Most gas turbines in service today are of the “simple cycle” type, which means they typically consist of only a compressor, a combustor (burner), and a turbine. In simple cycle engines, the exhaust leaving the turbine is still very hot. This rejection of unused heat to the atmosphere represents a waste of energy. For this reason, some gas turbines employ a heat exchanger, such as a recuperator or regenerator, to recover waste heat from the turbine exhaust. This heat can be used to preheat the air leaving the compressor, before it enters the burner. Thus, less fuel is needed to heat up the air to its target temperature, making the engine more efficient. Recuperated and regenerated engines are common and well known to those skilled in the art.
However, despite their advantages, recuperators are rarely used in gas turbines due to their cost and weight. Current recuperators typically use expensive metal alloys with high temperature oxidation and creep rates that limit their use to certain maximum temperatures. Other current recuperators use stainless steel, which is cheaper than some other metal alloys but has an even lower temperature limit. In some instances, to overcome these temperature limits, ceramic recuperators have been used. However, prior art ceramic recuperator designs have typically suffered from one or more of the following weaknesses: 1) relatively large size and weight; 2) a box-shaped design that can require complex ducting of the gases; 3) the need for a large amount of insulation, both around the inlet/outlet ducting and surrounding the hot parts of the heat exchanger itself, to prevent excessive heat losses; 4) insufficient accommodation for thermal stresses; and 5) fabrication from ceramic materials with problematic attributes.
Common materials used in ceramic recuperators have been magnesium aluminum silicate (cordierite), lithium aluminum silicate (LAS), silicon carbide, and silicon nitride. Cordierite and LAS can be advantageous due to their low cost and low thermal expansion coefficients; however, they have relatively low strength and low fracture toughness. Worse, the magnesium and lithium in these materials are prone to corrosion by compounds in the exhaust, resulting in short service lives. Silicon carbide is typically stronger and more corrosion-resistant, but still not very tough, and can be corroded by water vapor in the exhaust if the concentration is high enough. In addition, silicon carbide has a very high thermal conductivity, which increases heat conduction in undesirable directions and thus reduces the heat exchanger effectiveness. (Heat exchangers typically have very thin walls, which impose little resistance to heat transfer in the desired through-thickness direction, regardless of the wall conductivity.) Finally, silicon nitride can be strong, tough, and only moderately conductive; however, it has not been used on a widespread basis because it is very expensive, due to both the raw material cost and the expensive manufacturing processes needed to produce finished parts. Secondarily, silicon nitride, like silicon carbide, is prone to erosion by water vapor in the exhaust.
In the prior art, regenerators have been made from ceramic materials and used in gas turbines for many decades. In general, a recuperator is a heat exchanger with manifolds that distribute gases to alternating air channels, as opposed to a regenerator, which is a periodic flow device that exposes a heat storage medium such as a ceramic honeycomb to alternating flows of compressed air and turbine exhaust. Typically, recuperators are stationary devices that are mechanically simpler, but geometrically more complex, than regenerators are.
Regenerators are straightforward to manufacture because they typically employ rotating disks that are simple honeycomb-like ceramic extrusions. The disk rotates past seals, so that one side is exposed to a duct of exhaust gases flowing through axially in one direction, which cool down as they flow through. The other side is typically exposed to an air duct flowing through in the other direction, which heats up as it flows. The ceramic matrix assumes a relatively stable temperature gradient, in much the same way as a counterflow heat exchanger. Regenerators can have very small internal channels; and, as a direct result, their effectiveness can be very high relative to the size and weight of the ceramic component. However, the typical problem with regenerators is that there are moving parts and sliding seals, both of which have to operate at high temperatures. The moving parts complicate the system, reducing reliability due to the wear of the moving parts. In addition, although the ceramic matrix itself is lightweight for a given effectiveness, the associated mechanical components, ducts, seals, and insulation substantially increase the weight and volume of the system. Finally, regenerators have typically been made from cordierite or LAS, and thus have suffered the same corrosion and durability problems mentioned above. For these reasons, a fixed-surface compact radial counterflow recuperator design is preferred wherever feasible and practical. It is only the fact that no ceramic recuperator is available with equally tiny internal channels, consequently high effectiveness per unit weight, and yet low manufacturing cost, that explains why regenerators are still commercially competitive.
In summary, prior art heat exchangers are typically disadvantageous in size and weight, and are typically made from materials that are undesirably expensive, temperature-limited, prone to corrosion, weak, or insufficiently tough. Furthermore, the heat exchangers typically suffer from pressure and heat losses and are prone to thermal stresses that can cause reliability issues. Accordingly, there remains a need in the art for a heat exchanger that can overcome these and other limitations.