or focusing reflector plate) usually has a curved mirrored surface of some type that redirects and concentrates solar energy onto a focal point. At the focal point lies an absorption tube(s) that carries the heat transfer medium, or (as in the case of a parabolic tracking dish collector) the electrical power generator assembly (i.e. a stirling engine attached to a generator). As the mirrored surfaces concentrate solar energy at the focal point the absorption tube(s) gets very hot. When the heat transfer medium passes through the absorption tube, it acquires some of the heat (via conduction) and continues on to the storage unit or is used to generate power.

On the other hand, passive greenhouse-type collectors are composed of an insulated enclosure where one of the sides is actually a single or double glazed face that lets solar energy into the enclosure. The solar energy heats up an STE collecting structure (a simple metal or polymer component, sometimes referred to as a "blackbody") conducting the heat through the absorbtion tube(s) and into the heat transfer medium within them. This is typical of plate-type collectors. Batch collectors, in contrast, use a holding tank housed in the enclosure, as the STE collector blackbody. The tank simultaneously functions as the primary heat storage unit.

To illustrate the greenhouse effect for these collectors, consider what happens to a car on a hot summer day. Solar energy enters through the windows and windshields heating up the seats, dashboard and any other surface on which it falls. These surfaces act as STE collectors. As the surfaces heat, they in turn begin to heat the air within the car. With the windows rolled up, the temperature builds because the air can't escape. The temperature really begins to climb because the hot air is, in turn, heating everything it touches (even surfaces not directly touched by sunlight). In fact, even the windows will become hot because the heated air will also transfer heat to them. Therefore, air acts as the transfer medium, while the objects it heats act as heat storage components. When the car door is opened the hot air immediately escapes. However, the surfaces that have been heated by contact with the air continue to radiate stored heat, generally requiring a few minutes to cool before being utilized.

It's very simple to optimize this principle for collectors in residential solar systems. Typically, to absorb the maximum amount of STE the collector's interior (especially the blackbody component) must have the maximum amount of surface area exposed to sunlight, and it must be black or at least be of a very dark color. Why? Because black uniformly absorbs almost all visible light energy, reflecting very little. In other words, black isn't really a color. It's the visual representation of how we see an object that that has the property of simply not reflecting very much visible light. (or none at all, e.g. black holes).

For thermal solar collectors, the absorbed energy includes the sun's invisible infrared and ultraviolet parts of the electromagnetic spectrum as well (a lot of energy). This energy becomes heat that efficiently passes into the transfer medium via conduction (the same way engine coolant acquires heat from the engine block on its way to the radiator).

From a residential solar systems standpoint, a common STE collector used is the solar (thermal) panel. Sometimes referred to as a solar hot water panel or a flat-plate solar collector, this panel operates on the greenhouse method of STE collection. It typically has a solar energy collector plate with heat transfer tubing that runs through the body of the collector, carrying the heat transfer medium. Solar energy enters the glazed enclosure and strikes the plate, heating it. Heat is transferred to the tubing and transfer medium through conduction. Collectors of this type are commonly used for water heating and pre-heating applications (i.e. in home use, swimming pools, spas, etc.).

Some residential solar systems take an integrated approach to hot water production. Integral collector-storage systems (ICS) combine the storage unit with the collector enclosure, thus the storage unit itself collects solar energy while simultaneously providing primary storage. Such systems typically use the home's conventional hot water tank and system as back-up storage.

Heat Transfer Medium - There are several types of heat transfer medium, each fitted for a specific application. For instance molten salt or oil is used in industrial applications where maximum heat transfer efficiency and thermal retention is required. For residential solar systems, water is commonly used.

For colder climates glycol and water (antifreeze) is used as the heat transfer medium for indirect water heating applications. These applications are "closed" systems in that the anti-freeze/water transfer medium moves exclusively between the collector and a heat exchanger (submerged in the potable warier storage unit). This means that the potable water is heated indirectly.

Pump - some residential solar systems can move the transfer medium (typically water) under pressure by way of a circulating pump. In these systems a sensor tells an electronic controller when its solar panel has reached the correct temperature for hot water production to begin. The controller then activates the pump and water moves through the system. Such systems are called "active" systems. Contrast this with a "passive" system where circulation takes place due to cold water being heavier than warm water. For optimum passive circulation to take place, the cold water side must be located above the collector, but below the hot water return inlet, to allow the heavier cold water to "sink" into the collector, displacing the lighter hot water. This method is referred to as thermal siphoning. Passive systems are not quite as efficient as active systems, but their maintenance costs are significantly lower and they have a longer life since there are so few moving parts to maintain.

Storage unit - typically this is an insulated container where heated water is stored until needed. However, as mentioned in ICS systems, they also function as the blackbody collector and are therefore not insulated.

Solar Photovoltaic Systems

As mentioned earlier, solar photovoltaic systems convert light energy directly into electrical energy. There are two basic types of residential solar systems using photovoltaic technology, grid-tied (aka. grid-intertied) and off-grid. Each system type has its advantages, disadvantages and best application for meeting specific objectives. For instance, off-grid systems (as the name implies) are designed and built to support residential electrical energy needs without any connection to a commercial energy supply grid. Obviously, this is a critical characteristic when considering power supply issues for residences in remote locations (where commercial energy supplies are unavailable). On the other hand, grid-tied systems are connected to commercial supply grids. For residential solar systems designed this way, an energy storage subsystem is not required. Not needing to store electrical energy also means that for the most part, grid-tied systems are less expensive to implement. To get a better understanding of the system types we'll touch on the basic components each has in common, namely, solar PV cells, modules, arrays and power inverters. Other needed components (typically referred to as balance of system or BOS) are determined by each system's design objectives and therefore can vary by application. However, we'll touch on some of the more common components.

Basic components

Photovoltaic Cells (PV cells)

The heart and basic building block of photovoltaic residential solar systems is the photovoltaic (PV) cell (a.k.a. solar cell). These devices can be quite varied in composition and technology, but generally a single cell produces 1 - 2 volts of direct current when exposed to sunlight. When cells are wired together as a unit they are referred to as a module (also commonly referred to as a panel). Depending on system requirements, a number of modules are wired together to form an array (i.e.. solar array). The ultimate size of an array is driven by the average amount of solar energy available per day, how efficient a module is at converting sunlight into electrical energy, and finally, how much electrical energy is needed for the application. This software helps simplify the calculation. Before continuing, a quick review of how PV cells do their work can be helpful in gaining an understanding of the technology (see below).

The PV Effect - a summary

PV cells are able to generate electricity from sunlight because of the photoelectric or photovoltaic effect. PV cells take advantage of this phenomenon when certain materials, namely, a negative or "N"type semiconductor and a positive or "P" type semiconductor have been bonded together to form a P-N junction. The junction develops built-in electrical characteristics very much like a battery. A closer look reveals that this is possible because the base material, silicon (or germanium, etc.) for instance, can be atomically modified to carry a negative charge (known as "doping"). Through doping, silicon atoms are given "extra" electrons in their outermost orbits. This n-type (negatively charged) silicon is predisposed to getting rid of the extra electrons to reach a more stable atomic configuration. On the other hand, p-type (positively charged) silicon has been doped so that its outer electron orbit is almost full (almost stable) but is actually missing one or two electrons in the outer orbit. The missing electron can be thought of as a "hole(s)". This hole gives p-type silicon the characteristic of having strong attraction for electrons to fill the hole and thus reach a more stable atomic configuration. This means the p-n junction has a general condition where the n-type material on one side is trying to push extra electrons across the junction, while the p-type material on the other side is actively trying to pull electrons across the junction. The photovoltaic effect takes place as sunlight, in the form of photons, is absorbed by the n-type material (silicon in this case). The photons impart additional energy to the extra n-type electrons causing them to leave their atomic orbits (where they're not wanted anyway) and move toward and through the orbits of neighboring p-type atoms, on the other side of the junction. The n-type electrons flow across the junction as long as there is light supplying them energy to move from orbit to orbit. To be clear, the electrons move in a one-way direction, that is, toward the p-type silicon which is why a solar cell generates direct (one way) current flow.

PV Cell Technologies

There are several different technologies using different materials (for n-type and p-type semiconductors) to make solar cells for residential solar systems (monocrystalline silicon, multicrystalline silicon, multicrystalline ribbon silicon, thin film cadmium telluride, etc.). However, they all employ the PV effect in the same general way to generate electric power.

It's worth noting that the PV effect is in no way limited to the realm of solar technology. In fact, it's just the opposite. Solar technology uses one small facet of a much more general phenomenon. When light energy of the correct frequency lands on any number of various metals or non-metals, the energy will atomically excite those substances causing them to emit electrons in random directions (photoelectric effect). Solar (photovoltaic) technology simply further refines the photoelectric effect in a way that causes the electrons to flow in one direction, thereby producing electrical current.

Generally speaking solar technology utilizes the PV effect within the visible portion of the light spectrum only. The invisible ultraviolet portion immediately above and the invisible infrared portion immediately below the visible light spectrum is not used to produce power. Currently, research is underway to find and develop techniques that will utilize the energy from these unused portions of the spectrum.

Inverters

An inverter is a very common piece of equipment for residential solar systems. Its main purpose is to convert and condition the variable direct current flowing from the PV array into clean steady alternating current (pure sine wave form) that can be used with home appliances and/or even sent to the local commercial power grid to offset or eliminate residential power costs. Inverters are rated by the number of watts they can handle and can be generally classified by the type of residential solar systems they're used in. They can be described as:

Grid-tie - This inverter type converts DC power into AC power as it flows directly from the solar array. After conversion it directs the electricity into the main breaker panel for the home. In situations where the home is set up for net-metering the inverter will send electricity to the commercial grid when there is more power available for the home than the home can immediately use. This is accomplished most efficiently with inverters that have a microprocessor driven Maximum Power Point Tracker (MPPT) feature to regulate and route the power as needed.

Off-Grid - These inverters normally receive power directly from the storage batteries (charged by the solar array through a charge controller) and directs it into the home's main breaker panel. Some inverters of this type have an integrated battery charging controller feature, which obviates the need for a separate charger controller. This circuit makes sure that the proper amount of power is sent from the array to the batteries to maintain their charge and prevent overcharging. It also disconnects the batteries at night to prevent them from being discharged by the array. In addition, these units can accept connections from secondary power sources (wind power subsystems or gas driven generators) that engage when insufficient solar power is being generated.

On/Off Grid - Used with grid-tied PV residential solar systems, this type of inverter allows a connection from the home to the utility grid and use of power from a battery bank (generally as a power backup source). Should the main utility grid fail for some reason, the inverter will automatically switch to supplying power to the home from the battery back-up source. By contrast, an ordinary grid-tie inverter typically has no such fail-over capabilities.

Inverters are also categorized by the type of power they produce. Some converters produce a simulated sine wave output which is nothing more than stepped (square wave) alternating DC output. Some A/C device/appliances (light bulbs, heaters, fans, etc.) will tolerate this "dirty" output power. Other more sensitive devices will not (computers, microwave ovens, televisions, etc.). Such devices will require the smooth sinusoidal waveform of A/C output to operate properly. These inverters cost more, especially high capacity units.

Other Residential Solar Systems Components

Combiner Box - this device is essentially a junction point that physically and electronically aggregates the multi-module outputs coming from the array. A very common error for inexperienced installers is to connect the array modules to each other in series, that is, connecting each modules output terminal block to the next module output terminal block. This technique economizes wire runs. However, it also causes the amperage at the terminal blocks to increment with each added module in the circuit. The last module's terminal block in the "daisy chain" is forced to carry all the amperage from the modules preceding it. A very real danger materializes where the block possibly ends up carrying more electrical energy than its specified rating, which causes a heat build up on that block. If the block softens or melts altogether, then the wires are free to make contact with random points in the assembly, at least causing the array to fail and/or possibly damaging it. A combiner box encourages multiple parallel wire runs from the array keeping amperage down at a safe level for each run. The output side of the combiner box is usually designed to accommodate the very high accumulated amperage from the array (busbar connections, heavy gauge cabling, etc.).

A/C D/C Disconnect Circuit Breakers - these circuit breakers provide safety and on/off power control for easy system maintenance. They specifically isolate the inverter from the solar array and main utility breaker panel.

Charge Controller - used in systems where power is stored in battery banks, this unit specifically regulates the amount of energy being applied to the batteries to prevent over/under charging. The newest generations are microprocessor controlled and will optimize the flow of amperage into the battery bank thereby ensuring maximum utilization of the array output. This is a little different than simple over/under charge regulation. The units actually change the characteristics of the power coming from the array to optimize its injection into the battery bank. This is necessary because array output varies with weather conditions (sunlight and temperature) and the battery banks' ability to accept a charge varies with temperature and current depletion level. These units are commonly referred to as Maximum Power Point Trackers (MPPT) or MPPT Charge Controllers.

Grid-tied systems

Residential solar systems that are grid-intertied generate electricity for the home. In addition, they can also produce a power surplus. This surplus, sent to the utility power grid, provides a billing offset calculated using agreed to power rates. Basically, the utility company pays homeowners for the excess power their residential solar systems produce. This means that on hot summer days when power demand is typically at its peak, a properly designed grid-tied system can not only provide relief from high electrical energy costs, but also create a credit balance with the utility that is available to be used when solar energy is less plentiful and utility energy is less expensive. In a way, the credit balance can be loosely viewed as a sort of energy "storage" mechanism since the credit balance can be drawn upon later as a billing offset (depending on the utility's policy/agreement).

System Operation

The solar array collects the sun's energy producing DC (direct current) electricity (see Diagram A, below). The array's modules are carefully balanced into wire runs that terminate at the combiner box. The combiner box aggregates the separate current flows into a single high amperage D/C circuit. The DC power is routed from the combiner box (through the DC disconnect breaker) to the inverter which converts it into AC (Alternating Current) electricity. Properly conditioned, the A/C electricity then flows (through the A/C disconnect breaker) to the AC power panel, making it available to the home or power grid.

When the array is not producing enough power for the home, electricity from the grid is used. When more power than the home needs is produced by the array, it goes to the grid actually forcing the utility meter to run backwards. When the meter is read for the month, its net position represents whether the monthly bill will have a balance due, credit amount, or zero balance due.

Off-grid systems

Residential solar systems of an off-grid design are built to generate 100% of the power needed by the home. Typically there is no connection to the utility power grid in any way (most times because the grid is not available). This means that at night when the solar array is inactive, another source of power is needed. Generally, most off-grid PV systems employ multiple high capacity batteries as an alternate power source to meet nighttime or cloudy day home power requirements. Since it is the only source of power for the home, components for this system should be of relatively high reliability or the system design should have very few single points of failure.

SInce an off-grid system must handle all of the home's power needs and produce enough energy to store for nighttime operation, they are generally considerably larger and a bit more complex than grid-tie systems. Issues such as back up power, battery storage, charge current regulation and others must be resolved (generally through additional equipment). All in all it translates to a larger initial capital investment to become fully renewable and energy independent. However, in the long run the investment could save many thousands.

System Operation

The solar array collects the sun's energy producing DC (direct current) electricity (see Diagram B, below). The array's modules are carefully balanced into wire runs that terminate at the combiner box. The combiner box aggregates the separate current flows into a single high amperage D/C circuit. The DC power is routed from the combiner box (through the DC disconnect breaker) into the charge controller and then to the battery bank. The inverter draws power from the battery bank and converts it into AC (Alternating Current) electricity. Properly conditioned, the A/C electricity then flows (through the A/C disconnect breaker) to the AC power panel, making it available to the home.

When the array is not producing enough power for the home or fails completely for some reason, electricity from an auxiliary wind turbine or gas power generator can be used. When the array generates more power than the battery bank needs, power can be switched directly to the inverter, if the inverter/system design will allow it. Otherwise, a trade-off between battery bank size, charging cycles, and battery life will be required.