A 1976 study found that a gothic arch shape required 15-25% less heating than an even-span gable or a Quonset shape (Chandra 1976), however it should be noted that here the gothic arch shape was modified such that there are no vertical components as seen in other studies. This led to a decrease in overall surface area in contact with the ambient, affecting the heat balance equations used in his calculations. Similarly, in 2002, a similar study used the same modified gothic arch shape to show a 2.6 - 4.2% increase in effectiveness compared to even-span gable and Quonset shaped greenhouses in Delhi (Gupta and Chandra 2002).

As seen in Figure 3, a study on the thermal performance of five standard greenhouse shapes under the weather conditions of New Delhi, India was conducted (Singh and Tiwari 2010). The findings revealed that the uneven-span gable roof greenhouse exhibited the highest solar radiation absorption, attributed to the larger surface area of the uneven-span design, making it more effective in receiving solar radiation. Thus, the uneven-span gable roof greenhouse demonstrated the least requirement for additional heating to maintain optimal indoor temperatures. Similarly, in a theoretical investigation, another study explored solar radiation availability in greenhouse structures across three different climate conditions (10 N, 31 N, and 50 N) in India. The results indicated that the uneven-span design necessitated the least supplementary heating across all latitudes, generally mirroring the study mentioned previously in New Delhi, however with less comparability as latitude increased (Sethi 2009).

Conversely, Ahamed et al. compared the performance of five types of greenhouse designs in Saskatoon, Canada and found that the Quonset shape required 7.6% less annual heating to maintain suitable temperatures for plant growth in a single span construction (Ahamed et al. 2018a). In the same year, the group concluded that local climate was the most important factor in determining an ideal greenhouse shape, as the heat gain from solar radiation per unit area of cover may be less than the rate of heat loss through the fabric (Ahamed et al. 2018b).

Table 1 above provides an extensive overview of various studies that were systematically compared (Ahamed et al. 2019), which has helped to determine an optimal greenhouse shape suitable for the United Kingdom in this dissertation. The key criteria considered in this comparison included latitude, the primary objective of the study, and the key findings derived from each investigation. Figure 4 depicts a comparable latitude range between the UK and the vast majority of Canada, spanning between 50 N to 60 N.

Research has identified the Quonset shape as the most suitable for a single-span greenhouse in the UK, drawn from the studies acknowledging the Quonset shape's efficiency in Canada. Despite the climatic distinctions between Canada and the UK, primarily due to the Gulf Stream effect, there is a similarity in the amount of solar radiation gained from sun exposure – as seen in Figure 5, measuring Global Horizontal Irradiance (GHI). Though not entirely identical in value, this shared characteristic is pivotal in affirming the applicability of the Quonset shape to the UK.

To determine an optimised greenhouse design, it is imperative to extend beyond the selection of an ideal shape and address the critical aspect of temperature regulation within the structure. As such it is possible to incorporate a north wall with high thermal mass, specifically for the storage and controlled release of solar energy accumulated during daylight hours. A review conducted by Santamouris looked at various heat-storing north walls, revealing a 30-50% reduction in heating demand in greenhouses featuring a solid north wall compared to counterparts without this feature (SANTAMOURIS 1993). In the subsequent year, Santamouris et al. reported an 82% saving in heating energy within a 30 m² greenhouse with a 600mm thick insulated north wall, particularly when external temperatures ranged from 3-5 C, and the internal air temperature maintained an approximate 10 C above ambient temperatures (Santamouris et al. 1994).

A study done by Gupta and Tiwari compared the thermal performance of brick, concrete, and mud north walls. The findings indicated that while a concrete north wall exhibited the highest energy storage capacity, a mud north wall remained a viable option for colder climates where daytime heating may be required (Gupta and Tiwari 2005). The energy exchange dynamics between the room air and the wall, denoted as qwall-gh, are noted in Figures 6 and 7 below. It is observed that the maximum energy exchange displayed minimal variance between concrete and mud north walls, despite the substantial differences in overall energy gain.

Delving into the mass producibility aspect of the proposed optimised greenhouse, rammed earth emerges as a potential material for the north wall due to its low carbon footprint and cost-effectiveness. Despite potential concerns regarding water damage in the British climate, a waterproofing additive known as Plasticure, developed by Australian company Tech-Dry, presents a promising solution. Findings revealed the effectiveness of cement-stabilised rammed earth with a Plasticure additive, exhibiting lower microscopic pore network connectivity, thus resulting in reduced water absorption without compromising wall durability, as absorption rate and moisture content were significantly lower in this combination than other arrangements tested (Meek et al. 2020), illustrated in Figure 8. These observations affirm the viability of rammed earth, especially when augmented with the waterproofing additive, as a pragmatic choice for the proposed greenhouse's north wall.

Thermal screens, commonly referred to as night curtains, have been employed to mitigate the loss of thermal radiation during cold nights, especially during the winter months. These thermal screens have proven to be effective in reducing radiation losses by a substantial margin of approximately 40-70% (Andersson 2010).

A study by Sethi and Sharma highlights the impact of thermal curtains on energy conservation in greenhouses, indicating potential energy savings up to 60%, with the possibility of reaching an impressive 90% when coupled with passive heating systems, such as a thermal mass storage (Sethi and Sharma 2008). This value is of course heavily idealised and assumes minimal if not no heat loss from other sources; at this juncture there has been no research to quantitatively prove this claim.

Various types of curtains, including aluminised screens, polyester screens, polypropylene monofilament, and vinyl screens, have been explored to address improvements in greenhouse heating, with studies (Andersson, 2010, Teitel et al., 1999, Zhang, 1996) having delved into the energy-saving potential of these options. The collective findings suggest that energy savings can vary from 20% to 60%, depending heavily on factors such as location and the specific type of thermal screen employed.

Sethi et al. demonstrated the effectiveness of aluminised polyester sheets as night screens in a greenhouse in India, resulting in a 3-4°C increase in indoor temperature compared to an unshielded greenhouse (Sethi et al. 2003). Aluminised thermal screens stand out due to their high reflectivity, surpassing other materials like polyester, polypropylene, and polystyrene; however, it must be noted that these aluminised screens increase cost significantly as well – contradicting with the mass-producibility ethos of this research. In contrast, other studies focused on screens made of PE and polyester materials, and demonstrated energy-saving effectiveness of 16% and 19.8% respectively (Abak et al., 1994, Öztürk and Bascetincelik, 2003).

In regions characterised by higher solar radiation, direct sunlight can induce leaf burning in greenhouse crops on summer days. To mitigate this issue, plastic films have been used to enhance the proportion of diffuse radiation within the greenhouse, also known as turbidity.

Augmenting turbidity not only enhances light uniformity but also contributes to increased yields in Mediterranean countries (Castilla and Hernandez 2007). For instance, Magán et al. observed cucumber yields of 20.5 kg/m² in a greenhouse with a plastic diffusing film (52.8% diffuse light), compared to 16.7 kg/m² in a glasshouse (34.6% diffuse light) (Magán et al., 2011).

The positive effects of a diffusive light cover extend to Western European countries such as the Netherlands as well, with Hemming et al. conducting a comparison between diffusive glass and clear glass, concluding that under a diffusive glass, the crops were able to receive more light and therefore resulted in a higher assimilation rate (Hemming et al. 2008). Similarly, a study by Dueck et al. demonstrated a 9.2% increase in cucumber productivity in the Netherlands through the use of a highly diffusive cover with a turbidity of 70%, despite transmission values reducing by 3% (Dueck 2009). Though the optimal turbidity percentage for achieving maximum yield is yet to be determined in south and central European environments, the potential for increased productivity when utilising diffusive films is higher in regions where the direct radiation percentage is greater than the 30% found in this study.

It should be noted that this is a common arrangement even in low-tech setups such as the case in the Beeston Allotment, with many volunteers shielding their greenhouses with thin cloth to protect from direct sunlight – one user in particular allowing hops to grow over her structure during the spring so as to cover the roof in vines before the summer months.

Outside of direct changes to the greenhouse envelope, a lack of ventilation leads to a decreased thermal comfort within the space and therefore decreased use of the greenhouse especially during the summer months, as explored earlier in the Beeston allotment case study, as well as increased humidity levels, which adversely impacts crop yield and quality (Hand 1988) by promoting the growth of fungi such as Botrytis cinerea which may lead to fungal diseases. Additionally, it can induce leaf necrosis and calcium deficiencies, leading to stunted growth and delayed plant development (Mortensen 2000).

Regardless of location, natural ventilation remains the most straightforward, cost-effective, and practical method for users to modify the microclimate in greenhouses. This ventilation method influences the greenhouse's energy balance and gas concentrations, addressing water vapour, CO₂, and the removal of potentially harmful gases from various sources. Efficient natural ventilation goes beyond achieving a high number of air exchanges; it also requires effective mixing of internal and external air for uniform temperature distribution and improved air movement within the canopy to enhance heat and mass exchange with the plants (Bailey 2000).

Typically, greenhouses feature roof openings aligned longitudinally along the main span axis, often situated near the ridge or gutter in a multi-span greenhouse; commonly employing either roll-up or pivoting-door closure systems. However, Bournet and Boulard found that smaller greenhouses benefit from higher ventilation efficiency when equipped with sidewall vents instead of relying solely on roof vents (Bournet and Boulard 2010).

Similar findings emerge from research done by Kacira et al., advocating for a dual approach utilising both sidewall and roof vents. The study demonstrates a ventilation effectiveness of approximately 20 times greater than that achieved by solely employing roof vents. In Figure 9, Graph A represents this combined strategy which significantly outperforms Graph B in the same figure in terms of air exchanges per minute (KACIRA et al. 2004).

While effective ventilation is important to maintain the greenhouse microclimate, a common challenge arises when the circulation of air tends to concentrate at the upper regions of the greenhouse, particularly when only roof vents are utilised, as air often follows the path of least resistance within a structure. To counteract this limitation, the implementation of a deflector becomes necessary to redirect the airflow downward.

Nielsen's study demonstrated the positive impact of a vertical deflector positioned below a double roof vent in a single span greenhouse. This resulted in a notable reduction of average temperatures within the greenhouse by 2.1°C and an enhancement of air exchange rates along the greenhouse floor by approximately 50% (Nielsen 2002). It must be taken into consideration however, that this study was conducted in a controlled environment without crops, implying unhindered airflow along the floor which may be potentially damaging in the presence of vegetation.

One solution involves elevating the crops, allowing the air to pass underneath them, or reducing the deflector's length to facilitate the homogenisation of the stream with the internal air above the crops.

In Figure 10, CFD simulations conducted by Baeza depict the findings mentioned above, with the deflected airflow redirected downwards, as compared to the typical case of air circulating at the top of the conventional greenhouse (Baeza et al. 2006).

In greenhouse design, the incorporation of reflective surfaces helps to enhance productivity by homogenising distribution to parts of the greenhouse that may not necessarily receive as much sunlight as others. The ensuing increase in light transmissivity not only fosters a more favourable environment for plant growth but also contributes to resource efficiency. The following studies were done with reflective surfaces installed on a vertical north wall, an inclined north wall reflector as well as a reflective film installed horizontally on the ground:

A study by Thomas to investigate the impact of specularly-reflecting back walls on the transmissivity of greenhouses was conducted in 1978 which utilised computer simulations written in FORTRAN IV and was implemented on an IBM 370/165. This study also assumed a reflectance of 90%, comparable to aluminium foil, and incorporated weather data from the Kew Meteorological Office. The findings of the study revealed that the use of specularly-reflecting back walls could significantly increase the amount of light reaching the greenhouse floor, as seen in Table 2. Specifically, greenhouses with a transparent north wall exhibited an average transmissivity of 76% over the 11^th to the 15^th of six months between the years of 1959 and 1960, whereas greenhouses with a reflective wall demonstrated an average transmissivity of 91% during the same period examined (Thomas 1978).

For a single-roofed (slope back to front) greenhouse with vertical walls. Length = 10.0 m, width 3.5 m, height to eaves =1.5 m, height to ridge = 3.2 m. Front wall faces due south. A dash indicates that there was no direct radiation during the period examined.

Several studies have discovered that incorporating reflective materials on the north wall of a greenhouse at an angle is a cost-effective method to reliably enhance light intensity (Li, Kurata and Takakura, 1998, Cai, Nu and Zhao, 1994, Kong and Meng, 2004). However, another study devised a formula to further determine optimised dimensions and an angle of incidence at any given time in greenhouses in China between latitudes of 32°N and 42°N (Kong et al., 2008). Table 3 below shows the results of these calculations.

Ideally, such adjustments would be mechanically controlled through a sensor and algorithm, however due to the high economic cost of operating such a set-up in the proposed greenhouse design discussed in this dissertation, this aspect will be overlooked.

Boo et al. conducted a study measuring sugar concentration in aubergines at various developmental stages with different types of polyethylene mulch placed horizontally below the plants. Their findings revealed that across all four mulches, sucrose, glucose, and fructose concentrations increased during fruit development. However, the reflective and white mulches induced consistently higher activities of the sucrose synthase enzyme compared to the black and transparent mulches (Boo et al. 2010), as depicted in Figure 11.

In the anaerobic digestion process, organic materials such as food waste and wastewater sludge, are collected and introduced into a sealed digester. Within this controlled environment, microorganisms, particularly anaerobic bacteria, break down complex organic compounds through a series of stages. The residual material left after digestion, known as digestate, is a nutrient-rich substance which has significant agricultural value as a fertiliser (Nkoa 2013).

This microbial activity also results in the production of biogas, composed mainly of methane and carbon dioxide. This biogas is then captured and has the potential to be harnessed as an energy source. However, potential risks exist due to the flammability and explosiveness of methane in biogas, as well as the release of toxic gases like hydrogen sulphide that may lead to health issues such as paralysis, asthma, respiratory issues, contagion and death (Macor and Benato, 2020a, Macor and Benato, 2020b, Benato and Macor, 2021) should the biogas not be treated properly.

In light of this, it is unrealistic to expect a small community to have the facilities to process biogas safely, thus while anaerobic digestion has the potential to be a sustainable source of energy and fertiliser, at this juncture the risks outweigh the benefits of anaerobic digestion for a greenhouse at the scale put forward by this dissertation.

Compost heat recovery involves harnessing the heat generated during the aerobic decomposition of organic materials by microorganisms like bacteria and fungi in a compost pile. A basic use of this is as a heat generator for small spaces, though air quality becomes an issue without proper ventilation. In 2021, a study found that a compost heat recovery system (CHRS) may be used to cover more than 50% of the energy demand in a small building such that there is an increase in internal gains (Malesani et al. 2021).

Within the Beeston Allotment explored in the case study for this dissertation, it was found that the community has an agreement with one of the local stables to supply horse manure to use as fertiliser. A few studies as seen in Table 4 below were explored to calculate the energy output of a CHRS should the community decide to implement this in their greenhouses, with specific reference to research done in 2019 (Bajko and Jícha 2019). This particular study was chosen due to its similarity to what materials are available to the greenhouses at the Beeston Allotment.

A rough estimate of the energy produced per year by such a set up can thus be calculated using Eq.(1):

Where, is the average power output density by volume produced by the compost, (kW/m³);

V is the size of the compost in volume, (m³);

Thus, given the space constraints and energy produced, a CHRS may be an option to supplement part of the heating load required for the proposed new greenhouse, especially during the colder months to ensure the space is operable throughout the year.

A ground source heat pump (GSHP) is a sustainable heating and cooling system that harnesses the stable temperature of the ground’s subsurface to regulate indoor climate; relying on soil below the surface having a more consistent temperature throughout the year, remaining cooler than surface temperatures in summer and vice-versa in winter. The GSHP system consists of a loop of pipes buried in the ground or submerged in a stagnant water source; a heat pump then circulates a heat-exchange fluid through this loop, absorbing heat from the ground in winter and rejecting heat to the ground in summer, thereby regulating the temperature of the space.

Despite many studies (Fine et al., 2018, You and Yang, 2020) having confirmed that a GSHP allows for high efficiency as a passive form of heating, it is also important to note that continuous use leads to serious thermal imbalance in the soil, which leads to the deterioration of energy efficiency and thus increased costs over time (Yang et al. 2022). Similarly, since the system requires an extensive area or a deep borehole that goes down approximately 10-15 meters, initial costs of implementation also make it unfavourable for the average user; as such a GSHP system is unsuitable for the proposed design (Harjunowibowo et al. 2021).

Earth-Air Heat Exchanger (EAHE) systems are constructed as buried pipes or networks underground at a specific depth to circulate air within a space. The temperature difference between the air and soil is used to substitute cooled or heated air in a space, aiding in both summer cooling and winter heating. Typically situated at depths of 1–6 meters, the system leverages the relatively constant soil temperature at this depth, which is lower than summer temperatures and higher than ambient temperatures in winter. Due to its reliance on only a small fan for circulation, an EAHE proves to be an economical and practical solution for heat management.

In 1986 a study was conducted on a greenhouse's performance which utilised an early EAHE, comprised of 20 aluminium pipes, each with a length of 15 meters, a diameter of 0.2 meters, and a thickness of 0.2 millimetres. These pipes were installed at a depth of two meters within a greenhouse with a floor area of 150 square meters. To regulate the greenhouse air temperature, the air was circulated through the pipe system whenever it fell outside of an arbitrarily set 12-28°C boundary. Consequently, this approach led to a minimum mean night air temperature of 8.1°C inside the greenhouse. In comparison, the outside minimum mean air temperature was -0.8°C during the same period (Mavroyanopoulos and Kyritsis 1986).

Similarly, a study done in the Shaanxi region of China (34°N) tested the effects of an EAHE on the winter and summer temperatures of a greenhouse, and found that the EAHEs can increase night temperatures in winter by 1.41°C and decrease day temperatures in summer by 1.87°C, as well as effectively reduce indoor air temperature fluctuation. The setup had the following parameters as seen in Figure 12 below: “EAHE in the experimental greenhouse was buried in the north trough inside the greenhouse at a depth of 2.5m and consisted of 26.9 m long PVC pipe with 0.16 m inner diameter, including a 4.5 m long inlet pipe, a 19.1 m horizontal heat exchanger pipe and a 3.3 m outlet pipe” (Xiao et al. 2023).

Given the comparability in scale of the latter study and the design put forward by this dissertation, it is more reliable to refer to temperature differences found by Xiao et al. rather than Mavroyanopoulos et al. for the purposes of sizing an EAHE system. That being said however, the cost of implementation for a greenhouse of this size outweighs the negligible change to internal air temperatures.