cooling's 1.35, and allow for up to 100 KW of computing power (heat dissipation, TDP) per 19-inch rack, as opposed to air cooling, which usually handles up to 23 KW. Waste heat reduction Where powerful computers with many features are not required, less powerful computers or ones with fewer features can be used. a VIA EPIA motherboard with CPU typically dissipates approximately 25 watts of heat, whereas a more capable Pentium 4 motherboard and CPU typically dissipates around 140 watts. Computers can be powered with direct current from an external power supply unit which does not generate heat inside the computer case. The replacement of cathode ray tube (CRT) displays by more efficient thin-screen liquid crystal display (LCD) ones in the early twenty-first century has reduced power consumption significantly. Heat-sinks A component may be fitted in good thermal contact with a heatsink, a passive device with large thermal capacity and with a large surface area relative to its volume. Heatsinks are usually made of a metal with high thermal conductivity such as aluminium or copper, and incorporate fins to increase surface area. Heat from a relatively small component is transferred to the larger heatsink; the equilibrium temperature of the component plus heatsink is much lower than the component's alone would be. Heat is carried away from the heatsink by convective or fan-forced airflow. Fan cooling is often used to cool processors and graphics cards that consume significant amounts of electrical energy. In a computer, a typical heat-generating component may be manufactured with a flat surface. A block of metal with a corresponding flat surface and finned construction, sometimes with an attached fan, is clamped to the component. To fill poorly conducting air gaps due to imperfectly flat and smooth surfaces, a thin layer of thermal grease, a thermal pad, or thermal adhesive may be placed between the component and heatsink. Heat is removed from the heatsink by convection, to some extent by radiation, and possibly by conduction if the heatsink is in thermal contact with, say, the metal case. Inexpensive fan-cooled aluminium heatsinks are often used on standard desktop computers. Heatsinks with copper base-plates, or made of copper, have better thermal characteristics than those made of aluminium. A copper heatsink is more effective than an aluminium unit of the same size, which is relevant with regard to the high-power-consumption components used in high-performance computers. Passive heatsinks are commonly found on