Kai Whiting, Luis Gabriel Carmona

1Faculty of Engineering, Universidad EAN, Bogotá, Colombia

2Faculty of Environmental Sciences, Universidad Piloto de Colombia, Bogotá, Colombia

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Industrial Ecology Opportunities between CHP and Arable Farming in Alloa, Scotland

Kai Whiting1,?, Luis Gabriel Carmona2

1Faculty of Engineering, Universidad EAN, Bogotá, Colombia

2Faculty of Environmental Sciences, Universidad Piloto de Colombia, Bogotá, Colombia

Submission Info

Communicated by Zhifeng Yang

Symbiosis

Farming

Sustainable Agriculture

Scotland

This paper addresses the potential for industrial ecology application at Alloa, Clackmannanshire, Scotland, between a consortium of arable farmers and a waste-to energy company. For improved onsite production, the average annual energy required to maintain the farmers' proposed 3 ha greenhouse at optimal temperature （21°C during the day and 16°C at night） was calculated as 1448.8 MJ/m2. The coldest average temperatures, registered in February result in an energy requirement of around 3000 MJ/m2. The optimal quantity of carbon dioxide for the proposed glasshouse equates to 4032 kg of CO2per day. In addition, the fertiliser produced by the waste-to-energy company will reduce the pesticide and chemical based nitrogen/potassium/phosphate demand. The authors identified that the heat, carbon dioxide and fertiliser produced at the proposed 10 MW CHP plant could be utilised by the consortium to produce higher quality food products in a symbiotic manner. The calculations undertaken indicate that the project is technically viable.

? 2015 L&H Scientific Publishing, LLC. All rights reserved.

1 Introduction

The Scottish Government, as part of the Zero Waste initiative, is concerned with waste prevention and minimisation in preference to waste recovery. Waste-to-energy installations are extremely important in diverting waste away from landfill, not least due to the reduction of methane emissions, one of the principal gases involved in global warming （IE 2008）. Scotland's quota of this gas stems from waste and agriculture - at 29 percent and 51 percent respectively （Scottish Executive 2000； Scottish Executive 2011）.

Zero Waste Scotland was developed in response to targets set in Climate Change （Scotland） Act 2009：Over 2 million tonnes of food waste is produced every year from all sectors in Scotland. If just half of this food waste was captured and treated through anaerobic digestion， the electricity generated could power a city the size of Dundee for six months， provide heat for local homes and businesses and produce enough

fertiliser for ten percent of Scotland's arable crop needs. This is just one example of how the move to a zerowaste society will create real environmental benefits， new economic opportunities and contribute to the creation of green jobs in the Scottish economy. - Zero Waste Plan （2010）

The Zero Waste Plan thus represents an opportunity for the application of industrial ecology, whereby the wastes from one part of the industrial ecosystem provide the raw material for another. Energy and material efficiency is subsequently maximised. In this respect, material and energy flows form part of a larger closed loop system and mimic what occurs in Nature where all resources are cyclically optimised and nothing is wasted （Manahan, 1999）.

In this present paper, the proposed industrial ecology is between a farming consortium and a local wasteto-energy company who is looking to operate a 10 MW combined heat and power （CHP） plant. The plant proposal is towards the southern edge of the town adjacent to the local wastewater treatment works. The potential farm location is a nearby 28 ha site is in Alloa, Scotland. The energy company has secured the use of dried sludge from the neighbouring wastewater treatment plant as its prime material.

The idea for symbiosis involves the use of organic waste from neighbouring farms as a fuel, by the local energy plant, which in turn is converted back into the heat and carbon dioxide that support food growth on the farms. In this way, agricultural and industrial symbiosis can viably support local energy production, reduce the quantity of waste that enters landfill and ensure that carbon dioxide, which would otherwise contribute to global warming, is re-diverted into an economically beneficial use.

The farming development consists of four main components：

-underground 400m steam pipeline and associated network installations

-underground 400m carbon dioxide pipeline and associated network installations

-3 ha glasshouse

-25 ha poly-tunnels

-200 m road link to A907

Fig 1. Proposed development plans. Source： Authors

The agricultural consortium is particularly looking to benefit from the carbon dioxide （CO2） formed during the stages of methane （CH4） combustion within the CHP plant. Natural gas is normally burnt in glasshouses to produce CO2to optimise crop yield. This has been highlighted as an unnecessary cost given that any CO2produced by the CHP plant has been identified as a viable and appropriate source. It should help reduceemissions from the CHP plant and cut the local energy company's carbon footprint accordingly. Gas capture would be conducted by the waste-to-energy site employees and would be transferred by pipe.

The consortium would install a 3 ha glasshouse and a system of poly-tunnels. The former is expected to contain a variety of tomatoes all year round whilst the poly-tunnels, given their structure will be predominately used for the seasonal growth of berries. In winter, due to unpredictable weather and ground conditions, they will be temporarily taken down.

The project is vulnerable to flooding （Figure 2）. This proposed threat is likely to increase with time as records indicate that the average global sea level has increased by 3 mm pa since 1993 （Lowe and Hardy 2008）. Anticipated rises in Scotland could reach 20-28 cm higher in 2080 than the current datum level（Sniffer 2008） - enough to submerge half of Alloa （Macleod 2007）.

Accordingly, the construction of the glasshouse, as shown in Figure 1， will be as near to the waste-toenergy site boundary as possible, minimising the length of pipeline whilst also reducing the potential energy losses linked those to areas susceptible to flooding （Figure 2）. Due to their low construction and maintenance requirements, coupled with high manoeuvrability, the poly-tunnels will be situated in areas identified as having a higher potential flood risk.

Fig 2. Indicative river and coastal flood map of Alloa, Clackmannanshire. Source： Authors, based on SEPA （2010）

2 Methodology

The first step involved the definition of independent entries and exits for agricultural and energy flows respectively. Subsequently each of these flows were evaluated for their potential ability to interact between them. Considering that the project is at the planning stage, assumptions and calculations made were based on published data gathered from similar projects （in terms of geographical location and crops）. When undertaking the calculations, the authors considered three main variables： heat demand in the glasshouse, carbon dioxide requirements and nutrient needs.

The formula used to calculate the glasshouse requirements is：

where

Q is the energy required to heat the greenhouse to maintain 21oC during the day-time and 16oC during the night-time （MJ m-2）2

A is the ground area （m） of calculated area

U is a heat transmission coefficient calculated using the following equation, where Uris wind speed（m/s）： U=（5.28+0.375Ur）×（Ti-Ta）

F is the glass to floor ratio, assumed as 1.6 for all calculations

Tiis the internal temperature of greenhouse （K）, 294K during day and 289K during night

Tais the external temperature of greenhouse （K）； assumes a random temperature based upon maximum and minimum monthly temperatures, with upper half being used for day-time calculations and lower half for night-time.

K*is the net short-wave radiation （MJ/m2） using the following equation, where K is calculated as 0.75%（FAOUN.1998） of daily sunshine hours, night-time value is assumed at -1.066 MJ/m2：

K*=0.46K -1.066

Qgis the ground flux of heat （MJ/m2） from ground, structure and contents which was calculated using the following equation： Qg=0.152（Ti-Ta）.

The formula for the carbon dioxide requirement for the glasshouse is as follows：

where

C is the CO2required by the glasshouse （kg/ha）

r is the relationship between CO2and tomato production and is equal to 0.93 CO2-kg/tomato-tonnes

P is the Total Potential/expected tomato production （tonnes/ha）

The formula for the optimum reduction in fertiliser demand for the poly-tunnels and glasshouse：

where

ΔF represents the fertiliser reduction （potassium, nitrogen or phosphate） （kg/ha）

FDis the original fertiliser demand （kg/ha）

FWis the provision of nutrients by waste digestate （kg/ha）

Note： The 3 elements were analysed independently. The selected value represents the greatest value.

3 Results

This section includes the description of the project and its different components such as heat production, nutrients, waste, water etc. The preliminary concepts and calculations show that the project is technically viable.

3.1 Flow Schematic

The transfer of steam from the waste-to-energy company to the farms will occur in an underground pipeline system that connects all agricultural structures to optimise food production. The feeder pipeline between the two site boundaries is expected to be 400 m in length. The glasshouse is situated closest to the CHP plant to maximise heat transfer efficiency. After passing through the glasshouse the temperature drops sufficiently and the steam, now water, flows to a network of pipes that will heat the 25 ha of poly-tunnels. The cooled water once circulated through the poly-tunnels is then returned back to the system and recycled within the CHP plant forming a continuous cycle （Figure 3）. The carbon dioxide is likewise delivered by pipe. The di-gestate and the organic farming waste will be sent for/collected when needed as on working agreement between the two parties.

Fig 3. Schematic of the Industrial Symbiosis Source： Authors

3.2 Project Elements

It is expected that the use of renewable sources to generate heat will not only improve energy security but also food security, as it enables production to become a 24 hour, 7 day a week process. This development improves local availability and affordability of high value food products that currently are imported. Food production is set to grow by 70 percent to feed the additional three billion that will exist by 2050. If current estimates are correct the 2030 globe will require up to 50 percent more energy （Scottish Government. 2010）. Such statistics place the project in an increasingly important position, as the company and its competitors strive to generate food in a sustainable manner, whilst also increasing yields.

3.2.1 Using Carbon Dioxide to Help Meet Food Demand

Glasshouses and poly-tunnels are a means of protecting crops from adverse weather conditions and uncertainty, whilst securing farmers a stable income and a sustainable future. In 2004, Europe accounted for around 23 percent of the world's greenhouses. The majority of which are situated in Italy and Spain （Peet and Welles 2005）. The Netherlands has around 10,000 ha of glasshouses, which account for around 25 percent of the global total （Bot, 2001）. The latter currently achieves around 60 kg/m2of tomato production compared to the Spain's 28 kg/m2, despite lower levels of insolation （Peet and Welles 2005）. Such high levels of growth rates are attained due to advanced technology such as well conditioned computer controlled glasshouses. They are not solely reliant on natural light intensity. This higher figure also represents the target for the farming consortium. The calculations presented in this paper are based upon this possibility.

Carbon dioxide is an additional useful by-product from the CHP plant, as both an abundance of heat and CO2are required in optimum plant growth. CO2is widely used within the glasshouse industry to increase productivity. This addition can help to achieve 30 percent increase in marketable fruit and vegetables compared with crops grown in the absence of this method. In fact, this is one of the key methods in achieving the high production rates of tomatoes （60 kg/m2） in the Netherlands. To achieve a 30 percent increase in yields, 56 kg/ha of CO2is to be dosed each hour to enrich the concentrations within the glasshouse to 2.0 g/m3（Slack and Calvert. 1972）. This equates to 168 kg per hour and 4032 kg of CO2each day for a 3 ha glasshouse.

3.2.2 On-Site Renewable Energy

The potential of combining anaerobic digestion of organic waste to create energy, whether heat or electricity, should not be overlooked, particularly in the agricultural sector. The production of sustainable high value food crops requires energy. The energy requirements for agricultural business operations （the glasshouse） are based upon a simple calculation model derived from Wass and Barrie （1984）. The equation utilised data collected by the MET Office from 1971-2000 at Leuchars, Scotland. The whole process was calculated for each month and repeated 100 times with a resultant average calculated below. Figure 4 shows the variation of energy requirements required to maintain constant day and night time temperatures throughout the year. The average annual energy required was calculated as 1448.8 MJ/m2. The coldest temperatures experienced are in February and this results in an energy requirement of around 3000 MJ/m2. During the summer months the energy demand to heat the glasshouse drops to around 500 MJ/m2. Any excess energy that the greenhouse does not require would be diverted to the network of poly-tunnels used to grow berries.

Fig 4. Energy required （MJ/m2） to maintain a temperature of 21oC during the day and 16oC during the night in a glasshouse using average Leuchars, Scotland weather conditions （1971 to 2000）. Source： Authors.

Assuming initial waste heat estimations arising from the CHP plant are 24,997,350 kWh, an energy output of around 89,990,460 MJ could be produced. This suggests an approximate energy demand of 30,000,000 MJ/ha. Therefore the total output of 89,990,460 MJ, can sustain a glasshouse of 3 ha in size. Excess heat will be transferred to the network of 25 ha of poly-tunnels.

3.2.3 Using Waste to Reduce Nutrients and Pesticides

For the agricultural sector cutting carbon is only one of the challenges of climate change. Other greenhouse gases, including two of the most potent- nitrous oxides and methane are prevalent issues. Other practices such as the application of pesticides and water extraction for irrigation can be equally damaging to the environment. All chemically introduced nutrients could have a negative impact should they be introduced into the surrounding environment.

For 50-60 tonnes/ha of tomato crop to be grown in a glasshouse operating in a temperate climate, 50 kg of nitrogen/ha, 150 to 200kg of P2O5and 250 kg K2O would be required, followed by a further addition of 100 to 150 nitrogen/ha as topdressing （Benton Jones 2007）.

The farms will utilise waste digestate produced from the CHP plant as a fertiliser. In the first year of fertilisation, 42 tonnes of waste will be required per hectare for both the glasshouse and poly-tunnels, based on potassium requirements. To fully fertilise the combined 28 ha of agricultural land, 1167 tonnes be required（based on data from Hargreaves et al. 2008）. It is hoped that nutrient re-circulating systems will be imple-mented within the glasshouse. Such systems have been shown to decrease the cost of fertiliser by around 30-40 percent whilst the recycling of water can reduce usage by around 50-60 percent （Peet and Welles 2005）.

Prevalence of pesticides within the agricultural sector is heavily associated with adverse impacts on the natural environment. Accordingly, the consortium is minimising dependency and will strive to implement the following measures as a sustainable alternative：

-Using naturally pest resistant strains of tomatoes and berries

-Mechanical weeding systems

-Use of, protection and promotion of beneficial organisms such as insects, birds, mites that provide a natural biological control

-Use of mechanical controls such as traps, barriers, light and sound to inhibit pests.

The farming consortium can provide, if necessary, some of the biomass material to supplement waste-toenergy biogas process. These additions will also serve to safeguard business operations by ensuring efficiency whilst sustaining a constant flow of gas. Studies suggest that at this point in the production chain, around 1.5-4.4 percent of tomatoes produced end up as organic waste （Riggi and Avola. 2008）. Assuming a 60 kg/m2tomato yield, the total annual production of waste will be around 5.4 tonnes on average. Up to 10% of a given strawberry yield is considered waste during the picking and selection, hence the need for value added services（Waldron et al 2010） such as those presented here. Assuming full use of the 25 ha of poly-tunnels to produce strawberries and an annual yield of 20 tonnes/ha （Creed et al 2014）, the total amount of organic waste from strawberry production could be as high 50 tonnes. The combined waste produced within the glasshouse and poly-tunnels results in a total annual organic waste output of 55.4 tonnes.

Further waste reductions for Alloa and local surrounding residents could be realised if household waste is segregated and managed appropriately by the local council. The organic component, for instance could be collected and diverted into the biogas system rather than being disposed of in landfill, which can cause various environmental issues especially regarding methane gas and water pollution caused by the generation of leachate （Jones et al 2006； Dijkgraaf and Vollebergh 2004）. Landfill disposal means residents would be paying twice - for the initial disposal and for the clear-up operation. It is obviously beneficial to deal with the waste in a manner proposed by the principles of industrial ecology. It is also logical to assume that in solving business needs one is also meeting those of the surrounding community. Another point to consider is that organic waste will be banned from Scottish landfill in 2020.

3.2.4 Internal Climate Control

Controlling the climate is an integral part of harvesting a reliable food product. There are a number of key elements that need to be addressed to achieve the desired climatic conditions.

-Temperature will be regulated at 21°C during the day and 16°C during the night. As previously discussed the temperature will be maintained via the use of heat from the CHP plant. To help decrease the energy demand for heating the glasshouse, high insulating covers will be used. These covers must adequately manage temperature fluctuations （i.e. trapping thermal heat during the winter and preventing overheating during summers）. Silica Aerogel is an excellent material for insulation due to its vast surface area and porosity, a single inch can insulate as effectively as 32 layers of glass （Insite, 2010）.

-Vapour pressure deficit； the difference between the current air moisture and the maximum it could hold, is optimum at 4-8 millibars, in relation to temperature and humidity, for the growth of tomatoes（Peet and Welles, 2005）. Humidity can be regulated by implementing a series of cooling surfaces where condensation can occur, thus removing moisture from the air （Bot, 2001）.

-Air Circulation is essential to maintain uniformity of climatic conditions throughout the glasshouse. Traditional fans are expensive to maintain due to high electrical demand. The glasshouse will be designed to implement natural circulation of air. The cooling surfaces （used to remove humidity） and heating pipes （heat from CHP） can also act as a method of encouraging natural air movement by creating temperature imbalances within the glasshouse.

-Carbon Dioxide： this has been discussed at length previously. The addition of CO2will be undertaken via a 400 m pipeline connecting the CHP plant to the farm boundary.

-Innovative Computer Monitoring System： To ease the management of climatic conditions, computers which analyse data inputs from sensors （temperature, humidity and CO2levels） will be used throughout the glasshouse. The computer system can utilise a series of different models to calculate the operational costs and resultant yield to maximise productivity for the growth of the tomato crop（Bot, 2001）.

-Electricity Demand within the glasshouse is low. The cooling surfaces designed to remove excess air moisture content and the computer monitoring system which controls climate have the highest requirement. These two technologies are essential for modern glasshouse tomato crop production.

-Water Demand typically irrigation within glasshouses growing tomatoes ranges from 1-5 L/m2（Peet and Welles. 2005）. Rainwater catchers on the roof will utilise rainfall to irrigate crops within. Water use within the poly-tunnels for soft berries such as strawberries, varies from 1,500-2,300 m3/ha within the UK and is highly dependent upon climatic conditions （FDEA. 2009）. Water will be continuously monitored and adjusted before being recycled into the glasshouse. Electrical conductivity, pH and less frequently a chemical analysis of the water is also undertaken. Water is only released whenever the sodium ion concentration reaches the crop threshold, which in the case for tomatoes is 8 mol/m3. This leads to a ratio of around 20-30 percent between water supply and drainage （Massa et al. 2011）.

4 Conclusion

The potential symbiosis between a CHP plant and the arable farmer consortium has been identified as a technically viable and sustainable option from a technical perspective. It has added the benefit of promoting the local economy and supporting Scottish governmental policy and targets. This paper also provides a strong foundation for subsequent reports and investments which will serve to further protect local interests and the global environment.

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15 January 2015

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Email address： whitingke@yahoo.co.uk

ISSN 2325-6192, eISSN 2325-6206/$- see front materials ? 2015 L&H Scientific Publishing, LLC. All rights reserved.

10.5890/JEAM.2015.03.007

Accepted 6 May 2015

Available online 1 October 2015