This assumption is valid for the relatively small changes in pressure and temperature associated with composting [ 1 ]. Heat energy values were calculated for each time step using estimated compost material concentrations and standard specific heat capacity values Table 1.
Cumulative energy over the day composting period was also calculated. A heat exchanger was designed for the collection of energy generated during composting. The heat exchanger selected for the purpose of this study was a pipeline made of stainless steel that run suspended from the top of the in-vessel tunnels in the airspace above the composting piles.
At the initial calculation stage the length and diameter of pipe required was unknown so the initial pipe layout shown in Figure 3 was investigated. The design of the heat exchanging element of the proposed design was carried out using the principles and methods discussed by Shah and Sekulic [ 20 ]. Nomenclature used in the following section is summarized in Table 2. The aim of the design process was to determine the outlet temperature for both the hot and cold fluid for a suggested heat exchanger surface area.
In this case study, the hot fluid was the warm, moist air that is released by the composting process and circulated throughout the tunnel. The temperature of this air was dictated by the temperature of the compost itself; the majority of the air was recirculated.
The temperature of the air approached that of the compost after the process has been running for a certain period. The only other effect on the temperature of the air was through heat conductive losses through the concrete walls and roof of the tunnel. These blowers were controlled in real time by computer to regulate oxygen and moisture levels. It was proposed to use water as the cold fluid that runs through the pipe work of the heat exchanger.
This was due to its low cost, ease of availability, and its thermal properties which were optimal for absorbing and storing thermal energy [ 21 ].
The mass flow rate of the water was fully controllable by the design team. A pump, pressure, or gravity fed system was designed according to requirements. The heat capacity rates for each fluid were calculated using [ 20 ] Subsequently the heat capacity ratio, , could be calculated using 5.
The heat capacity ratio is simply the smaller-to-larger ratio of the heat capacity rates of the two fluids The next step was to calculate the ratio of the overall thermal conductance to the smaller of the two heat capacities, which is defined as the number of transferred units NTUs. This was found with the following equation: Once these values were obtained the exchanger effectiveness, , was calculated. The equation for exchanger effectiveness depends on the type and flow direction associated with the particular exchanger being designed.
In this case study 7 was used as it is appropriate for the counter-flow conditions that existed in the tunnel [ 20 ] Once the exchanger effectiveness was calculated the fluid outlet temperature was found using. Compost heat as a source of renewable heat was compared to solar thermal systems and to ground-source heat. The solar thermal system was designed using the good-practice guidelines discussed by DGS [ 17 ], which was originally written with a single-family house in mind which proved transferable to the purpose in this case study.
Design methods were discussed separately for both domestic hot water supply and spatial heating. Supplying domestic hot water is the most common use for solar thermal systems. The following sizing calculations allowed a full design to be proposed. In order to calculate the area of the solar collector required the desired solar fraction, SF, and the overall average system efficiency, , of the solar collector were found.
The SF is the ratio of solar heat yield to total energy required by the building and is shown by It showed what percentage of the yearly heat energy demand is to be supplied by solar rather than conventional means where SF is the desired solar fraction, is the solar heating requirement kWh , and is the auxiliary heating requirement kWh.
If aiming to counter this by using a large area of solar collectors to better cope with winter months, it will result in an oversupply of hot water during the summer months, thus a much less efficient design. For these reasons as the solar fraction increases the system efficiency decreases. When coupled with the high set-up costs associated with a scheme of that kind it proved to be an option that limits the economic attractiveness of solar thermal systems [ 17 ].
The average system efficiency is the ratio of solar heat yield to global solar irradiance experienced by the absorber surface and is linked to the solar fraction. Guidelines state that initial calculations should assume a of 0. This data was then used to calculate the required area of solar collector using In order to calculate the optimal diameter for the piping of the solar circuit it is vital to regulate both the speed of flow and the volumetric flow.
In order to minimise noise nuisance and prevent abrasion a flow speed, , of 0. The level of volumetric flow is key in keeping the collector cooling at an efficient rate, preventing overheating and therefore wasting energy. Subsequently the optimum pipe diameter, , could be calculated using This calculation allowed an appropriate size of commercially available pipe to be proposed. The recommended method for calculating the collector area required to fulfill spatial heating demand is currently far less developed than for domestic hot water supply [ 17 ].
This is due to the highly variable nature of the thermal insulation of buildings, individual preferences for the comfortable temperature of a room, and whether the building uses conventional or underfloor heating systems. The calculation method used was of that described by DGS which is based solely on the living area required to be heated.
Regarding a ground source heat pump design, generic design guidelines for specifying ground source heat pumps are currently at an underdeveloped stage. Figure 4 shows the seasonal average temperature temporal profiles between January and December This time is similar 2—4 days to that observed when composting with one direction of airflow [ 22 ], but higher than that observed 0.
Average temperatures at the end of day 12 of composting were above during spring, summer, and autumn, and about during the winter, which was close to the values reported by Harper et al. Energy values were calculated using these average temperature profiles, and they are presented in Figure 5 , where Figure 5 a presents daily energy values and Figure 5 b presents cumulative energy values.
This value was higher than that reported by Harper et al. Negative values in Figure 5 a can be explained by the overall energy losses being greater than the energy emitted on those particular days, as the in-vessel systems were not hermetically closed.
These values related well to those reported by Ekinci at al. Differences in cumulative energy values are due to differences in decomposition rates under different conditions and heat of combustion values of the different composting substrates, which lead any direct comparisons being difficult to arrive upon and ultimately ineffectual. However, it is clear that, as these values are in line with a section of the existing research, they are reliable figures which may be utilised appropriately for subsequent calculations.
The main reason for seasonal variation is likely due to the difference in material that is available during that particular season. For example, there will be less nitrogen rich material such as grass cuttings during the autumn and winter months, thus resulting in a lower overall energy content.
All design parameters are summarised in Table 5 , and the pipe layout in the in-vessel unites shown in Figure 6. These layouts provide enough length whilst managing to avoid contact points such as temperature probe holes and exhaust air outtakes. Using this particular pipe dimension leads to the hot and cold fluid exit temperatures that are presented in Table 6. These have been calculated for varying cold water inlet temperatures, due to the potential seasonal variability, and an initial air temperature of in each case.
According to these values, the hot water could be transferred to a storage vessel site office and used to supplement the hot water supply. This would remove the need for gas or electricity to provide hot water. However, in order to meet storage legislation [ 25 ] the temperature of the stored water must exceed at all times. Currently the heated water leaving the tunnel heat exchanger system is at a temperature between and , depending on the temperature of the cold water entering the system.
However, if the water exiting a tunnel is put through another tunnel in which degradation is also underway then higher temperatures can be achieved. For example, calculations based on an initial cold water feed of and initial hot air temperatures of result in temperatures of , , , and by passing water through 0, 1, 2, and 3 tunnels in series.
Thus, by passing the same volume of water through 2 or 3 in-vessel tunnels, the required storage temperature of can be achieved. As the hot air is at an approximate temperature of , then passing the water through more than three in-vessel tunnels has little improved effect and is likely inefficient practice.
The main issue with this use is that it requires at least two tunnels to be operating at the same time. Another issue is that this system will require additional infrastructure to facilitate the level of control needed to direct the water into the correct tunnels. Two-way switch valves can be installed which could be controlled manually or by computer if required.
If underfloor heating is provided in the new site office then it may be feasible to provide the hot water for the system. Standard operating procedure for underfloor heating is to have an inlet temperature of and a return flow at. Standard underfloor systems require a heated water flow rate in the region of 1. Water could be stored thus allowing the pumps to be run for fewer hours each day or the pump could be run at a lower capacity thus improving temperature gain further. These supply and demand parameters led to the quantification of the solar thermal system components as shown in Table 7.
The method utilised to size and cost a suitable ground source heat pump system is very simplified and will not provide wholly accurate or reliable answers. This process was carried out in order to provide a comparison of typical cost and performance level of alternate renewable sources and is therefore for guidance purposes only. If a more detailed design is to be carried out a full site survey will be required to determine ground conditions and the levels of thermal insulation provided by the site office.
Table 8 summarises the cost per kWh of energy generated for each of the three possible methods. This cost is based solely on the annual operating cost required to run the system.
Capital costs are provided for information and comparative purposes only and are not a factor in the cost per kWh calculation. The fraction of spatial heating provided is based on supplying a standard underfloor heating system with an input temperature of. Capital costs do not include the underfloor heating system or the heating network itself. These should be similar if not identical for each system. There is no such thing as a failed compost, it just needs adjusting and eventually it will work.
That size is around 1 cubic meter or about 4 feet deep, 4 feet high and 4 feet wide. This is the perfect size to heat up and keep the right about of moisture to break down quickly. Start by making sure that your compost is shredded down to as small as pieces as possible.
I recommend using autumn or fall leaves as they are a great material for making any compost, hot or cold. These are easily shredded even finer using a leaf blower. If you are adding kitchen scraps for your green material, make sure these are all in small pieces.
Adding whole fruit or veg will take too long to break down for this process, so chop or shred any kitchen scraps you might be adding.
For any compost the secret ingredient to make the process even quicker is to add a shovel full of soil or even better is to add a shovel full of compost from your previous compost pile. Adding this to your compost adds more beneficial microbes which will make the process quicker.
I have added various types of microbe mixes to my compost, even straight to my veggie garden to improve the soil quality. This just ensures you have a good mix of microbes to facilitate the breakdown process. You will find with hot composting that you need to add extra water most days.
Depending on the weather, especially if you live in a warm climate, extra water will be needed to keep the pile moist. The microorganisms are like us and need moisture and food to survive.
You are aiming for a damp sponge level of moisture. Get your shredded green and brown materials and mix them together with your secret ingredient and water. It is important that you have at least 1 cubic meter of materials for this process to work. As the compost breaks down it will reduce in size but it is important to start with this much because it is the magic size that will help it break down quickly.
After you make your compost you may want to cover your compost pile with a permeable tarp. That way it will keep any material from blowing away, it will keep any mice , rats or possums out of the compost whilst still allowing moisture in and out.
Each day you will need to monitor the temperature of the mix. It will gradually rise in temperature over the first 5 days and get up to degree Fahrenheit. What will then happen is it will then start to cool.
Once it gets below degrees Fahrenheit you get your fork and give it a mix which will introduce more oxygen and the pile will heat back up again.
You will need to turn your compost every days and after about 4 weeks you will see your compost heap look like the dark compost you are used to seeing. The temperature will be low now, around 85 degrees Fahrenheit or less than 29 degrees Celsius. The next step is just to let your compost rest for around 2 weeks. This just completes the process, allows the temperature to come down and it is ready for your garden.
The basic reason compost heaps heat up is because of the activity of the microorganisms that break down the organic material in your compost. As they chew up and break down the green and brown material, they use up oxygen and release heat. This heat gets trapped in your compost pile and gradually warms up. If you are making a hot compost this will take a few days to get up to heat and level out. If the compost gets too hot, eg. This stage also is important for destroying thermosensitive pathogens, fly larvae, and weed seeds.
In outdoor systems, compost invertebrates survive the thermophilic stage by moving to the periphery of the pile or becoming dormant. Regulations by the U. As the compost begins to cool, turning the pile usually will result in a new temperature peak because of the replenished oxygen supply and the exposure of organic matter not yet thoroughly decomposed.
After the thermophilic phase, the compost temperature drops and is not restored by turning or mixing. At this point, decomposition is taken over by mesophilic microbes through a long process of "curing" or maturation. Although the compost temperature is close to ambient during the curing phase, chemical reactions continue to occur that make the remaining organic matter more stable and suitable for use with plants.
The temperature at any point during composting depends on how much heat is being produced by microorganisms, balanced by how much is being lost through conduction, convection, and radiation. Through conduction , energy is transferred from atom to atom by direct contact; at the edges of a compost pile, conduction causes heat loss to the surrounding air molecules.
Convection refers to transfer of heat by movement of a fluid such as air or water. When compost gets hot, warm air rises within the system, and the resulting convective currents cause a steady but slow movement of heated air upwards through the compost and out the top.
In addition to this natural convection, some composting systems use "forced convection" driven by blowers or fans. This forced air, in some cases triggered by thermostats that indicate when the piles are beginning to get too hot, increases the rates of both conductive and convective heat losses. Much of the energy transfer is in the form of latent heat -- the energy required to evaporate water. You can sometimes see steamy water vapor rising from hot compost piles or windrows.
The third mechanism for heat loss, radiation , refers to electromagnetic waves like those that you feel when standing in the sunlight or near a warm fire. Similarly, the warmth generated in a compost pile radiates out into the cooler surrounding air. The smaller the bioreactor or compost pile, the greater the surface area-to-volume ratio, and therefore the larger the degree of heat loss to conduction and radiation.
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