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Alternatives for Alternative Energy

1.Introduction

"Alternative energy" has a different meaning depending on your perspective. Many sources treat "alternative" as a synonym for "renewable" or "clean". While not completely disagreeing with this, a somewhat wider definition is proposed herein:

Alternative energy is a source of energy that is environmentally cleaner and potentially more renewable than traditional utility generation.

This white paper is intended for electric utilities, facility energy managers and possibly partnerships between a facility energy manager and the electric utility or ESCO serving that facility.

The most popular forms of alternative energy are wind and solar, and these are becoming less costly than some traditional generators, and are also rapidly expanding their generation market-share.

This paper is not about wind or solar. Neither is it about geothermal nor hydroelectric generation. Instead it is about alternative methods of electric generation (per the above definition) that the reader might consider when wind, solar, geothermal and hydro all have shortcomings that make them unsuitable. Wind and solar are intermittent. If the application requires the ability to dispatch the generation, storage would need to be added to wind and solar to provide this capability. Storage is still expensive, although its price is rapidly decreasing. Wind and solar also require large amounts of minimally used land (or roof-surfaces in the case of solar). Geothermal and hydro have very unique site requirements, and thus we will assume that very few sites are suitable.

We will also not consider very large generators (larger than 5 MW). The following information is intended for those looking for point-solutions that can support one or a few facilities or a medium-to-small-small electric distribution system serving a number of small consumers.

2.Renewable Fuel Production and Procurement

One method of making combustion-based generation renewable is by making your own fuel in a renewable (and clean) fashion. Another method is by procuring such a fuel. These methods are explored below.

2.1.Waste- and Biomass-to-Energy

Waste and biomass are two similar and overlapping fuels: most agricultural and much other waste is biomass. There several technologies using these fuels as seen in the chart below.[1]

The first three technologies are primarily used for waste, but can also be used for biomass like dried stover, wood, and many types of dried grasses. The last technology can be used for most types of biomass, but also for organic waste. Many technologies have secondary energy/products that can be stored or used by alternative processes (vehicles or stationary engines), but all can use the full process to produce electricity. With proper pre-treatment and after-treatment all can have low conventional emissions.

All four technologies have many projects. Combustion, pyrolysis and biochemical conversion are widely used. Plasma gasification is typically used to process waste containing hazardous materials, and a given project may or may not produce energy and/or fuel-gasses.

2.2.Biomethane

Biomethane (a.k.a. renewable natural gas) comes from biogas, which is generally produced by anaerobic digestion of bio-waste (last sub-process in the above figure) and also includes landfill gas and biogas from wastewater processing. Biomethane is produced by purification of biogas to a composition that meets the specifications of a pipeline transmission system. In other words it’s not significantly different in composition than any other “natural gas”.

There are a set of recently-defined regulations in California that allow biomethane to be delivered via the public gas transmission pipeline network. This will greatly expand the number of sources and destination-consumers that can use biomethane. I looked for additional states that have or are considering similar regulations, but could find none. However, since biomethane is nearly identical in composition to purified natural gas that comes from wells, it could be that other states or transmission networks already allow pipeline delivery.

3.Alternative Energy Generators

Note that the term "gen sets" is used for combustion-fueled engines and electric generators sets in this section.

3.1.Reciprocating Engine-Generators

Reciprocating engines provide much of the electricity in the world, especially in remote areas. These are mainly diesel gen sets. In the U.S., due to increasingly stringent environmental requirements, diesels are frequently limited to back-up service. Natural-gas (spark-ignition) gen sets use automotive technology, including emission controls, and thus can meet the most stringent emissions requirements. Also gas-powered gen sets tend to be smaller and quieter than diesel generators. The largest of these are typically 400 kW, and noise emission is 71 dB at 7 meters for a Kohler 400REZXB, 400 kW gas generator with a Weather and Sound Enclosure. Thus it is much easier to get them approved. Other advantages of natural gas powered units include:

Gas gen sets are typically less expensive than diesel gen sets. The installed price of gas generators is around $0.40 per watt for a 150 kW gen set.

  • No diesel fuel storage tanks or issues with stale fuel
  • Gas gen sets can be fueled by natural gas or liquefied petroleum gas.
  • Gas generators can use biomethane or biogas (with modifications), making them potentially renewable.

One characteristic that diesel and gas gen sets share is their very fast start and ramp response, making diesels very good for back-up applications, and gas gen sets very good for both backup and firming-up variable renewables.

3.2.Steam Gen Sets

Steam gen sets can use natural gas fired boilers, but their boilers can also be fired by just about any fuel, including biomass, industrial process waste, municipal waste or office-waste. Industrial steam gen sets have designs ranging from 50 kW to 50 MW. Steam gen sets are very useful in facilities that already use steam for industrial processes, environmental heating and/or cooling processes. In such cases steam gen sets are frequently used at the front-end (between the boiler and the process) or back-end (between the process and the condenser) of such processes. In many of these cases the application is combined heat (for environmental heat or another process) and power (CHP) or combined cooling, heat and power (CCHP).

Efficiency depends on the specifics of an application, but is generally in the range of 10% to 30% (note that this includes the thermal-efficiency of the steam generation and condensation). CHP or CCHP applications can provide up to 90% efficiency.

Other advantages of steam gen sets include:

Most boilers achieve maximum efficiency at around 50%-load, and the efficiency decreases with reduced load, ending up below zero if the boiler is banked (no steam output, but kept hot). Operating a steam turbine to generate power on the low end of the efficiency curve increases overall steam-output efficiency and results in the most efficient power generation.

Backpressure turbines can be used where the pressure of steam coming from a boiler is too high for the process. The normal practice is to use pressure reducing valves, but this wastes the energy removed during the pressure reduction. Per DOE backpressure turbines can be considered where a pressure reducing valve has constant steam-flow of at least 3,000 pounds per hour and when the steam pressure drop is at least 100 psi.

3.3.Microturbines

Microturbines were derived from automotive turbochargers, air transport auxiliary power units and small aircraft jet engines. They offer power ratings from 25 to 500 kW, and often several microturbines are used in a given facility resulting in better availability and simpler logistics.

Microturbines are in a single enclosure that is relatively compact and lightweight. For instance, the enclosure for a Capstone C200 Microturbine (200 kW electrical output) is 67” W x 150” D x 98’ H and weighs 6,000 to 7,500 lbs. (depending on options).

The simplest microturbines are also the least expensive (less than $0.70/watt for the hardware only), but also the least efficient (around 15%). Microturbines with recuperators (heat exchangers that transfer heat from the exhaust to the inlet) approach the efficiency of small industrial gas turbines (around 30%) but are somewhat more complex and expensive ($1.10/watt for the hardware only).

Microturbines can be used in a combined heat and power (CHP) or combined heat cooling and power (CCHP) application. Depending on the demand for electricity and heat, CHP/CCHP can use the simplest microturbines, which will provide more heat than a unit equipped with a recuperators. In a CHP application the overall efficiency can be as high as 85%.

Another advantage of microturbines is that they are reasonably responsive. They can go from a cold start to full power in a few minutes. They can ramp up or down 30% in under 30 seconds.

3.4.Fuel Cells

Most commercial fuel-cells use natural gas, which is reformed into hydrogen. Advantages fuel cells have over PV (and wind) include:

They do not require large outside areas with unobstructed line-of-site to most of the sky.

Their output can be controlled.

They can generate heat for combined heat and power (CHP) and combined cooling heat and power (CCHP) applications.

Fuel cells are eligible for renewable self-generation incentives in some states (at least in California), even if they use non-renewable natural gas. The table below compares the project costs for fuel-cells, against other comparable sources.

Generator             Min Project Size     $/Watt        Intermittent            Heat Source

Photovoltaic         5 kW                       $2.00*        Yes                         No

Fuel Cell               50 kW                     $6.00*        No                          Yes

Microturbine         50 kW                     $2.00*        No                          Yes

*Complete installed project cost for a medium project (>100 kW) in 2015.

3.4.1.Fuel Cell Technologies

Most renewables use different competitive technologies, but with fuel cells the nature of a fuel cell’s technology strongly impacts how that fuel cell can be used. The following are the technologies currently being used for stationary fuel cells (descriptions below are directly from referenced source[2]):

3.4.1.1.Polymer Electrolyte Membrane Fuel Cells (PEMFC)

Polymer electrolyte membrane (PEM) fuel cells, also called proton exchange membrane fuel cells, use a proton conducting polymer membrane as the electrolyte. Hydrogen is typically used as the fuel. These cells operate at relatively low temperatures and can quickly vary their output to meet shifting power demands. PEM fuel cells are the best candidates for powering automobiles. They can also be used for stationary power production. However, due to their low operating temperature, they cannot directly use hydrocarbon fuels, such as natural gas, LNG, or ethanol.  These fuels must be converted to hydrogen in a fuel reformer to be able to be used by a PEM fuel cell.

3.4.1.2.Direct-Methanol Fuel Cells (DMFC)

The direct-methanol fuel cell (DMFC) is similar to the PEM cell in that it uses a proton conducting polymer membrane as an electrolyte. However, DMFCs use methanol directly on the anode, which eliminates the need for a fuel reformer. DMFCs are of interest for powering portable electronic devices, such as laptop computers and battery rechargers. Methanol provides a higher energy density than hydrogen, which makes it an attractive fuel for portable devices.

3.4.1.3.Alkaline Fuel Cells (AFC)

Alkaline fuel cells use an alkaline electrolyte such as potassium hydroxide or an alkaline membrane that conducts hydroxide ions rather than protons. Originally used by NASA on space missions, alkaline fuel cells are now finding new applications, such as in portable power.

3.4.1.4.Phosphoric Acid Fuel Cells (PAFC)

Phosphoric acid fuel cells use a phosphoric acid electrolyte that conducts protons held inside a porous matrix, and operate at about 200°C. They are typically used in modules of 400 kW or greater and are being used for stationary power production in hotels, hospitals, grocery stores, and office buildings, where waste heat can also be used. Phosphoric acid can also be immobilized in polymer membranes, and fuel cells using these membranes are of interest for a variety of stationary power applications.

3.4.1.5.Molten Carbonate Fuel Cells (MCFC)

Molten carbonate fuel cells use a molten carbonate salt immobilized in a porous matrix that conducts carbonate ions as their electrolyte. They are already being used in a variety of medium-to-large-scale stationary applications, where their high efficiency produces net energy savings. Their high-temperature operation (approximately 600°C) enables them to internally reform fuels such as natural gas and biogas.

3.4.1.6.Solid Oxide Fuel Cells (SOFC)

Solid oxide fuel cells use a thin layer of ceramic as a solid electrolyte that conducts oxide ions. They are being developed for use in a variety of stationary power applications, as well as in auxiliary power devices for heavy-duty trucks. Operating at 700° to 1000°C with zirconia-based electrolytes, and as low as 500°C with ceria-based electrolytes, these fuel cells can internally reform natural gas and biogas, and can be combined with a gas turbine to produce electrical efficiencies as high as 75%.

3.4.2.Applications of Different Technologies

Low-temperature technologies (PEMFC, DMFC and AFC) have a much quicker startup and are much more responsive than high-temperature designs (MCFC and SOFC). However low-temperature designs can only be fueled by relatively pure hydrogen with few contaminants, whereas high temperature designs can accept a wider range of fuels. High temperature designs are best suited to combined heat and power (CHP), or combined cooling heat and power (CCHP) applications. PAFC is a medium-temperature design and thus combines some characteristics of high-temperature and low-temperature designs.

4.Electric Generation and Heat

The subsections below describe two technologies for using heat in addition to generating power.

4.1.Combined Cooling Heat and Power

This subsection will cover combined heat and power (CHP), and combined cooling, heat and power (CCHP).

As with any thermal cycle, the efficiency is the useful energy produced divided by the maximum amount of thermal energy the input fuel could generate (heat content), converted to a percentage.

A standard cubic foot (SCF) of natural gas has a heat content of about 1,000 BTUs. If a process only producing electricity consumed one SCF of natural gas per hour, and generated 293 watts it would be 100% efficient (1,000 BTUs/hr. = 293 watts).

Most thermal electricity generation methods producing less than 5 MW are 10% to 40% efficient. Most of the wasted energy from the process is emitted as exhaust heat, and if this heat is used in another process (such as environmental heating), then the total efficiency is:

Total Efficiency = (Electric output + heat applied to process) / input fuel heat content

Thus, the total efficiency is much closer to 100% than if the process just produced electricity. Typically CHP has an efficiency of 60% to 90%.

CHP technologies can have the following configurations:

A manufacturing process produces waste heat at a high enough temperature (300˚C to 700˚C) to produce steam that will drive a steam turbine producing electricity. For lower temperatures see the next subsection on recovered heat generators.

An electric generator is driven by prime mover that produces process heat via its exhaust. This is typically a gas turbine, micro-turbine, some types of fuel cells or less commonly a reciprocating engine.

A boiler that produces process steam that is also used to generate electricity via a steam turbine.

Cooling can be provided using an absorption refrigeration / chiller, which uses waste heat to produce, respectively, chilled air or water. However this is generally only cost-effective if the heat used would otherwise be wasted.

4.2.Recovered-Heat (Organic Rankine-Cycle) Generator

This is a technology that is only applicable to industrial processes with a low-quality (temperature 50°F to 100°F over ambient) waste-heat output. The Organic Rankine cycle can use a relatively low-temperature heat source (150˚F to 200˚F) and still produce useful power. Industrial processes that have waste-heat streams that can use this generation include gas compressor stations, incinerators, and processes at paper mills, oil refineries, cement factories, chemical plants, glass plants and metal refineries.

5.Simulation and Microgrids

Many applications require (or benefit-from) using several different electric generation technologies. Others require a combination of generators and storage, or combined heat and power. The only way to define if such an application is cost-effective, and which components will result in the most effective solution is through computer simulation.

Complex applications with multiple generators and storage and multiple objectives (like minimizing electric costs plus providing long-term back-up power) are frequently called microgrids. These require simulation as described above to define the best design, plus an optimizing microgrid controller during operation to assure the process meets the owner's goals.

 

[1] Jerry Davis, Scott Haase, and Adam Warren, NREL, “Waste-to-Energy Evaluation: U.S. Virgin Islands”, Technical Report NREL/TP-7A20-52308, August 2011, http://www.nrel.gov/docs/fy11osti/52308.pdf

[2] U.S. Department of Energy (Energy.gov) – Fuel Cell Basics, http://energy.gov/eere/energybasics/articles/fuel-cell-basics

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