Wood kiln

Abstract

The present invention provides a kiln (1) that is unvented, relies on passive air circulation via natural convection, requiring no fan or similar means to provide a forced circulating airflow and uses passive gravity driven dehumidification. The wood is placed in a sealed kiln chamber (10) with no ventilation, which is well insulated. This improves the efficiency of the heating system and results in a drying method that uses the minimum amount of energy during the drying process to achieve the maximum dryness of the wood. The kiln (1) uses passive, gravity driven dehumidification, so the dehumidification device has no moving parts and is not powered in any way.

Description

WOOD KILN

[0001] The. use of wood as a fuel source for heating is as old as the technology required to produce a spark to produce fire. Heating using wood is still common throughout much of the world, although it has been replaced with coal, oil or natural gas heating in most places. However, wood heating has been singled out as a serious health hazard in many regions of the world, despite the advances made in the devices in which wood combustion takes place. It is also now well known that most of these problems, such as smoke and particulate production, are caused by the moisture content found in the wood causing its incomplete combustion. Technology now exists to mitigate these problems and the moisture content can be reduced significantly.

[0002] The invention discloses 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 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.

[0003] 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.

[0004] 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.

[0005] and granted versions PTL2, PTL3 relate to a solar powered drying, heating and air-conditioning system comprising an enclosure that defines a kiln chamber for drying a product, more particularly a charge of timber using air heated by solar radiation. However, the design of solar kiln described includes an "airflow system" using at least one fan for generating a forced, circulating airflow within the kiln chamber.

[0006] describes a drying kiln where fresh air is added to the flow of circulating air. The drying kiln described has a horizontal floor that carries a heater battery and circulation fans and part of the circulating air in the kiln is vented from the kiln outside as exhaust airflow via an evacuation chimney.

[0007] NPL3 teaches away from the conventional assumption that wood acts like a simple porous material, e.g. wet sand, when being dried. The surface of the sand becomes dry, and evaporation happens inside the pile as the 'Dry Front' moves deeper into the material. However, in this conventional model, 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, more efficient 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, 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 as discussed in NPL3 which explains 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.”

[0008] The present invention provides a kiln that is unvented, relies on passive air circulation via natural convection, requiring no fan or similar means to provide a forced circulating airflow and uses passive gravity driven dehumidification. 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 drying method that uses the minimum amount of energy during the drying process to achieve the maximum dryness of the wood with an unmeasurable moisture content (indicated 0%), whilst having highest drying efficiency (close to an indicated 100%). Although the kiln chamber is sealed, the kiln itself is not hermetically sealed as water is allowed to escape from the kiln during the dehumidification process. 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 also 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 using solar energy to produce dry wood for use by a domestic consumer in a wood-burning stove for example. A prototype kiln was constructed using existing materials and techniques and a load of logs of "green wood" was placed into the kiln.

[0010] Moisture probes were inserted into 6 logs of varying sizes and the mass of each log was recorded during the test. The load of logs was inserted into the kiln and the moisture content of the 6 wooden logs was taken over a period of 70 – 73 days starting on 18 March 2023, whilst the kiln was running. Temperature and humidity were recorded for both the log’s surface conditions and the ambient conditions in the kiln chamber, which were taken regularly throughout this period.

[0011] Being a natural load of logs, they all were of different sizes and had varying water content. Part-way through the drying process the mass of each log, 1 to 6, was measured along with its moisture content. These results are shown in the columns “Green Mass” and Green Moisture Content” of Table 1. At the end of the test each log was weighed again. All were dry and the moisture content was unmeasurable (or an indicated 0%). The “Dry Mass” is shown in column 4 of Table 1. The difference in mass from “Green” to “Dry” is shown in the last column as “Water Removed” in grams.

[0012] Log NumberGreen MassGreen Moisture ContentDry MassWater Removed12,169g38.0%1,779g370g2993g49.0%791g202g3736g33.5%556g180g41,990g100.0%1,361g629g52,559g81.0%1,686g873g61,594g49.0%1,267g327g

[0013] The ambient kiln chamber conditions initially showed that the temperature was kept low and the relative humidity (RH) kept high, cycling between 85% and 100% daily. This indicated that the moisture in the load was being released into the chamber. The drops in RH were observed and indicate that the heat from the sun during the day warmed the saturated air in the chamber at a rate that briefly exceeded the rate of moisture evaporating from the wood. Overnight, the moisture from the wood continued to evaporate until the humidity in the chamber reached 100% once more. The external surface of the heat exchanger was kept below the dew point of the air passing through the dehumidification channel throughout the day and droplets formed continuously. This effect ensured that the moisture in the wood was drawn out and moved to the dehumidifer drain outlet by natural convection. The next phase of the drying showed the humidity dropping and staying below 100% and evaporation to continue overnight. This allowed the temperature in the chamber to rise to balance the energy input from the sun. Finally, the load became so dry that there was no appreciable moisture left to evaporate. The wood was then fully dried and the chamber temperature rose dramatically. This is a simple way to indicate to the end user that the load is dry.

[0014] The improved efficiency of the kiln and heating method according to the invention results 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 ground footprint of the 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 specialised 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.

[0015] The same air is reused throughout the drying process to carry water from the surface of the wood to a heat exchanger or dehumidifier where water is removed from the water laden air and allowed to exit the kiln via a wicking material that removes water whilst preventing loss of the air (and hence enthalpy) from the system.

[0016] The dehumidification device 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 of the wooden logs more easily and efficiently than in a conventional kiln. A continuously hot outside surface leads to a vapour pressure on the outside of the wood that is higher than the inside, hence the moisture becomes trapped inside the logs and the hot dry air will stop wicking and trap moisture inside the wood. This is supported by the discussion in NPL3.

[0017] In a first embodiment of the invention, the kiln uses a solar thermal array to harness solar energy to dry the wood directly unlike the solar kilns described in the Background Art. The solar kilns described in the Background Art use photovoltaic solar panels to convert the solar energy into electric current, which is inefficient and has significant energy losses as heat! This electric current is then used to power heaters and fans to try and dry the wood within the kiln as quickly as possible. However, as described in the preceding paragraph, this simply leads to a constantly hot outside surface of the wood trapping any moisture within the logs and prevents further removal of water. The solar thermal array used in the invention simply uses the solar energy to heat a fluid medium directly. The heated fluid medium flows to a thermal store at the base of the kiln. This heats the floor of a sealed and insulated kiln chamber encased within the insulated kiln much like under-floor heating. The hot floor of the kiln chamber thus heats the air at the bottom of the chamber which rises and circulates through the chamber and wooden logs therein by natural convection currents, so its flow is passive, with no fan needed. In this way the elevated temperature in the kiln chamber is provided directly by the heat of the sun.

[0018] The fluid 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. In other words both the power to the pump (hence flow rate of the fluid medium) and the heat output of the panel are directly proportional to the solar irradiance, so they rise and fall together. Therefore, the power of the pump is balanced to the output of the solar thermal collectors so that the system is automatically balancing and needs no differential temperature control.

[0019] 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 the "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 fluid medium such as water for example, which is then fed to the base of the main kiln 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 kiln chamber.

[0020] Hot wet air enters a dehumidification device at the top of the kiln chamber which is passively controlled by hygrostats or similar means to maximise the efficiency of the dehumidification process by controlling the humidity level in the channel leading to the dehumidifier before hitting a nucleation point to initiate condensation in a channel that runs outside the thermally insulated kiln chamber and the thermal insulation prevents the energy in the warm air surrounding the wood from being removed by the cooler / cold air after condensation. The simplest way to drive the condensation is to have a point that is thermally connected to the outside 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 50oC to 15oC). The dehumidification device uses condensation, and the latent heat of condensation causes the air to cool. As the air cools, it falls to the base of the outside wall of the kiln chamber under gravity and is then returned to the base of the kiln chamber to be reheated and reused.

[0021] 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.

[0022] The airflow through the dehumidification device is controlled to minimise enthalpy losses beyond that of the latent heat of condensation. The return air is hygrostatically controlled to be as near to 100% relative humidity as possible.

[0023] For environmental reasons, we need to reduce the carbon footprint of domestic heating. Most households in many countries including UK, Europe and the USA still rely on fossil fuels for the generation of electricity and / or for direct household heating. The main renewable sources for production of electricity are currently Wind using onshore and offshore Wind turbines and Solar Photo Voltaic (PV) panels and farms composed of many Wind turbines or Solar PV panels are required. However, Wind is inconsistent, Solar PV is only about 20% efficient and cannot be stored for longer than a few days. It costs a large sum of money to provide the batteries required to meet the base load power needs and storing thermal energy is similar in the scale of investment, for example using a cellar to provide a thermal store. Therefore, at present there is no possibility of storing Energy in Summer to use for heating In Winter.

[0024] Dry firewood has been considered as a possible solution for combining both the heating and storage needs together. However, kiln drying of wood requires a heat source. Current heat sources include fossil fuels, renewable sources used to generate electricity or some of the wood itself and the conversion of heat energy into electricity and the vented processes currently offered are very inefficient. Commercial kilns use four times the amount of energy to dry the wood than is recovered when burning the drier wood.

[0025] The "calorific content" of firewood depends mainly on how dry it is. "Green" wood is about 10 MJ / kg, air seasoned wood about 16 MJ / kg, while kiln dried wood is about 18 to 20 MJ / kg depending upon the type of wood. Allowing the fuel (wood) to burn completely without producing particulates has been found to be possible by reducing the water content of the wood and compressing wood pulp into pellets or artificial logs provides an excellent means of reducing emissions. Reducing the water content of the wood also has the advantage of saving transportation costs as the water content of wood increases its weight considerably, so more dry wood can be transported further reducing the reliance on fossil fuels (as most lorries used to transport wood currently use fossil fuels).

[0026] Lots of facilities are available to traditionally season firewood, but traditional seasoning takes 12 months or longer and this has a lower limit of 15% moisture content. Therefore, there is a huge benefit to starting with traditionally seasoned wood and kiln drying it down to an unmeasurable moisture content. We could consider a happy medium, of three months of traditional seasoning and then perhaps two months in the kiln, just before the wood is needed. However, a high efficiency drying process is the key to being able to use a renewable energy source to produce kiln dried wood. The present innovation is directed at improving the efficiency of kiln drying, so that it can be considered by an end user.

[0027] The wood kiln according to the invention is an alternative and highly efficient way of drawing water out of the wood, resulting in nearly 100% wood density with lower than 2% water content. All kiln drying of wood requires a heat source, and this high efficiency dehumidification process enables energy to be drawn from renewable sources rather than traditional fossil fuels. There are several options for providing renewable energy, but at temperate latitudes, the solar irradiance is enough for solar energy to be provided by a solar thermal panel. The kiln cycle is designed so that the footprint of the load of wood is the same as the footprint of the solar panel. Thus, a solar powered process is scalable from domestic to industrial capacity units.

[0028] The use of kiln dried wood also removes the need to process the wood into wood pulp and such processes may themselves require the use of electrically powered machinery, which as stated previously the production of such electricity also has implications for increased use of fossil fuels and is inherently inefficient. This has advantages both in reducing emissions when the wood is burnt and reducing the fossil energy consumed in transporting the wood

[0029] The invention will now be described in more detail, by way of example only, with reference to the accompanying drawings in which:Fig.1

[0030] is a graph showing the results of a series of tests of the kiln carried out by the inventor over a period of about 3 months showing the experimental results from tests carried out on the dates shown on the X axis. This graph shows the distribution of temperature (oC), humidity (%RH), %RH (1) and Temp (1) around the kiln chamber 10 where the suffix (1) identifies the results obtained for one of the logs 25 identified as log (1). The surface conditions of log (1) are shown to follow the ambient conditions in the kiln chamber 10. The lines show the log humidity 10Hand log temperature 10Tcompared with the chamber humidity 25Hand chamber temperature 25T. These results show that by providing the heat to a floor beneath the wood, hot air naturally rises and circulates and evens out the conditions throughout the Kiln. There is no need for a fan or chimney etc.Fig.2

[0031] 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 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. 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.3A

[0032] illustrates an isometric view of a first embodiment of the solar firewood Kiln 1 according to the invention, showing the North facing wall 7 (rear), left hand side (looking from the rear) and the solar panel 50 sloping towards the lower South facing wall 5 of the kiln. A detailed view is provided of the lower part of the North facing wall 7, showing how the North facing wall terminates in a sheet that is folded inwardly to accommodate wicking material 80 that extends the width of the kiln.Fig.3B

[0033] illustrates the front view of the solar firewood Kiln shown in. Essentially, this is looking at the South elevation of the Kiln with the viewer looking Northwards. The drawing is a Third Angle Projection with a cross-section A-A indicated.Fig.3C

[0034] illustrates Section A-A aligned exactly with the front view shown in. The Killn 1 has a front (South facing) wall 5, spaced from a rear (North facing) wall 7. The sloping solar collector 50 between these two walls is also labelled along with the pump 60 that drives the fluid medium (not labelled) through the solar collector 50. The thermally insulated body 12 of kiln 1 is shown and the kiln chamber 10 having a floor 15 encased therein, containing a load of wood 25 is also illustrated. The thermal store 40 arranged below the floor 15 of the kiln chamber 10 is shown with the hot air 16 rising and circulating through the wood until reaching the top of kiln chamber 10 as hot, water laden air 17 where it is directed to all the nucleation point where it hits the inside surface of the colder rear wall 7 that initiates condensation and separation of the water from the air. As the water is removed from the air and cools via the latent heat of evaporation the cooling air 82 falls under gravity to the base chamber (as shown in the detail views) and when it reaches the base of chamber it is reintroduced into the kiln chamber as cold air 18. In this embodiment the water can then condenses and falls down the inside wall and is collected in the wicking material (shown and labelled in).Fig.4A

[0035] illustrates an isometric view of a second embodiment of the solar firewood Kiln 1 according to the invention, showing the South facing wall 5 having insulated doors 30 and left-hand side (looking from the front). In this embodiment of the invention, firewood is loaded through the doors 30 and adjoining solar panel 50. Also shown is the Photo Voltaic (PV) panel 65 that drives an electric pump and is power balanced to Solar irradiance to enhance efficiency.Fig.4B

[0036] illustrates the South facing (front) view of the second embodiment of the solar firewood Kiln 1 previously shown in. The solar panel 50, PV panel 65 and doors 30 are clearly visible, but this view shows greater detail of the catch arrangement 35 (particularly the top section adjacent to the solar panel 50), which has to be released allowing the solar panel to be lifted before the section holding the doors together can be released and the doors opened providing an opening through which the firewood may be loaded.Fig.4C

[0037] illustrates a view of the North facing wall 7 (rear) of the solar firewood Kiln shown in Figures 4A and 4B. Essentially, this is looking at the North elevation of the Kiln with the viewer looking Southwards. The rear wall 7 is composed of 3 or more slates 37 slightly overlapping with a small gap between them, much like the structure of roof tiles for example. The drawing is a Third Angle Projection with a cross-section B-B indicated.Fig.4D

[0038] illustrates Section B-B aligned exactly with the rear view shown in. The hot air rising and circulating through the load of wood 25 to the top of the kiln chamber is the same as that described with regard toalthough not specifically labelled so that the kiln chamber 10, its floor 12, load of wood 25 and their relation to the thermal store 40 is clearer. The detail shows the very thin dehumidifier channel 20 that runs between kiln chamber 10 and the rear wall 7 of the kiln. The hot / warm, moist air 17 is shown passing down the dehumidifier channel 20 and as it contacts the rear wall 7, the external surface of which provides cooling initiating condensation of the water from the warm, moist air 17.

[0039] The water runs down the internal surface of the rear wall 7 and is absorbed by the wick 80, in the narrow gap left by the overlapping plates 37. The wick 80 is air sealed but permeable to water, allowing the water to exit the kiln 1 whilst retaining the air therein. The top panel 37 of the dehumidifier channel will condense water that will run down through the wick 80 and then over the outside of the panel 37 below. The condensate water will evaporate on the outside of the lower panel, taking the latent heat of evaporation with it. This will cool the lower panel to below ambient. The saturated air on the inside of the dehumidification channel will be cooled by the lower panel 37 and more water will be condensed to bring the air back to saturation at this lower temperature. The third panel 37 will operate in the same way, cooling and condensing more water as the condensate from the middle panel evaporates on its outer surface.

[0040] As the air gradually cools it falls by passive gravity slowly becoming cooler until it reaches the base of the dehumidification channel 20 and re-enters the kiln chamber 10 as cold air 18. The dehumidifier channel is separated from the back of the kiln chamber 10 by further insulation 45, as labeled in the main drawing.

[0041] As the load dries out, the temperature in the kiln chamber 10 will rise. The dehumidifier will reach a new balance point where the air is saturated, but at a higher return temperature.Fig.4E

[0042] illustrates a side view of the solar firewood Kiln 1 shown in Figures 4A to 4D described above. The side wall of the kiln (not referenced) passes between the South facing front wall 5 and the North facing rear wall 7. Doors 30 are shown as well as the solar panel 50 and catch arrangement 35 allowing sequential lifting of the solar panel 50 and opening of the doors 30 so that a load of wood can be inserted into the kiln chamber.Fig.5A

[0043] illustrates a plan view of the solar collector 50 having a small photovoltaic (PV) panel 65 that drives the pump 60 and is power balanced to the solar irradiance. A heat absorber material 57 (for example black nylon) is shown and holes in the heat absorbing material create localised turbulence that improve heat transfer from the heat absorber material into the fluid medium 55. The solar collector 50 has a lower plenum 58 and a top plenum 59 and a row of holes 54 leaving the lower plenum restricts flow and ensures that it is laminar, whereas a similar row of holes 54 into the top plenum restricts flow and ensures that it is laminar too.Fig.5B

[0044] illustrates a partial cross-sectional view along the line C-C at a scale of 1:2 highlighting the important features of the solar collector. The cross-sectional view shown inprovides more detail of the construction of the solar collector 50, which comprises a standard double-glazed unit (Argon or vacuum filled) on its outer surface. A transparent sheet 52 (for example glass or polycarbonate) and an inner sheet of low emissivity glass 52' encases a gas or vacuum cavity 53. Below this lies a fluid-filled heat absorbing unit comprising a lower plenum 58, to which the cold fluid medium is pumped before passing through holes 54 to ensure laminar flow in the heat absorption chamber where the fluid medium flows through the heat absorber sheet 57 ensuring heating of the fluid medium by the sun before being. pumped back to a thermal store 40 at the bottom of the kiln through pipes (not shown).

[0045] Two different embodiments of the wood kiln are described: One in which the water is collected inside the kiln and one where the water is directed outside the kiln. Both embodiments may use the same solar collector arrangement, or they may be designed to work with other sources of renewable energy.Examples

[0046] It will be apparent to those skilled in the art that other sources of heat can be used and other systems used for pumping the fluid medium from the heat source into the thermal store. But the kiln would operate in the same way and the dehumidifier would still need to be exposed to cooler conditions on its outer wall. There might need to be a very low power electrical supply to the pump used to circulate the fluid medium.

[0047] However, if solar energy is not used, there is no need for an angled roof or for the kiln to point in any direction, be located outside, etc. It would simply be a box operating to the same design principles, but the fluid medium sourced from elsewhere.

[0048] If the heat was obtained from compost in a garden, this could be a solar PV panel positioned correctly. It should be noted that the balance between solar irradiance and heat generated would not be relevant for a non-solar heat source, and so a mains powered water pump with a differential temperature controller might be needed.

[0049] 1 Kiln5 South facing wall7 North facing wall10 Kiln chamber12 Thermally insulated body14 Hygrostat15 Floor of Kiln Chamber16 Hot air17 Hot / Warm moist air18 Cold air20 Dehumidifier Channel25 Load (wooden logs)28 Handles30 Doors35 Catch arrangement37 Plates40 Thermal Store45 Insulating material50 Solar Panel51 Double Glazed Unit52 Outer Transparent Sheet52’ Inner sheet53 Gas or Vacuum Cavity54 Holes55 Fluid medium56 Heat Absorbing Unit57 Heat Absorber Material58 Lower Plenum59 Upper Plenum60 Electric Pump65 Photo Voltaic (PV) Panel75 Nucleation Point80 Wicking Material82 Cool Air

[0050] Citation list follows.

[0051] International patent publication no. WO2011146993 (SOLARKILNS PTY LTD) published on 01 December 2011.

[0052] European Patent no. EP2577199B1 (National Phase of PTL1) Granted on 26 April 2017.

[0053] US Patent no. US9250015B2 (from the National Phase of PTL1)) Granted on 02 February 2016.

[0054] International patent publication no. WO2013158009 (ALENT DRYING AB) published on 24 October 2013.

[0055] Technical Development report reference FCPR045 published in April 2011.

[0056] Energies 2019, 12(9), 1590 "Wood Chip Drying through the Using of a Mobile Rotary Dryer" by Dr. Angelo Del Giudice et al. published on 26 April 2019.

[0057] Physical Review Applied 14, 05405 "How Bound Water Regulates Wood Drying" by Hélène Penvern et al. published 20 November 2020.

Claims

A kiln for drying wood using renewable energy having a kiln chamber (10) into which wood is placed and the renewable energy is used to heat air within the kiln chamber, characterised in that the kiln (1) is unvented, encapsulates an insulated kiln chamber (10) and a liquid medium (55) is heated by the renewable energy before being passed to a thermal store (40) at the bottom of the kiln (1), the thermal store (40) heats the floor (15) 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 leaves the kiln chamber (10), 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).A 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) )and the water is directed outside the kiln (1) whilst the air is retained within the kiln (1) and falls in a channel (20, 82) under gravity where it cools before re-entering the bottom of the kiln chamber (10) as cool air (18).A 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 fluid 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).A kiln for drying wood according to claim 3, wherein the solar panel (50) is contained within the same ground footprint as the kiln (1).A kiln for drying wood according to claim 3 or claim 4, wherein the solar panel (50) comprises an outer sheet (50) and inner sheet (52') encapsulating a gas or vacuum cavity (53) covering a heat absorbing unit (56) containing the fluid medium (55) and a heat absorption material (57) that enhances the heat retained by the fluid medium (55) from the solar energy.A kiln for drying wood according to claim 5, wherein the heat absorbing unit (56) has a lower plenum (58) to which cold fluid medium (55) is pumped by the pump (60) and a top plenum (59) from which the hot fluid 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 fluid medium (55) into a chamber housing the heat absorption material (57) through which the fluid medium (55) flows absorbing solar energy as it rises from the lower plenum (58) to the top plenum (59).A kiln for drying wood according to any of the preceding claims, wherein the fluid medium (55) is water.A kiln for drying wood according to any of the preceding claims, wherein the outer wall of the dehumidification channel (20) forming the cooler wall of the kiln (1) is composed of a plurality of horizontally arranged, overlapping panels (37) spaced from the panel (37) below by wicking material (80) to absorb condensate water exiting the kiln (1).A kiln for drying wood according to claim 8, wherein as the condensate water falls over the outside of the panel (37) below, it evaporates cooling the lower panel (37) and thereby multiplying the cooling effect of each panel (37).A kiln for drying wood according to any of the preceding claims, wherein the kiln (1) includes a simple indicator linked to the temperature of the wood, calibrated to show when the wood is dry.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.A method for drying wood in an unvented and insulated kiln as described in claim 11, wherein the same air is reused throughout the drying process.A method for drying wood in an unvented and insulated kiln as described in claim 11 or claim 12, wherein the source of renewable energy is solar, wind, tidal or waste heat from an industrial process.A method for drying wood in an unvented and insulated kiln as described in any of claims 11 to 13, wherein the wood includes fine wood for cabinet making, wooden logs, wood chips or other forms of lumber.A method for drying wood in an unvented and insulated kiln as described in any of claims 11 to 14 using air circulation by natural convection currents and a passive gravity driven dehumidification device that has no moving parts and is not powered in any way

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