NOVELTY - An elongated curved mirror, for use in a two-axis solar concentrator, comprises a flat reflective flexible substrate shaped to be substantially cylindrically curved in one direction about a widthwise axis, where the substrate is supported at least at its lengthwise ends by a substantially continuous mechanical support.    USE - The elongated curved mirror is used in a two-axis solar concentrator. The system is used for supporting densely-packed two-axis trackers, for controlling photovoltaic power, for supporting a row of an even number of curved mirror structures, and for concentrating solar energy in dimensions onto a receiver (all claimed).    ADVANTAGE - The present invention provides cost-effective systems for using large reflective elements that track the sun on two axes to concentrate solar energy onto a receiver that can convert the sun's optical energy to a form usable for extensive displacement of combustion of fossil fuels. The structures of the tracker frame, tracking mechanism and tracker supports are co-optimized with the optical elements and the receiver for high efficiency, low cost, and ease of assembly, making moderate and large-scale implementations cost-competitive with fossil fuels for peak power, and, with suitable storage, for base-load power and dispatchable peaking power in sunny locations. Improvement to small-tracker two-axis systems and one-axis tracking systems that focus in two dimensions are also included, as are improvements in systems for space-based solar power.    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are:    (1) a two-axis solar concentrator that directs and concentrates sunlight directly and/or indirectly via a secondary concentrator onto a receiver with a concentration greater than 500 suns, the concentrator comprising: mirrors, each one of the mirrors; a frame supporting the mounting base of the mirrors to focus light onto the receiver with focal spots of the mirrors substantially overlapping; and a two-axis mounting for supporting and orienting the frame with respect to the earth;    (2) a concentrated solar photovoltaic receiver comprising electrically conductive heat exchange tubes arranged side by side and in electrically insulated groups; and photovoltaic cells having a backplane and a front surface, the backplane having a first electrical contact and the front surface having a second electrical contact, in operation, at a potential with respect to the first electrical contact, the cells being mounted with the backplane in good thermal contact with at least one of the tubes, and the cells being electrically connected in series such that the groups of tubes are, in operation, at different electrical potential with respect to one another;    (3) a photovoltaic cell adapted to receive light at a front surface, the cell being characterized by a light absorption enhancement structure comprising one or more of: a raised, substantially triangularly shaped raised profile on cell contacts provided on the front surface to increase light reaching a photoreceptive body of the cell that is otherwise lost to the cell contacts: and an off-axis silicon layer sliced from a silicon ingot at an angle such that the lattice constant of the silicon ingot divided by the cosine of the angle is substantially equal to the lattice constant of a material of a non-silicon junction of the cell to allow junction materials that are lattice matched to each other but not to silicon to be used in a multi-junction cell able to absorb more light than a single junction cell;    (4) a solar tower system comprising a large number of two axis heliostats each directing sunlight onto a tower receiver, the system being characterized by receiver towers cooperating with the heliostats by one or more of: at least some of the heliostats are controlled to change from directing sunlight from one of the receiver towers to another of the receiver towers as a function of a position of the sun so as to maximize an amount of light reflected from the at least some of the heliostats reaching the receiver; at least some of the heliostats have a field-adjustable focus to allow them to focus tightly when being switched from directing sunlight from one receiver tower to another; and at least some of the heliostats located near a base of one of the receiver towers reflect light across one side of the one of the receiver towers to reach another of the receiver towers, where the at least some are within the azimuthal angular range of heliostats focusing on the receiver tower;    (5) a system for supporting densely-packed two-axis trackers, comprising a substantially rectilinear array of poles or lattice towers, with the poles or lattice towers interconnected by trusses substantially parallel to the rows and columns of the substantially rectilinear array, where each of the two-axis trackers is supported at one end by a first one of the trusses, and at its other end b a second one of the trusses that is substantially parallel to and adjacent to the first one of the trusses, in a manner that allows the tracker to rotate relative to the first and second trusses;    (6) a method for controlling photovoltaic power generated by a parking lot canopy structure providing shade within a parking lot area located in a location exposed to insulation, the method comprises connecting motor vehicles each having a rechargeable battery to a source of photovoltaic power generated by the structure; automatically assessing in accordance with predetermined logic an availability of excess power generated from the structure that is not required to supply an adjacent building or power grid; and switching power to the motor vehicles as a function of the assessing;    (7) a system for supporting a row of an even number of curved mirror structures, that each focus sunlight in two dimensions, on top of a single pole or lattice tower, at a height that is within 2% of the width of the aperture of the mirror structures from the height of the center of gravity of the mirror structures, without requiring a gap in or between the mirror structures for the pole or lattice tower to pass through when tracking the sun through a range of altitudes;    (8) a system for concentrating solar energy in two dimensions onto a receiver, where the system comprises a primary concentrator that concentrates the solar energy substantially only in a first dimension and is rotated substantially only in that dimension to track the sun in the first dimension, the receiver being adapted to track the sun in a second dimension, preferably orthogonal to the first dimension; and    (9) a system for concentrating solar energy in one dimension comprising a set of parabolic trough mirror that all focus onto a common substantially linear focus, where each of the parabolic troughs has an adjustable focal length to match elevation.