1. Field of the Invention
The present invention relates to a solar cell string having at least a part of an electric line thereof not housed in an insulating envelope, a solar cell array having the solar cell string, and a solar photovoltaic power system having the solar cell array.
2. Related Background Art
In recent years, awareness of ecological problems has been raised worldwide. Among others, the global warming resulting from CO2 emission is seriously concerned about, and clean energy has been desired increasingly. In such a circumstance, a solar cell shows great promise to serve as a source of clean energy in terms of its safety and operability. The solar cell includes a photoelectric conversion layer for converting light into electricity, typical materials of which include single-crystalline silicon semiconductor, polycrystalline silicon semiconductor, amorphous silicon-based semiconductor, III-V compound semiconductor, II-VI compound semiconductor and I-III-VI2 compound semiconductor.
A solar photovoltaic power system including such a solar cell exists in a wide variety of scales from several watts to several thousands kilowatts. For example, a battery may be used to store energy generated by the solar cell, or a DC-AC converter may be used to supply output energy of the solar cell to a commercial system. FIG. 1 is a block diagram of a typical solar photovoltaic power system disclosed in Japanese Patent Application Laid-Open No. 2000-207662. In this solar photovoltaic power system, four solar cell strings 104 to 107 are connected in parallel to constitute a solar cell array 101, each of the solar cell strings being composed of a plurality of solar cell modules connected in series. An output of the solar cell array 101 is led to a power conditioner 102 having a controller for performing control of a maximum output and the like, and then supplied to a load 103. The load 103 may be an electric power system, and such a system of flowing the power of the solar cell back to the electric power system is referred to as a utility connected system.
The solar photovoltaic power system is grounded through apparatus grounding or electric line grounding. The former apparatus grounding is generally carried out, and includes the grounding of a frame of the solar cell module and the grounding of a housing of the power conditioner. The latter electric line grounding varies from country to country. In Japan, the electric line of the solar cell array is typically insulated from the ground. On the contrary, in the United States, as described in the IEEE standard 1374-1998: xe2x80x9cGuide for Terrestrial Photovoltaic Power System Safetyxe2x80x9d or the National Electrical Code Article (NEC) 690: xe2x80x9cSolar Photovoltaic Systemsxe2x80x9d, the electric line of the solar cell array is generally grounded. In the above-described IEEE standard and NEC, it is described that the grounding of the electric line includes middle point grounding for connecting an electrical middle point of the electric line between the strings, positive-terminal grounding for connecting a positive-side electric line to the ground and negative-terminal grounding for connecting a negative-side electric line to the ground.
FIG. 2 shows an example of the solar photovoltaic power system having the negative-side electric line grounded. In FIG. 2, four solar cell strings 204 to 207 are connected in parallel to constitute a solar cell array 201, each of the solar cell strings being composed of a plurality of solar cell modules connected in series. An output of the solar cell array 201 is led to a power conditioner 202 having a controller for performing control of a maximum output and the like, and then supplied to a load 203. Similarly to the case of FIG. 1, the load 203 may be an electric power system. In addition, the electric line on the side of the negative terminal is connected to the ground via a grounding point 208. Here, in the present application, this example is not identified as a prior art.
FIGS. 3A and 3B show an example of a typical solar cell module used in the solar photovoltaic power system. In this drawing, FIG. 3A is a perspective view of a solar cell module 301 and FIG. 3B is a cross-sectional view taken along the line 3Bxe2x80x943B of FIG. 3A. As shown in FIG. 3B, the solar cell module 301 generally comprises a photovoltaic element 302 for converting received light into electricity, a front cover 303 made of glass, a translucent resin or the like disposed on the side of a light-receiving surface of the photovoltaic element, a back cover 304 made of glass, a resin, a metal or the like disposed on the side of a non-light-receiving surface of the photovoltaic element, an output cable 305, a frame member 307 for reinforcing and fixing the solar cell module, and an adhesive 306 for bonding of the frame member.
One of the largest problems of the solar photovoltaic power system intended for electricity market is reduction of the power generation cost. In order to introduce the solar photovoltaic power system into the electricity market on a full scale, the cost reduction is essential, and it is needed to attain a cost comparing with the cost of conventional thermal power generation or nuclear power generation. However, as reported in an interim report (Jun., 11, 1998) from the Supply and Demand Party of the Advisory Committee for Resources and Energy of the Ministry of Economy, Trade and Industry, the energy cost of the solar photovoltaic power generation is 2.5 to 6 times than the electricity rate in Japan, and a radical cost reduction is needed for full-scale introduction thereof.
In view of such circumstances, particularly with a view to simplify an environmental resistant coating which significantly increases a cost of a solar cell module and an insulating coating of a member for interconnecting solar cells in series and parallel, an object of this invention is to significantly reduce power generation cost and to provide a solar cell string, a solar cell array and a solar photovoltaic power generation system with improved safety of an exposed electric line resulting from the simplification of the environmental resistant coating and the insulating coating.
In order to attain the object described above, the inventors have earnestly studied and obtained the following findings.
As shown in FIGS. 3A and 3B, besides a photovoltaic element which is an essential component for power generation, a conventional solar cell module requires many members including a front cover, a back cover, a frame member and the like. These members are necessary to protect the solar cell module against thermal stress, optical stress and mechanical stress in an outdoor environment and ensure electrical insulation thereof. The inventors has investigated a novel requirement of the solar cell module, which is intended to protect the photovoltaic element against the environmental stresses but not to ensure the electrical insulation so that a charging part thereof is partly exposed. As a result, a first finding has been obtained that the front cover and the back cover can be significantly reduced in thickness, and therefore, a significant cost reduction can be expected. Furthermore, by also excluding from the requirements the insulation performance for the interconnection member for interconnecting the solar cells in series and parallel, a significant cost reduction can be expected.
By excluding the electrical insulation as described above from the requirement, as expected as a matter of course, safety should be ensured because an insulation resistance between the solar cell array circuit and the ground is small, and the electric line for charging the solar cell array is exposed. Therefore, it is a major premise that such a system having such an exposed charging part is installed in a controlled area to which those other than an administrator are forbidden to enter. Furthermore, the inventors have investigated auxiliary means as an additional measure for safety and noted the IEC standard 60479-1: xe2x80x9cEffects of Current on Human Beings.xe2x80x9d According to this, for a direct current, a human body is affected substantially the same in the cases where the current flows from the ground to an upper part of the human body and where substantially double the current thereof flows from the upper part of the human body to the ground. That is, if a resistance of the human body is uniform, the case where a human body on the ground touches a negative potential with a hand is substantially equivalent where the human body touches a positive potential twice as high as the negative potential. The inventors have conceived that this finding is applied to the auxiliary measure for safety of the solar cell array. That is, the inventors have obtained the second finding that in order to improve safety of the exposed electric line as much as possible, grounding is provided at an arbitrary point between an electrical middle point and a negative terminal, and a maximum potential of the electric line with respect to the ground (if the potential is negative, an absolute value thereof) is determined to satisfy the condition that an absolute value of the maximum positive potential is equal to or more than an absolute value of the maximum negative potential, whereby safety can be improved. Specifically, the highest safety can be attained when the grounding is provided at a position where the ratio of the positive terminal to the negative terminal in absolute value of potential with respect to the ground is substantially 2 to 1.
With reference to FIG. 4, the above finding will be described in more detail. A position where the electric line of the solar cell string is grounded can be any position between the negative and positive terminals of the string. The potential of the electric line with respect to the ground is the same as the ground at the grounding point, and the closer to the positive or negative terminal, the higher in positive or negative direction the potential of the electric line becomes, respectively. Therefore, regardless of the grounding point, it can be said that the maximum absolute value of the voltage to ground is attained at one of the negative and positive terminals of the string. Thus, only for the case where a part of a human body comes into contact with the negative and positive terminals, a current flowing between the human body and the ground is to be investigated. If the resistance of the human body is uniform, the current flowing therethrough is in proportion to the amplitude of the potential at a part to be touched. Thus, when the human body touches the positive terminal, the current is zero if the positive terminal is grounded, and is at the maximum if the negative terminal is grounded, as indicated by a solid line in FIG. 4. As described above, in terms of influence on a human body, the current that flows when the human body touches the negative potential is equivalent to substantially double the current that flows when the human body touches the positive potential. Therefore, comparison of the current flowing in case of touching the positive terminal with the current flowing in case of touching the negative terminal in terms of the influence on the human body can be made by doubling the value of the latter current. A broken line in FIG. 4 is a plot of the doubled value of the current flowing when the human body touches the negative terminal. The equivalent current that flows in this case is zero when the negative terminal is grounded, and is at the maximum when the positive terminal is grounded, the maximum value being twice as large as the current flowing in case of touching the positive terminal when the negative terminal is grounded. As can be seen from FIG. 4, at any grounding point, concerning a larger one of the equivalent currents that flow through the human body in case of touching the positive terminal or the negative terminal, the equivalent current is smaller when the grounding is provided between the negative terminal and the middle point than when the grounding is provided between the middle point and the positive terminal. It is also shown that the larger one of the equivalent currents that flow through the human body when the positive terminal or the negative terminal is touched is at the minimum when the grounding is provided at a position where the ratio of the absolute value of the voltage to ground of the positive terminal to that of the negative terminal is substantially 2 to 1 (xe2x80x9cgrounding at a point of ratio 2 to 1xe2x80x9d, in FIG. 4).
As a result of further investigation, the inventors have obtained the third finding that in the case of a plurality of strings, by not grounding all the electric lines of the strings but only one of them, an undesirable loop current or a current flowing over the different strings can be reduced. Now, with reference to FIGS. 5 and 6, this finding will be described in more detail.
FIG. 5 is a schematic circuit diagram of a solar cell array 501. In FIG. 5, a solar cell string is composed of six solar cells 502 interconnected in series and a reverse-current blocking diode 503 connected thereto in series. Five solar cell strings having the same arrangement are interconnected in parallel to form a solar cell array 501 having a positive terminal 504 and a negative terminal 505. Each of the five solar cell strings has one point connected to the ground at a common grounding point 506. With such grounding, a loop current denoted by reference numeral 507 or a current flowing through the different strings, denoted by reference numeral 508, may appear due to variations of characteristics of the solar cells and a condition of a solar radiation. This may cause a reverse bias or current equal to or larger than the rated current in the solar cell. Such an unexpected current can be avoided by grounding only one of the solar cell strings as shown in FIG. 6.
As a result of further investigation, the inventors have obtained the fourth finding that in the case of the utility connected system, an insulation transformer is provided between a power conditioner and a power system to completely separate the solar cell array circuit from the power system circuit in terms of direct current, whereby the power system can be prevented from being affected by a ground-fault current of the solar cell array.
From the above-described findings, the inventors have come to conceive a principal concept of this invention.
A solar cell string of this invention comprises a plurality of solar cells interconnected in series and/or parallel, one positive terminal, and one negative terminal, in which at least a part of an electric line of the solar cell string is not housed in an insulating envelope, and the electric line of the solar cell string is grounded at an electrical middle point between the positive and negative terminals or a point closer to the negative terminal than the electrical middle point.
The solar cell string of this invention, in which the insulating coating of the solar cell or the serial/parallel interconnection member for serial/parallel interconnection is reduced, and a part of the electric line is not housed in the insulating envelope, makes it possible to significantly reduce the cost of material and processing. Besides the major premise that such a solar cell string having an exposed charging part is installed in a controlled area to which those other than an administrator are forbidden to enter, as an auxiliary measure for safety, the electric line of the solar cell string is grounded at the electrical middle point between the positive and negative terminals or a point on the side of the negative terminal with respect to the electrical middle point. In this way, safety can be improved.
In the above-described solar cell string, the electric line of the solar cell string is preferably grounded at a point on the side of the negative terminal with respect to the electrical middle point and on the side of the electrical middle point with respect to the negative terminal, whereby safety can be improved further. More preferably, the electric line of the solar cell string is grounded at a point where a ratio of the positive terminal and the negative terminal in absolute value of voltage to ground is substantially 2 to 1. This can improve safety further. Here, the ratio is described as xe2x80x9csubstantially 2 to 1xe2x80x9d, because when the point of the ratio 2 to 1 lies in an internal circuit of the solar cell, the grounding is provided at a point between the solar cells having a potential closest to that of the point of the ratio 2 to 1.
In addition, at least a part of an electrode or wiring member arranged on a light-receiving surface side of the solar cell is not desirably housed in the envelope for insulating the electric line. This can lead to a significant cost reduction. For further cost reduction, it is desired that the solar cell has a photoelectric conversion layer, a collector electrode and a front wiring member arranged on a light-receiving surface side of the photoelectric conversion layer, and a coating member arranged on the light-receiving surface side of the photoelectric conversion layer, and a part of the collector electrode or front wiring member is a non-coated part where the coating member is absent. The coating member can be formed of a resin material by coating to reduce the cost further.
In addition, at least a part of an electrode or wiring member arranged on a non-light-receiving surface side of the solar cell is not desirably housed in the envelope for insulating the electric line. This can lead to a significant cost reduction. For further cost reduction, it is desired that the solar cell has a photoelectric conversion layer, and a back electrode and a back wiring member arranged on a non-light-receiving surface side of the photoelectric conversion layer, a back support member for supporting the solar cell is arranged on the non-light-receiving surface side of the solar cell, at least a part of a surface of the back support member is made of a non-insulating material, and the surface of the non-insulating material is not insulated from the back electrode or back wiring member.
A serial/parallel interconnection member for interconnecting the solar cells in series and/or parallel can be formed of a conductor having no insulating coating, thereby realizing further cost reduction.
A first solar cell array of this invention comprises a plurality of solar cell strings interconnected in parallel, each of the solar cell strings comprising a plurality of solar cells interconnected in series and/or parallel, and is characterized in that the array further has at least one solar cell string according to this invention. Such a configuration can provide an inexpensive and highly safe solar sell array having the above-described advantages of the solar cell string of this invention.
A second solar cell array of this invention comprises a plurality of solar cell strings interconnected in parallel, each of the solar cell strings comprising a plurality of solar cells interconnected in series and/or parallel, and is characterized in that the array further has only one solar cell string according to this invention. Such a configuration can provide an inexpensive and highly safe solar sell array having the above-described advantages of the solar cell string of this invention can be provided, and the undesirable loop current and the current flowing through the different strings.
A first solar photovoltaic power generation system of this invention is characterized in that the system comprises the first or second solar cell array of this invention, a power conditioner for converting a direct current output of the solar cell array into an alternating current output, and an insulation transformer connected to an output of the power conditioner. Such a configuration can provide an inexpensive and highly safe solar photovoltaic power generation system having the above-described advantages of the solar cell array of this invention. Furthermore, the electric power system can be prevented from being affected by the ground-fault current of the solar cell array.
A second solar photovoltaic power generation system of this invention is characterized in that the system comprises the first or second solar cell array of this invention, an electricity storage apparatus for storing electric energy generated by the solar cell array, and a charging control apparatus connected between the solar cell array and the electricity storage apparatus. Such a configuration can provide an inexpensive and highly safe solar photovoltaic power generation system having the above-described advantages of the solar cell array of this invention.