Power plants – Motive fluid energized by externally applied heat – Process of power production or system operation
Reexamination Certificate
2001-11-30
2003-07-29
Nguyen, Hoang (Department: 3742)
Power plants
Motive fluid energized by externally applied heat
Process of power production or system operation
C060S671000
Reexamination Certificate
active
06598397
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention generally relates to a cogeneration system for the supply of electrical power, space heating (SH) water and domestic hot water (DHW), and more particularly to a small scale Rankine-type cogeneration system that utilizes a scroll expander and an organic working fluid.
The concept of cogeneration, or combined heat and power (CHP), has been known for some time as a way to improve overall efficiency in energy production systems. With a typical CHP system, heat (usually in the form of hot air or water) and electricity are the two forms of energy that are generated. In such a system, the heat produced from a combustion process can drive an electric generator, as well as heat up water, often turning it into steam for dwelling or process heat. Most present-day CHP systems tend to be rather large, producing heat and power for either a vast number of consumers or large industrial concerns. Traditionally, the economies of scale have prevented such an approach from being extrapolated down to a single or discreet number of users. However, increases in fuel costs have diminished the benefits of centrally-generated power. Accordingly, there is a potentially great market where large numbers of relatively autonomous, distributed producers of heat and electricity can be utilized. For example, in older, existing heat transport infrastructure, where the presence of fluid-carrying pipes is pervasive, the inclusion of a system that can provide CHP would be especially promising, as no disturbance of the adjacent building structure to insert new piping is required. Similarly, a CHP system's inherent multifunction capability can reduce structural redundancy.
The market for localized heat generation capability in Europe and the United Kingdom (UK), as well as certain parts of the United States, dictates that a single unit for single-family residential and small commercial sites provide heat for both SH (such as a hydronic system with radiator), and DHW (such as a shower head or faucet in a sink or bathtub), via demand or instantaneous system. Existing combination units are sometimes used, where heat for DHW is accumulated in a combination storage tank and boiler coil. In one configuration, SH water circulates through the boiler coil, which acts as the heating element for the water in the storage tank. By way of example, since the storage capacity required for instantaneous DHW supplying one to two showers in a single family residence (such as a detached house or a large apartment) is approximately 120 to 180 liters (roughly 30 to 50 gallons), it follows that the size of the storage tank needs to be fairly large, sometimes prohibitively so to satisfy thermal requirements of up to 25 kilowatts thermal (kW
t
) for stored hot water to meet such a peak shower demand. However, in newer and smaller homes there is often inadequate room to accommodate such storage tank volume. In addition to the need for instantaneous DHW capacity of up to 25 kW
t
, up to 10 kW
t
for SH is seasonally needed to heat an average-sized dwelling.
Furthermore, even in systems that employ SH and DHW into a single heating system to consolidate spacing, no provision for CHP is included. In the example given above, it is likely that the electrical requirements concomitant with the use of 35 kW
t
will be between 3 and 5 kilowatts electric (kW
e
). The traditional approach to providing both forms of power, as previously discussed, was to have a large central electricity generating station provide electricity on a common grid to thousands or even millions of users, with heat and hot water production capacity provided at or near the end-user on an individual or small group basis. Thus, with the traditional approach, the consumer has not only little control over the cost of power generation, as such cost is subject to prevailing rates and demand from other consumers, but also pays more due to the inherent inefficiency of a system that does not exploit the synergism of using otherwise waste heat to provide either additional electric generation or heating capacity.
Large-scale (in the megawatt (MW) range and up) cogeneration systems, while helpful in reducing the aforementioned inefficiencies of centrally-based power generation facilities, are not well-suited to providing small-scale (below a few hundred kW) heat and power, especially in the small-scale range of a few kW
e
and below (micro-based systems) to a few dozen kW
e
(mini-based systems). Much of this is due to the inability of the large prime mover systems to scale down, as reasonable electrical efficiency is often only achieved with varying load-responsive systems, tighter dimensional tolerances of key components and attendant high capital cost. Representative of this class are gas turbines, which are expensive to build for small-scale applications, and sacrifice efficiency when operating over varying electrical load requirements. Efficiency-enhancing devices, such as recuperators, tend to reduce heat available to the DHW or SH loops, thus limiting their use in high heat-to-power ratio (hereinafter Q/P) applications. A subclass of the gas turbine-based prime mover is the microturbine, which includes a high-speed generator coupled to power electronics, could be a feasible approach to small-scale cogeneration systems. Other shortcomings associated with large-scale CHP systems stem from life-limited configurations that have high maintenance costs. This class includes prime movers incorporating conventional internal combustion engines, where noise, exhaust emissions, lubricating oil and spark plug changes and related maintenance and packaging requirements render use within a residential or light commercial dwelling objectionable. This class of prime mover also does not reject a sufficient amount of heat for situations requiring a high Q/P, such as may be expected to be encountered in a single family dwelling. Other prime mover configurations, such as steam turbines, while generally conducive to high Q/P, are even less adapted to fluctuating electrical requirements than gas turbines. In addition, the steam-based approach typically involves slow system start-up, and high initial system cost, both militating against small-scale applications.
In view of the limitations of the existing art, the inventors of the present invention have discovered that what is needed is an autonomous system that integrates electric and heat production into an affordable, compact, efficient and distributed power generator.
BRIEF SUMMARY OF THE INVENTION
These needs are met by the present invention, where a new micro-CHP system is described. In micro-CHP, a compact prime mover can provide both electric output, such as from a generator coupled to a heat source, as well as heat output to provide warm air and hot water to dwellings. What distinguishes micro-CHP from traditional CHP is size: in the micro-CHP, electric output is fairly small, in the low kW
e
or even sub-kW
e
range. The system of the present invention can provide rapid response to DHW requirements, as the size of tanks needed to store water are greatly reduced, or possibly even eliminated. The size of the micro-CHP system described herein can be adapted to particular user needs; for example, a system for a single-family dwelling could be sized to produce approximately 3 to 5 kW
e
, 10 kW
t
SH and 25 kW
t
DHW. For small commercial applications or multi-dwelling (such as a group of apartment units) use, the system could be scaled upwards accordingly. The heat to power ratio, Q/P, is an important parameter in configuring the system. For most residential and small commercial applications, a Q/P in the range of 7:1 to 11:1 is preferable, as ratios much lower than that could result in wasted electrical generation, and ratios much higher than that are not practical for all but the coldest climates (where the need for heating is more constant than seasonal). Since the production of electricity (through, for example, a generator or fuel cell) is a byproduct of the prime mover heat generation process,
Anson Donald
Coll John Gordon
Hanna William Thompson
Stickford, Jr. George Henry
Energetix Micropower Limited
Killworth, Gottman Hagan & Schaeff LLP
Nguyen Hoang
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