Energy seems to be the subject at the heart of many of the greatest issues and debates facing the world today. Global warming is a huge issue that promises to change the face of the planet in unimaginable and irreversible ways. It is without question that governments and industry in the developed world must do everything in their power to try and stop this from happening; however this objective is at odds with something of even more importance to those people — money. Cheap energy is the foundation of industry, and industry is the foundation of making money. If energy costs more, then it ends up costing everybody, and this is the reason why almost half of the world’s electricity production comes from burning cheap but environmentally harmful coal (see Figure 1).

World Electricity Generation by Fuel







403% ^________________

Renewables and Other 2.2%

Natural Gas Petroleum 19.7% 0.q%


Ii ii Source: International Energy Agency’s Key World Energy Statistics 2007 Updated: 1/OS

Figure 1: World power generation from various sources [1]

It can be both enlightening and shocking to look at the number of coal-fired power stations both currently operating and the number being built on a daily basis. Figure 2 shows over 100 planned coal-fired power plants in the US alone over approximately the next two decades. China and India are much worse, planning to build 562 and 213 new coal power plants respectively in the 7 year period from 2005 — 2012 alone [2]. These planned coal power plants totally swamp any savings that were to be realised from the Kyoto Protocol, as illustrated by the graph in Figure 3. Many experts believe that it is necessary to not only reduce, but to actually reverse carbon emissions to avoid a major environmental change taking place [3], and instead of this happening the opposite is true.


Figure 2: Planned coal plants in the US by 2030


Figure 3: Savings in greenhouse gas emissions from Kyoto compared to extra emissions from new power stations [2]

Renewable energy is the answer. This is energy that comes from sources that will not expire (ignoring the fact that the sun will die out in a few billion years). It is generally also clean emission free energy as it does not involve burning anything. These sources of energy are great for the environment, however they cost a lot more to set up than more mature but environmentally irresponsible technologies. Some of this extra cost is made up for by the typically lower operating costs due to lack of fuel requirements, and with this come greater security of supply, independence from foreign fuel suppliers and immunity to fuel price fluctuations.

Currently several main renewable energy technologies are in use around the world, and their uptake is slowly on the rise. The currently available technologies or sources of energy are shown in Figure 4, with hydro-electric power on large and small scales making up almost 2/3 of the total.

IntroductionRenewable Energy, end of 2006 (GW)

□ Large hydropower

■ Biotrass heating *

□ Solar colectors for hot water/heating (glazed)*

□ Wind turbines

I Small hydropower

□ Biorrass power

■ Ethanol production **

□ Geothermal healing *

■ Geothermal power

■ Biodiesel production”

□ Solar PV, grid connected H Solar PV, off-grid

■ Concentrating solar thermal power (CSP)

■ Ocean (tidal) power

Figure 4: World Renewable energy sources, 2006 [4]

While solar energy provides a significant portion for hot water heating it is minimal in terms of electricity production. Wind turbines provide a significant amount as well, being a somewhat more established technology than some of the newer techniques such as tidal power.

What is surprising is the minimal amount of energy produced by geothermal energy. There is a lot of potential for energy production by using hot water or steam from beneath the earth’s surface, and it is both renewable and relatively clean as well as being available 24 hours a day, 365 days a year.

It is the aim of this project to design and build a research prototype of a type of heat engine called a Stirling engine which will be capable of generating electricity from sources of hot water such as geothermal water. It will not even require very hot water, being able to run on low temperatures down to just 50 or 60°C. New Zealand has a lot of geothermal energy available, being situated on the ‘Pacific Ring of Fire’ tectonic zone. The Alpine fault line in the South Island and the upper half of the North Island are home to large regions of hot springs with temperatures ranging from 30 — 100°C [5]. Currently some of this hot water is put to direct use although the vast majority is unused.

The central North Island is home to more intense geothermal activity, with high temperature underground systems and visible geothermal phenomena such as geysers resulting from high rates of heat flow and a thinner crustal layer at depths of less than 5 km [5] [6]. This region, known as the Taupo Volcanic Zone, is home to 29 individual geothermal areas and 6 power stations producing around 6% of New Zealand’s electricity.

Other countries with large geothermal potentials include The United States, Australia, China, Philippines, Iceland, Hungary and Mexico.



Volcanically heated groundwater


Ngawha — High temperature

‘ resource — Taupo

__ / volcanic zone

Zone of low temperature springs

Strongly mineralised warm springs


Zone of springs associated with Alpine Fault

Figure 5: Geothermal resource map of New Zealand [5]

The beauty of the Stirling engine in this project is that it is able to run on any source of hot water, not just geothermal heat. This means that there are many areas in which it could be applied, either in scaled-up versions producing grid level electricity, small versions producing local supply or distributed generation, or as direct prime movers for pumps or motors.

One such alternative application could be in an existing installation where hot waste water is a by-product of the process. An independent agency has estimated that if all waste heat was harnessed, it could provide 20% of the entire power generation needs of the Western world [7]. Such installations include but are not limited to geothermal, coal, gas or nuclear power stations, industrial processes and food processing plants. Often this hot water is simply discharged back into a river or lake, which can raise questions about effects on local wildlife. Powering a Stirling engine off the heated water would reduce the injection temperature of the water into the river or lake, as well as being a low cost source of power which could be used to pump the water in question or drive some other load.

Another potential application for a Stirling engine of this type is in the offshore oil and gas drilling industry. In this industry it is common for subsea wells of oil and gas to be located many tens or hundreds of kilometres offshore. They use pumps and equipment which rely on electric power transmitted from somewhere on shore through an ‘umbilical’ cable. At greater distances these become very costly and also inefficient as much power is lost in transmission. Sometimes diesel or gas generation is used on-site, though it is expensive to run and requires a constant fuel supply. One of the features of oil and gas wells is the high temperatures at which the fluids exit the ground. This heat, which is often a nuisance to pumps, could be instead piped through a Stirling engine heat exchanger to power the Stirling engine which would drive the pump and/or generate electric power. The reason this application would be so well suited to a Stirling engine is that the engine would be submerged at the bottom of the ocean, as much as several kilometres deep. The water pressure at these depths would naturally equalise the pressure inside the Stirling engine (and as will be discussed later, the pressure inside the engine directly relates to power output) allowing a large engine to be built and pressurised using relatively inexpensive materials. In addition, the water surrounding the engine is a natural heat sink at a lower than typical temperature of only a few degrees Celsius.

The ability to run on low temperature geothermal water means that vast regions of land are suitable for use with this engine design, rather than just concentrated geothermal hotspots. For instance, as show in Figure 6, at a crust depth of 5 km the entire area of Australia is at a temperature of 50°C or more.


Figure 6: Subterranean temperatures in Australia at a depth of 5 km [8]

In the coming chapters a thorough history and theory of Stirling engines is presented to enable better understanding of what makes them work and how they can best be applied to make use of free or cheap heat sources. The design for the new engine concept is presented and analysed, and some preliminary results are also given.

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