Wood kiln

Abstract

An arrangement for drying wood including wooden logs, wood chips or other forms of lumber. The arrangement comprises an insulated kiln 1 defining a kiln chamber 10 in which wood is placed. A liquid me

Description

Technical Field

[0001] The present invention relates to a method for drying wood or more precisely removing the water content from wood including fine wood for cabinet making and more specifically designs for a device for drying "green wood", in a novel design of wood kiln. One particular application is for drying firewood either in the form of logs, wood chips or other forms of lumber. This dry wood is burnt to provide heating, from small-scale domestic heating using wood-burning stoves, through to large-scale heating used by commercial users on an industrial scale. Background Art

[0002] NPL1: Technical Development report reference FCPR045 published in April 2011, discloses how the use of wood as a fuel, particularly woodchips, is adversely affected by the water content of "green wood". Moisture content directly affects the calorific value of the wood as the water contained within the wood must be evaporated during combustion (the latent heat of evaporation). Therefore, the net calorific value of a fuel will decrease with increasing moisture content. Fuel with a higher moisture content also lowers the combustion temperature leading to incomplete combustion and increased emissions. This document also highlights that fuels, in this case wood, with a higher moisture content cost more to transport, as for any given volume of wood the weight will be higher as water is being transported too and therefore larger volumes of dryer wood can be transported in larger containers or at a higher bulk density.

[0003] NPL2: Energies 2019, 12(9), 1590 "Wood Chip Drying through the Using of a Mobile Rotary Dryer" by Dr. Angelo Del Giudice et al. was published on 26 April 2019 and discusses the exploitation of biomass (wood) for energy production and the negative affect of high moisture content on the efficiency of power generation in combustion and gasification systems. This publication discusses different types of dryers and in particular rotary dryers for use in the drying process of woodchips.

[0004] International patent publication no. WO2011146993 (SOLARKILNS PTY LTD) published on 01 December 2011, relates to a solar-powered drying, heating and air-conditioning system comprising an enclosure that defines a kiln chamber for drying a product or more particularly a charge of timber using air heated by solar radiation. This design of solar kiln includes an "air flow system" comprising at least one fan for generating a circulating air flow within the kiln chamber. This International patent document has resulted in granted patents EP2577199B1 (Granted 26April 2017) and US9250015B2 (Granted 02 February 2016).

[0005] International patent publication no. WO2013158009 (ALENT DRYING AB) published on 24 October 2013, describes a drying kiln where fresh air is added to the flow of circulating air. The drying kiln has a horizontal floor that carries a heater battery and circulation fans. Part of the circulating air in the kiln is vented from the kiln outside as exhaust air flow via an evacuation chimney.

[0006] Convention says that wood acts like a simple porous material e.g. wet sand. The surface of the sand becomes dry, and evaporation happens inside the pile as the 'Dry Front1 moves deeper into the material. However vapour moves from the 'Dry Front' to the sand’s surface by diffusion as a vapour, it does not flow to the surface as a liquid, leading to the conclusion that using a faster airflow and / or converting a wood log into wood chips having a larger surface area to volume ratio will result in faster drying, but this is not true. The model used for drying of simple porous material shows the drying rate decreasing as the 'Dry Front' moves deeper into the material and vapour diffusion becomes slower. However, with wood, actual results show that liquid water can flow from the middle of a log to the surface at a constant rate as it is wicked out along cell walls at the molecular level.

[0007] NPL3: This phenomenon is discussed in the document "How Bound Water Regulates Wood Drying" by Helene Penvern et al. Physical Review Applied 14, 05405 Published 20 November 2020 that discusses how optimal drying conditions can be maintained if: “(i) Sufficiently wet conditions around the free surface of the sample are preserved, i.e., similar to those prevailing at the beginning of drying, (ii) Sufficient water is continuously transported towards the free surface to maintain these wet conditions.” Summary of invention

[0008] The present invention provides a kiln that is unvented and does not contain a fan or similar means to provide a forced circulating airflow. The wood is placed in a sealed kiln chamber with no ventilation, which is well insulated. This improves the efficiency of the heating system and results in a method that uses the minimum amount of energy during the drying process to achieve the maximum dryness of the wood with a moisture content as close as possible to 0%, which requires a drying efficiency of close to 100%. The driver for looking at using very dry wood to provide a clean and sustainable energy source means that the energy used during the drying process should have a low carbon footprint. Renewable energy sources such as solar, wind, tidal and so on are the first that come to mind. However, capturing and reusing the waste heat from an industrial process is also a possibility.

[0009] Initially the inventor concentrated on the use of solar energy for producing dry wood for a domestic consumer and initial trials resulted in a system that can be run at most times of the year at UK latitude and weather conditions with a solar panel that is contained within the same ground footprint as the drying kiln. This makes the device scalable from very large industrial units, right down to a domestic size which is suitable for use by an untrained end user with no special knowledge. This makes it possible for end users to kiln dry their own firewood at home for the first time. A simple indicator linked to the temperature of the wood may be calibrated to show when the wood is dry.

[0010] The same air is reused throughout the drying process to carry water from the wood to a heat exchanger or dehumidifier where water is removed from the air and allowed to exit the kiln preferably via a wicking material that removes water whilst preventing loss of the air from the system.

[0011] The dehumidification device used in a first example of the invention is driven by passive gravity, has no moving parts and is not powered in any way. The external air temperature is hot during the day and then cools overnight, whereas the temperature inside the wood cycles more slowly. The temperature in the wood naturally rises and falls leading to the temperature differential reciprocating between the centre and the surface of the wooden logs. This drives the moisture out more easily than in a conventional kiln where a continuously hot outside surface leads to a vapour pressure on the outside that is continuously higher than the inside, hence the moisture often becomes trapped inside the logs as the hot dry air will stop the natural wicking process. This is supported by the discussion in NPL3.

[0012] In a first embodiment of the invention, the kiln uses a solar thermal array to harness solar energy to dry firewood, but unlike the Solar Kilns described in the Background Art, the solar thermal array used in the invention simply uses the solar energy to heat a liquid medium directly. The heated liquid medium is used to heat the floor of the kiln much like under-floor heating thus heating the air in the kiln which circulates up through the chamber by natural convection currents and its flow is passive, with no fan needed. So the elevated temperature in the kiln is provided directly by the heat of the sun.

[0013] The liquid medium is pumped through the solar thermal array using an electric circulation pump that is powered by its own solar PV panel. The size of the PV panel is selected to balance the power of the pump with the power from the solar thermal collector. The pump starts as soon as the sun is powerful enough to provide heat and stops immediately when the sun goes in, maximising the heat collected without drawing heat out of the thermal store. In other words both the power to the pump (hence water flow rate) and the heat output of the panel are directly portion or to the solar irradiance, so they go up and down together. Therefore, the power of the pump is balanced to the output of the solar thermal collectors so that it is automatically balancing and needs no differential temperature control.

[0014] This is a much more efficient way of using the solar energy directly, rather than using photovoltaic solar cells to produce electricity, which is inherently inefficient. This electricity is then used to drive fans to force warm / hot air over "green wood" to drive out the water held therein as quickly as possible. Conversely, in the Solar Kiln according to the invention the solar energy is used for heating a liquid medium such as water for example, which is then fed to the base of the main kiln chamber to provide warm / hot air that circulates around the wood using normal convection currents slowly absorbing water from the wood and increasing the relative humidity of the warm air as it rises up through the main chamber of the kiln.

[0015] Hot wet air enters a dehumidification device at the top of the kiln chamber which is passively controlled by nylon strip hygrostats and the air flows without aid under its own gravity flow. The airflow is driven by the temperature difference between that inside the kiln chamber and the ambient outside temperature (roughly 50°C to 15°C). The dehumidification device uses condensation, and the latent heat of condensation causes the air to cool. The cool air falls to the base of the chamber where it is heated and reused.

[0016] The heated water laden air passes through a thermally insulated channel, hits a nucleation point to initiate condensation and the thermal insulation prevents the energy in the warm air surrounding the wood from being removed by the cold air after condensation. The simplest way to drive the condensation is to have a point that is thermally connected to the outside, to allow liquid water to be removed from the kiln without allowing any air flow. The heat transfer through this thermal connection takes away the latent heat of vaporisation, but too much and the thermal connection will take dry heat out of the system unnecessarily. This separation both drives the convection and prevents re-evaporation of the water in the dehumidifier. Brief description of drawings

[0017] The invention will now be described in more detail, by way of example only, with reference to the accompanying drawings in which: Fig. 1 shows a side view of a solar firewood kiln according to a first embodiment of the invention; Fig. 2a is a graph showing the distribution of temperature (°C), humidity (%RH), %RH (1) and Temp (1) around the kiln chamber where the suffix (1) identifies results for log 1 shown in fig. 3; Fig. 2b is a graph of the experimental results obtained by the inventor contrasting the Kiln Temperature (T) &Humidity (H) variation vs. the kWh provided into the Kiln from the solar energy &the Evaporation Potential (kWh that could be taken out of the Kiln by evaporation of the water removed from the wooden logs). What this graph is showing is that Evaporation Potential is in balance with solar energy during Stages A &B, and out of balance in Stage C in line with the theory in document NPL3; Fig. 3 provides a plot of the data retrieved from moisture probes inserted into 6 wooden logs over a period of 75 days starting from 18 March 2023 when they were stored in the Kiln showing that all the logs had reached a moisture content of 0% within this period; Fig. 4a shows is a front view of a second embodiment of the solar firewood Kiln according to the invention. Essentially, this is looking at the South elevation of the Kiln with the viewer looking Northwards. The drawing is a Third Angle Projection with the cross-section A-A shown in Fig. 4b aligned exactly with the Front View. Fig. 5a shows a plan view of the solar collector and Fig. 5b shows a section along the line A-A (scale 1 : 2) indicating the features of the double glazed panel used in the solar collector.

[0018] Fig. 1 will now be discussed in more detail highlighting the various features illustrated. A solar firewood kiln 1 having a kiln chamber 10 located within a thermally insulated body 12. The kiln 1 has one or more doors 30 (not shown) to provide access into the kiln chamber through which a cage 20 of wooden logs 25 (not shown) may be loaded. A transparent rain resistant sheet 52 such as glass or treated polycarbonate much like that used in a double-glazed panel covers and protects a solar thermal collector 50 comprising a thermally insulated chamber with a transparent side facing the sun. A black Nylon sheet is supported in the middle of a flow of liquid medium 55 such as water or similar that collects the energy from the sunlight and transfers it to the liquid medium.

[0019] A pump (not shown) then takes the hot water (liquid medium) back to a thermal store 40 at the bottom of the kiln through pipes (not shown). The thermal store 40 may comprise a large water chamber, below the base of the kiln chamber which may be composed of a concrete slab or similar heat storing material. The heated base of the kiln chamber 15 heats the air 16 in the kiln chamber 10 and the heated air 16 rises and circulates via convection currents around the wooden logs (not shown) collecting water from the surface of the logs as it passes. The heated, water laden air rises to the top of the kiln chamber 10 and passes into an insulated channel separated from the main kiln chamber 10 by a thermally insulated divider 70 that prevents the energy in the warm air from being removed. This separation both drives the convection and prevents re-evaporation of the water as it hits the dehumidifier, which in this example is simply provided by a hydrophobic material forming a nucleation point 75 for condensation.

[0020] The simplest way to drive the condensation is to have a point that is thermally connected to the outside. The heat transfer through this thermal connection takes away the latent heat of vaporisation allowing water to leave the kiln, whilst preventing the loss of dry heat unnecessarily. This may be achieved by use of a wick 80 of glass fibre rope for example that is conventionally used for door seals on Wood Burners and is readily available. The water content of the air may also be collected by a "passive dehumidifier" which uses condensation to separate the water from the air and the latent heat of condensation cools the air which then falls under gravity and the cool air 18 is returned to the base of the kiln chamber 10 to be reheated and start the cycle all over again. The water droplets from the dehumidifier may be collected in a small channel and then either just drained away or passed over the external surface of the dehumidifier to augment the cooling effect by evaporation into the environment and then drained away.

[0021] The inventor has carried out a series of tests of the kiln over a period of about 3 months and the experimental results recorded during these tests are presented in Figs. 2a, 2b and Fig. 3. Fig. 2a is a graph showing the inventor's experimental results from tests carried out on the dates shown on the X axis. In this graph the surface conditions of a log (1) are shown following the ambient conditions in the kiln chamber. The lines show the log humidity 10 h and log temperature 10 t compared with the chamber humidity 25 h and chamber temperature 25 t. These results show that by providing the heat to the floor beneath the wood, air naturally circulates and evens out the conditions throughout the kiln. There is no need for a fan or chimney, etc.

[0022] Fig. 2b provides a graph for the same period showing the temperature and humidity in the kiln versus the power provided into the system by solar energy and the power taken out of the system by Evaporation potential. The evaporation potential (Evap) is the energy that the "free surface of water" on the outside of the logs can consume through evaporation given the conditions within the kiln. At Stage A the wood is wet, and the water moves to the surface as fast as it evaporates. At Stage B the wood is drying as the water is moving to the surface, just fast enough to keep the chemical potential gradient steep in the fibres. Finally, at Stage C "bound water" is the only water in the wood and therefore the capacity of the air to dry the wood is much greater than the capacity of the wood to provide water to the free surface. This causes the temperature of the wood to rise rather than water being able to evaporate from the wood. These results follow those discussed in NPL3.

[0023] 6 logs were fitted with moisture sensors and were recorded daily, and these results are shown in the graph in Fig. 3. The results of these experimental measurements bear out the inventor's belief that the kilns described in the Prior Art which seek to force the drying of wooden materials using high heat and fans to increase circulation of the hot air are not efficient, because they do not take into account the mechanism by which wood dries which can be carried out much more efficiently if the drying process is allowed to occur more slowly. It should be noted that the kiln according to the invention requires only solar energy and may achieve a percentage moisture content of 0%; well below the minimum that can be achieved (15%) by traditional seasoning.

[0024] Figs. 4a and 4b show an example of the kiln 1 according to a second embodiment of the invention having a South facing wall 5, an opposing North facing wall 7 and a solar collector 50. The kiln 1 is unvented and has a thermally insulated body 12 that encases a kiln chamber 10 in which wood (not shown) is dried without using a heater, fan or any forced circulating airflow. Instead the air is heated directly from a solar collector and circulates through the wood via convention currents collecting water from the surface of the wood and as the air collects water it rises as wet, heated air up through the chamber 10 to a dehumidifier that passes down the outside of the North facing wall between the insulation and the external surface. The water is removed from the air by the dehumidifier and the cool air falls under gravity to the base of a chamber via the channel 82. Unheated water is pumped through the solar collector using an electric water pump 60, that is photovoltaically powered, and the water collects heat before being fed to a thermal store 40 such as water bags or similar, which transfer heat to the base 15 of the kiln chamber 10 by conduction.

[0025] Figs. 5a and 5b show a plan view of the solar collector and a partial cross-sectional view along the line A-A at a scale of 1 : 2 highlighting the important features of the solar collector. Fig. 5a shows a small photovoltaic (PV) panel 65 that drives the water pump and is power balanced to the solar irradiance. A heat absorbing material 57 (for example a sheet of black Nylon) and holes in the nylon sheet create localised turbulence and improve heat transfer from the nylon sheet into the water. The solar collector has a lower plenum 58 and a top plenum 59 and a row of holes leaving the lower plenum restricts flow and ensures that it is laminar, whereas a similar row of holes into the top plenum restricts flow and ensures that it is laminar too.

[0026] The cross-sectional view shown in Fig. 5b provides more detail of the construction of the solar collector, which comprises a standard double-glazed unit (Argon or vacuum filled) on its outer surface. A glass layer 52 and a layer of low emissivity glass 52' encases a gas or vacuum cavity 53. Below this lies a water-filled heat absorbing unit comprising a lower plenum 58, to which the cold water is pumped before passing through holes 54 to ensure laminar flow in the heat absorption chamber where water flows through the black Nylon heat absorber sheet 57 ensuring heating of the water by direct solar collection and the heated water then passes through more holes 54 to ensure laminar flow into a top plenum 59 ready for the hot water to be returned to the kiln.

[0027] Although the examples shown and described in the drawings relate to generally small-scale domestic devices, it will be apparent to those skilled in the art how these devices may be scaled up for larger industrial users and how the principles may still be applied to alternative sources of energy having a low carbon footprint. 05 03 24

Claims

1. An unvented and insulated kiln for drying wood using renewable energy, wherein the kiln (1) encases a kiln chamber (10) into which wood is placed and the renewable energy heats a liquid medium (55) which passes to a thermal store (40) at the bottom of the kiln (1), the thermal store (40 heats the base or floor of the kiln chamber (10) thus heating the air therein and the heated air (16) rises and circulates around the wood (25) by natural convection, collecting water from the wood (25) as it rises to the top of the kiln chamber (10) as hot, water-filled air (17), where it hits a nucleation point (75) that removes the water from the hot, water filled air (17) and the water leaves the kiln (1) separate from the air via a wicking means (80).

2. An unvented and insulated kiln for drying wood according to claim 1, wherein the nucleation point (75) forms part of a dehumidification device that removes water from the water filled air (17) at the top of the kiln chamber (10) and the water is directed outside the kiln (1) whilst the air is retained within the kiln (1) and falls in a channel (82) under gravity where it cools before re-entering the bottom of the kiln chamber (10) as cool air (18).

3. An unvented and insulated kiln for drying wood according to claim 1 or claim 2, wherein the renewable energy is solar energy collected by a solar panel (50) through which the liquid medium (55) is pumped by an electric circulation pump (60) that is powered by its own solar PV panel (65) selected to balance the power of the pump (60) automatically with the power obtained from the solar panel (50).

4. An unvented and insulated kiln for drying wood according to claim 3, wherein the solar panel (50) is contained within the same ground footprint as the kiln (1).

5. An unvented and insulated kiln for drying wood according to claim 3 or claim 4, wherein the solar panel (50) comprises an outer sheet (50) and inner sheet (521) encapsulating a gas or vacuum cavity (53) covering a heat absorbing unit (56) containing the liquid medium (55) and a heat absorption material (57) that enhances the heat retained by the liquid medium (55) from the solar energy.05 03 246. An unvented and insulated kiln for drying wood according to claim 5, wherein the heat absorbing unit (56) has a lower plenum (58) to which cold liquid medium (55) is pumped by the pump (60) and a top plenum (59) from which the hot liquid medium (55) is returned to the thermal store (40) and both the lower plenum (58) and the top plenum (59) has a plurality of holes (54) to ensure laminar flow of the liquid medium (55) into a chamber housing the heat absorption material (57) through which the liquid medium (55) flows absorbing solar energy as it rises from the lower plenum (58) to the top plenum (59).

7. An unvented and insulated kiln for drying wood according to claim 5 or claim 6, wherein the heat absorption material (57) is black nylon.

8. An unvented and insulated kiln for drying wood according to any of the preceding claims, wherein the liquid medium (55) is water.

9. A method for drying wood in an unvented and insulated kiln using renewable energy as defined in claim 1, wherein the hot water filled air (17) enters a dehumidification device at the top of the kiln chamber (10) where the dehumidification device uses condensation to separate the water from the air and the water leaves the kiln (1), whilst the latent heat of condensation causes the air to cool, fall under gravity and the cool air (18) is returned to the base of the kiln chamber (10) to be reheated.

10. A method for drying wood in an unvented and insulated kiln as described in claim 9, wherein the same air is reused throughout the drying process.

11. A method for drying wood in an unvented and insulated kiln as described in claim 9 or claim 10, wherein the source of renewable energy is solar, wind, tidal or waste heat from an industrial process.

12. A method for drying wood in an unvented and insulated kiln as described in any of claims 9 to 11, wherein the wood includes fine wood for cabinet making and wooden logs, wood chips or other forms of lumber.

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