Pellet stove

A pellet stove is a stove that burns compressed wood or biomass pellets to create a source of heat for residential and sometimes industrial spaces. By steadily feeding fuel from a storage container (hopper) into a burn pot area, it produces a constant flame that requires little to no physical adjustments. Today’s central heating systems operated with wood pellets as a renewable energy source can reach an efficiency factor of more than 90%.

Principles of operation
A pellet stove normally consists of these components, whether basic or complex:

A hopper
An auger system
Two blower fans: combustion and convection
A firebox: burn pot and ash collection system, sometimes lined with ceramic fiber panels
Various safety features (vacuum switch, heat sensors)
A controller

To properly function, a pellet stove uses electricity and can be connected to a standard electrical outlet. A pellet stove, like an automatic coal stoker, is a consistent heater consuming fuel that is fed evenly from a refillable hopper into the burn pot (a perforated cast-iron or steel basin), through a motorized system. The most commonly used distributor is an auger system that consists of a spiral length of metal encased in a tube. This mechanism is either located above the burn pot or slightly beneath and guides a portion of pellet fuel from the hopper upwards until it falls into the burn pot for combustion.

Fan systems are necessary for clean, economical performance. The flame produced is concentrated and intense in the small area of the burn pot as a combustion blower introduces air into the bottom of the burn pot, while also forcing exhaust gases into the chimney. While some pellet stoves will be hot to the touch (especially on the viewing window), most manufacturers utilize a series of cast-iron or steel heat exchangers that run along the back and top areas of the visible firebox. With a convection blower, room air is circulated through the heat exchangers and directed into the living space. This method allows for a much higher efficiency than the radiant heat of a hand-fed wood or coal stove, and will in most cases cause the top, sides, and back of the stove to be at most warm to the touch. Along with convection air, an exhaust fan forces air from the firebox through special venting specifically made for pellet fuel. This cycle of circulation is an integral part of the combustion system as well, for the concentrated high temperature flame will quickly overheat the firebox. The possible problems associated with overheating are electrical component failure and flames traveling into the auger tube causing a hopper fire. As safeguards, all pellet stoves are equipped with heat sensors, and sometimes vacuum sensors, enabling the controller to shut down if an unsafe condition is detected. For daily maintenance, an ash vacuum is recommended. These are similar to shop vacs, but are designed for the removal of ash materials. These vacuums are available with a pellet stove kit which enables the cleaning of the interior areas of the stove which improves efficiency.

Pellet stoves can either be lit manually or through an automatic igniter. The igniter piece resembles a car’s electric cigarette lighter heating coil. Most models have automatic ignition and can be readily equipped with thermostats or remote controls.

The pellet fuel is delivered from the storage facility or the day tank (single stoves) into the combustion chamber. With the heat generated, circuit water is heated in the pellet boiler. In central heating systems the hot water then runs through the heating circuit. The heat distribution is the same as other central heating systems. Unlike oil or gas heating, the inclusion of a hot water reservoir is recommended with pellet heating systems to save hot water until it is needed.

In principle, pellet heating systems are to be distinguished as installations for the operation of central domestic heating systems including control and regulation technology (ie pellet central heating systems) of individual pellet stoves with direct heat emission into the living space.

Single ovens
Pellet single ovens are usually systems in the power range of max. 6-8 kW and less, which are placed directly in the living room. They usually have a small fuel storage tank that holds one to a few daily rations. Fuel supply and the control of combustion are controlled automatically, the ash discharge is done by hand. The heat is usually emitted directly to the room air. Water-bearing stoves use some of the energy to heat water, which is located in pockets around the combustion chamber. Thus, the performance of an existing central heating can be supported or replaced in individual cases. The range of individual pellet stoves is as varied as that of log firing ovens, and versions with viewing windows are also offered, which open up the view of the fire.

Central heating
Pellet central heating systems are eligible for nominal heat output or the heat requirement (= so-called heating load, calculation according to EN 12 831) from 3.9 kW upwards. Pellet heaters are not only suitable for use in detached and semi-detached houses (up to approx. 30 kW), but also for larger living or operating units, which are supplied by larger systems or by several in-line pellet heating systems (cascade systems) with a few hundred kW can. Hybrid or combination plants can also be charged with other firewood (such as firewood or wood chips).

Pellet burner systems run optimally under full load and can be regulated up to about 30% of the rated output. Since the duration of the energetically less efficient warm-up phase is longer with pellet heaters than with oil or gas firing, short burning phases have a negative effect on the fuel efficiency. An increase in energy efficiency and a reduction in pollutant emissions is therefore achieved by combining pellet heaters with heat buffer systems, generally via water tanks.

Cultivation pellet burner
Separate pellet burners, which are mounted on an existing oil or wood boiler, are offered as a low-cost alternative to a complete heating conversion. However, combustion efficiency is reduced in these solutions. Unlike pellet-tuned heaters or furnaces, investment costs are not supported by public funds.

Similar to woodchip heaters, the fuel is delivered periodically and automatically introduced by means of feeders from the pellet warehouse (heating systems) or the day tank (individual ovens) as needed in the combustion chamber. For combustion in wood heaters usual combustion chambers are used. With the heat generated in pellet central heating, water from the heating circuit in the pellet heating boiler is heated. Heat distribution is the same as with other central heating systemsthrough the heated water. Unlike with oil or gas heaters, the integration of a hot water tank into the heating system makes sense in pellet heating systems, which stores the heat generated in the firing process with little loss until heat is demanded by the heating system.

The firing system designed as a fixed bed reactor is automatically supplied with firing material. The control technology of the plant supplies the fuel incrementally in the amount that corresponds to the required heat output. Depending on the version, the supplied wood pellets are either ignited automatically with hot-air blowers, or a firing bed is permanently obtained in the combustion chamber.

Wood Pellet Heaters work with different feeding techniques. Today, the chute or pellet firing system specially developed for pellet burning, underfeeding firing, cross insert firing or the use of a roller grate system are currently in use.

When Fallschachtfeuerung the pellets slide down a chute into a burner pot. By using a burner pot, the combustion range is precisely defined, therefore combustion can be precisely controlled.
When Underfeed the pellets by means of a feed screw pressed from below of a burner plate, burned there and the remaining ash falls outside the box into the underlying ash container.
The transverse insert firing works in a similar way to the underfeed firing, except that the fuel is pushed from the side onto the firing plate via a screw conveyor. In this case, both the burner plate and the air supply to adapt to partial services can be specially shaped.
In the roller grate system, the pellets fall from above onto several, slowly rotating steel discs with a small gap. A scraper comb cleans the interspaces with each revolution, so that likewise the ash can fall freely downwards and combustion air can be fed upwards.
Picture: Partitions burner of the company SOLARvent
In the case of the fall fire technology, however, the pellets fall from above onto a grate in a combustion chamber. The flames are pulled down with the help of a suction fan through the grate. This system produces the least amount of ash.

In order to optimize the efficiency and pollutant content of the exhaust air, modern pellet burners control the combustion either via a temperature or flame chamber sensor in conjunction with a continuously adjustable combustion air supply or a lambda sensor via an induced draft fan. The hot combustion gases are fed into the chimney via a heat exchanger with manual or automatic cleaning of the reheating surfaces or turbulators (also called turbulators).

The resulting ash falls into an ash pan. In order to reduce the intervals in which an ash removal is necessary, the ash is partially compressed in the ash box. Occasionally, ash discharge systems are used, in which the firing residues are transported by means of screw conveyors into collection containers.

Heat transfer and storage
As with the use of other fuels, the combustion of the energy source in the boiler heats the water, which serves as a heat exchanger of a heating and / or hot water system and transports the heat energy via pumps and pipelines to the place of consumption. Since a largely complete combustion of the wood pellets is possible only in normal operation and during the warm-up and the burn-up phase larger losses and higher emissions arise in heating systems, the heated water is usually passed as in firewood heaters first in a buffer storage, from where it of the Consumers will be accessed as needed. This ensures a sufficiently long uninterruptible firing period.

Measurement, control and regulation
The measurement, control and regulation technology of pellet heating is usually more complex than the comparable heating systems with fossil fuels. On the one hand, the integration of one or more heat accumulators requires regulation of hot water storage, delivery and subsequent delivery, and on the other hand, the regulation of fuel supply, combustion air supply and firing is more complicated.

Safety devices
Because of the peculiarities of the fuel, pellet heaters have safety devices other than oil or gas burners. All modern wood pellet heating systems are equipped with a burn back protection, which prevents re-firing into the feed / storage area of the pellets. Negative pressure controls in the combustion chamber prevent the escape of toxic or flammable gases into the boiler room, overheating protection for some systems> 25 kW or combi boiler is made possible by safety heat exchangers, which automatically conduct cold water through a heat exchanger when overheated.

Power range and efficiency
Pellet heaters are available in all power ranges from approx. 3.9 kW, as single ovens between approx. 4 and 20 kW. Most systems available today have power control over the fuel and combustion air supply, so they can operate at both full load and part load. Currently, pellet boilers reach a combustion efficiency of around 85-95% in full-load operation (nominal heat output) in calorific value mode. With pellet boilers in condensing technology, boiler efficiencies of up to approx. 106% can be achieved. Here, the condensation energy of the steam in the exhaust gases in addition to the evaporation energy (at least partially) recovered. This achieves an exhaust gas temperature of only 30 ° C-40 ° C. As a material for the necessary heat exchanger corrosion-resistant materials such as stainless steel or graphite are used. There are special measures in the fireplace and the discharge of condensed water necessary (350 liters per ton of pellet).

Apart from a few exceptions, the efficiency drops when the pellet boiler operates in the partial load range. The firing efficiencies described here can deviate greatly from the actual plant efficiencies, for which reason the plant concept plays a major role. The use of a sufficiently large buffer memory makes sense.

Level of automation, support and maintenance
Modern pellet heating systems operate almost fully automatically, so that only regular cleaning and maintenance work at intervals of several weeks (ash disposal) or a few months (cleaning of the combustion chamber) are necessary. The regular work on the heating is limited to the filling of the warehouse, the removal of the ashes and for simpler models, the cleaning of the flues. A user friendliness comparable to oil or gas heating is an important development goal of manufacturers. For individual pellet central heating systems, care intervals of only once a year suffice.

Fuel reference
The fuel is offered as bagged goods (15-20 kg) for manual filling, in 1-2 m³ plastic bags (big bags) or loose. While bagged goods are especially suitable for single ovens or mini-systems, the use of big bags requires appropriate suspension systems and lifting technology.

The delivery of loose wood pellets is usually done by similar silo vehicles as the delivery of feed pellets. The pellets are blown in, except in dust-tight bag silos, and storage space is usually at the same time air sucked in order to avoid the dust load. Typical deliveries for end users are 3-10 tonnes.

Storage and discharge
The wood pellets are stored in bulk in a tank or storage room and fed by a conveyor system to the burner. The storage room must be dry, as the pellets react strongly hygroscopically to wall or too high air humidity during storage with crumbs.

Compared to oil, wood pellets require about three times the storage volume, but with less technical effort for the room, since pellets, in contrast to heating oil, are not water-polluting substances. For storage, the pellets can be stored in a simple storage room. The floor is built in funnel shape – usually in wooden construction – at the bottom of the funnel is the inlet of the screw or there are sampling probes for the fan. Several removal points in the storage room ensure unobstructed operation even in the event of malfunctions of a withdrawal point. Alternatives to a storage room are prefabricated tanks made of fabric or sheet steel. Buried underground tanks or free-standing siloscan be used if there is insufficient space in the building. In damp rooms, dense tank systems must be used to secure the pellet quality.

For feeding, trickle, suction fan or screw systems can be used. The choice depends primarily on the distance of the bearing to the boiler room, for distances over 2 m usually multi-stage or flexible screw conveyors are necessary. Blower systems can be used flexibly and transport up to more than 20 m. The discharge from the storage room or container is supported by an inclined container bottom or funnel outlet.

Burner systems with active discharge control provide themselves with the required fuel dose, otherwise, in addition, a small buffer is required from which the burner operates.

See also hazards due to wood pellets

Pellets as pressed sawdust for energetic use and pellet stoves were developed in the 1970s in the United States. Since the late 1970s, European boiler manufacturers, especially in Scandinavia and Austria, have entered the development of pellet heating. The market for wood pellet heating in Germany developed later, after the use of wood pellets in Germany was released in 1997. Today, Germany is the strongest sales market for wood pellet heating systems.

According to industry reports, the share of pellet heating in new buildings is 35%, the pellet heating system is increasing by more than 10% per annum, currently (January 2013) it is just over 100,000 units. With 12.6 pellet heating systems per 1,000 inhabitants, Austria has the highest density of pellet heating systems in Europe. In Germany were installed from 1999 to 2008 around 100,000 pellet plants.

Efficiency and operational costs
The initial cost of a pellet plant is higher than comparable gas and oil heating, but the operating costs are – depending on the fuel and current fuel price – in Central Europe often cheaper than fossil fuels. For larger systems, the proportion of investment costs decreases compared to the operating costs, so that there already a cost savings after less years of operation begins than in small plants. Once again, there are additional regenerative alternatives available for supplying heat to larger properties, with even lower operating costs compared to pellet heating, including woodchip heating or waste heat from biogas plants, Currently, the available financial support in favor of pellet heating systems.

See also: Building heating – for the economic efficiency calculation and dimensioning of heating systems in general
In Germany, the Federal Ministry for Economic Affairs and Energy is promoting the installation of pellet heating systems as part of a Market Incentive Program (MAP). The subsidy amounts to 1400 € for pellet stoves with water pocket (water-bearing stoves), 2400 € for pellet boilers without buffer storage (boiler output 5 to 66.6 kW) and 2900 € for pellet boilers with newly built buffer tank with at least 30 liters per kW. Boiler output (here 5 to 80.5 kW). Additional bonuses are granted if at the same time the installation of an eligible thermal solar system (bivalent heating) and / or a solar hot water systempreparation or the energy efficiency (thermal insulation) of the heated object is brought to a certain standard. The installation of an exhaust gas heat exchanger for increasing the efficiency and / or a particulate filter for separating the contained in the exhaust particulate matter is an additional innovation funding support.
In Austria, there are subsidies from the federal government, the federal states and individual municipalities for new central heating systems or the conversion to pellet heating.
In Switzerland, wood pellet heating systems are also being promoted. This is cantonally regulated differently.
In Belgium, the Walloon Region will pay the following bonuses per installation in 2008-2009: € 1,750 to 50 kW (+35 € per kW to 100 kW), 3,500 € to 100 kW (+18 € per kW to 500 kW), 10,700 € to 500 kW (+8 € per kW up to a maximum of 15,000 €).

Wood pellets (DIN plus) have a calorific value of 5 kWh / kg, and an oil equivalent of 2.16 kg / l or 3.33 l / l OE. The energy content of one kilogram of pellets equals that of a half-liter of heating oil, in volume (in bulk volume meters) one third of a liter.

Price development
The pellet market has seen strong growth in supply and demand in recent years. After an initially quite high price after market launch in the late 1990s, a phase of relatively low prices around 3.5 cents / kWh in Germany from 2002 to 2005 and several months of high pellet prices of more than 5 cents / kWh in the winter of 2006/07 Supply bottlenecks have greatly expanded the manufacturers’ capacities, so that the trade value has fallen since 2007 to a level between approximately 3.5 and 4.5 cents / kWh. In 2008, the minimum was reached, since then the price of pellets rises again. In 2015, prices between approx. 4.7 and 6.3 cents / kWh were achieved.

Compared to natural gas, the price development in Austria can be observed objectively based on the Austrian price indices of the products: In January 2013, the pellet price index was 139.91 compared to the base value January 2006, ie the pellet price only rose by 1 in these 7 years, 40 times against the Austrian gas price index of 143.75, d. H. the price of gas rose by 1.44 times over the past seven years (see the development of gas prices to illustrate the usual price fluctuations).

For the delivery of loose goods, a Einblaspauschale of net about 30 € will be charged; Less than 3 tons of delivery quantity often results in shortage surcharges. The 15 kg bagged goods cost between 7 and 20% more than loose goods and surcharges are also calculated for big bags (750 to 1000 kg).

Raw material origin and fuel alternatives
In addition to the originally used sawdust, wooden pellets are increasingly being used to make pellets, which are also in demand by the paper industry and the woodworking industry. In addition to woodchips, these also include forest residues and logs. The growth rates and overall consumption of wood pellet use increase demand in the area of lower wood qualities. Strategies to increase the supply of raw materials include the use of whole trees, the increased use of fast growing wood species and the plantation of short rotation plantations on agricultural land. While energy wood production on arable land can be a gain from an environmental perspective, target conflicts with natural, environmental and / or soil protection can occurwhen intensifying the raw material yield in the forest. For raw material origin see also wood pellet # criticism.

Alternative fuels for the operation of pellet heating systems are under development and testing. In addition to straw pellets, residual value pellets (eg mill residues) and further pelleted biomass such as B. Giant miscanthus or rapeseed cake possible raw materials. Non-woody biomass is used in pellet form for biomass cogeneration plants, but is currently used for pellet heating i. d. R. unsuitable, since these fuels both in combustion technology (eg., Due to high silicon content and sintering) and in the exhaust gas purification require increased effort. Grain as fuel requires suitable equipment for this (grain combustion) and is restricted according to the Ordinance on Small and Medium-sized Firing Plants.

Most pellet stoves are self igniting and cycle themselves on and off under thermostatic control. Stoves with automatic ignition can be equipped with remote controls. Recent innovations include integrated microcontroller monitoring of various safety conditions and can run diagnostic tests if an imminent problem arises.

A properly cleaned and maintained pellet stove should not create creosote, the sticky, flammable substance that causes chimney fires. Pellets burn very cleanly and create only a layer of fine fly ash as a byproduct of combustion. The grade of pellet fuel affects the performance and ash output. Premium grade pellets produce less than one percent ash content, while standard or low grade pellets produce up to six percent ash. Pellet stove users should be aware of the extra maintenance required with a lower grade pellet, and that inconsistent wood quality can cause serious effects to the electronic machinery over a short period of time.

A pellet stove is normally associated with pelletized wood. However, many pellet stoves will also burn fuels such as grain, corn, seeds, or woodchips. In some pellet stoves, these fuels may need to be mixed with wood pellets. Pelletized trash (containing mostly waste paper) is also a fuel for pellet stoves.

Unlike wood stoves which operate exclusively on a principle of chimney draft, a pellet stove must use specially sealed exhaust pipe to prevent exhaust gases escaping into the living space due to the air pressure produced by a combustion blower. Pellet stoves require certified double walled venting, normally three or four inches in diameter with a stainless steel interior and galvanized exterior. Because pellet stoves have a forced exhaust system, they have the advantage of not always requiring a vertical rise to vent, although a 3-to-5-foot (0.91 to 1.52 m) vertical run to induce some draft is recommended to prevent leakage in the case of a power outage. Like a modern gas appliance, pellet stoves can be vented horizontally through an outside wall and terminated below the roof line, making it an excellent choice for structures without an existing chimney. If an existing chimney is available, manufacturers urge use of a correctly sized stainless steel liner the length of the chimney for proper drafting. Modern building techniques have created tightly sealed homes, forcing many pellet stove manufacturers to recommend their stoves be installed with outside air intake to ensure the stoves will run efficiently and prevent potential negative pressure within the home.

Pellet stoves are approved for use in mobile homes, while standard wood burning stoves are not.

In many states pellet fuel is exempt from sales tax.

Environmental compatibility
Central aspects of the environmental compatibility of the pellet heating system are the origin of the raw materials, the pollutant emissions resulting from combustion and the climate balance.

Climate protection
Since pellets consist of the renewable and therefore CO 2 -neutral raw material wood, their climate balance can be more favorable than with fossil fuels. The amount of CO 2 that is released during combustion corresponds exactly to the amount of CO 2 that was incorporated into the wood as it grew. Even with firewood or wood pellets from guaranteed sustainable forestry, however, arises first a few decades lasting CO 2-Belast the air, as it takes decades for the relatively short amount of wood burned in old, harvest-ripe trees to grow back on an appropriately sized replacement forest area, thus removing the corresponding amount of CO 2 from the air.

The CO 2 emission of wood pellets is about 42 g / kWh, for fuel oil the value is about 303 g / kWh. According to the Global Emissions Model of Integrated Systems (GEMIS), the life cycle of wood pellets (including transport and material input), as a by- product of higher value wood use, accounts for about 13% of the useful energy of non-renewable energy. Due to the subsidies, the demand for pellets was significantly fueled. This results in side effects such as price increases and increased use of forest wood instead of wood waste.

Energy security and regional value creation
By using the biogenic energy source wood pellets, the dependence on fossil fuels is reduced. In addition, in contrast to fossil fuels, the use of wood pellets may in some cases make use of regionally produced fuels. Expenditure on fuel then largely remains in its own region, where it adds value.

In the case of pollutant emissions (carbon monoxide, volatile organic hydrocarbons, nitrogen oxides), pellet heaters are in the range of other heating systems – with differences depending on pollutant and heating system. The fine dust emissions of modern pellet heating systems are slightly higher than comparable oil or gas heating systems, but far below the applicable limit values. Even after the planned lowering of the exhaust emissions in 2015 according to 1. BImschV, the plants can comply with the limit values.

Sulfur dioxide (SO 2)
Wood pellets in accordance with DIN plus or ÖNORM M 7135 have a maximum sulfur content of 0.04 % by weight, which is between natural gas as specified by DVGW (maximum 30 mg / m³ or 8 mg / kWh plus sulfur content from average odorization) and light fuel oil (0.1% by weight according to § 10 of the 10th BImSchV). According to the Global Emissions Model of Integrated Systems (GEMIS), the release of SO 2 over the entire life cycle of wood pellets from waste wood recycling is approximately 0.53 g / kWh. Heating oil (condensing technology) and natural gas account for 0.73 g / kWh and 0.18 g / kWh, respectively.

Ozone pollution
The ozone exposure by releasing ozone predecessor substances (nitrogen oxides, carbon monoxide, methane and volatile organic compounds) is reported for the combustion of wood pellets from the residual wood in GEMIS with 0.88 g / kWh, approximately by a factor of two greater than in the combustion of Heating oil with condensing technology (0.41 g / kWh) or natural gas (0.35 g / kWh). Since the increased formation of photooxidants due to the necessary intense solar radiation mainly in the summer months is a problem (” Summer smog”), While space heaters naturally work predominantly in winter, but this emission is attributed comparatively little problem potential.

Fine dusts
The particulate matter emission of modern pellet boilers in normal operation is about 8 mg per MJ heat quantity, corresponding to 29 mg / kWh. In the meantime, there are pellet heating systems which, through optimized combustion, are below the particulate emission levels and can also be installed in areas where stricter regulations apply. These systems work with condensing technology and have a relatively dust and low-carbon exhaust gas (about 4 mg of particulate matter per MJ = 14.5 mg / kWh). Otherwise, the so-called. Partitionsbrenner (transverse insert burner with special shape of the air control) achieved this low particulate matter, although this does not require condensing technology. The comparative value for the fine dust emission lies with single ovens (open chimney,Tiled stove) at about 150 mg / MJ (= 544 mg / kWh), at log boilers at about 90 mg / MJ (= 326 mg / kWh), and at oil heaters at 3 mg / MJ (= 11 mg / kWh). (B.energie AG, Switzerland).

A study commissioned by the Institute for Economic Oil Heating (IWO) also takes dynamic operation into account. In the winter typical day profile, a pellet boiler emitted 114 mg / kWh fine dust, in contrast to measured 74 mg / kWh in continuous operation. The comparison values for the oil burner were 0.10-1.40 mg / kWh in the experiment (with hardly any differences between continuous and interval operation).

Corn stove
A corn stove is designed for whole kernel shelled corn kernel combustion and is similar to a pellet stove. The chief difference between a pellet stove and a dedicated corn stove is the addition of metal stirring rod within the burnpot or an active ash removal system. These vary in design slightly, but usually consist of one long metal stalk with smaller rods welded at a perpendicular angle, in order to churn the burn pot as it spins. An active ash removal system consists of augers at the bottom of the burn pot that evacuate the ash and clinkers. During a normal burn cycle, the sugar content within corn (and other similar bio-fuels) will cause the ashes to stick together, forming a hard mass. The metal stirring rod breaks apart these masses, causing a much more consistent burn. While there is demand to create stoves that are able to burn multiple fuels with minimal adjustments, some pellet stoves are not designed to stir fuel and cannot burn corn fuel.

A study by the Austrian Society for Environment and Technology, the cost of capital and operating costs of heating systems under different heating consumption and energy price scenarios compares, “expect” to pellet at lower or steady energy prices compared to fossil heating systems for above-average heat consumers, the more Energy (especially in low-energy houses) is saved by thermal insulation, the more the high installation costs in the overall price over the life cycle beat and then pellet heating systems”Assuming constant low energy prices. even the most expensive heating system “. Pellet boilers, in addition to firewood heaters, would bring the lowest total costs if fossil fuel costs continue to rise.

Source from Wikipedia