Energy is becoming an increasingly important factor in determining greenhouse enterprise profitability as energy prices rise and the economy inevitably moves towards full accounting for carbon dioxide emissions.
By JEREMY BADGERY-PARKER
In recent years, there has been a lot of discussion about energy—its use, its cost and its emissions. Energy use efficiency and energy use reduction are complex objectives and there are many options, practices and technologies that can contribute, alone or as an integrated plan to reduce energy use or improve its efficiency.
Many changes that are taking place in controlled environment systems involve aspects such as reducing heat loss or substituting a combustible fuel with one, which is cleaner and/or cheaper.
Ground source heat pump systems are an important development opportunity for the greenhouse vegetable industry. This technology has been the focus of a recent industry project managed by the NSW Department of Primary Industries with support from the vegetable industry levy, which aims to help Australian growers identify practical and affordable solutions to reduce energy costs and carbon dioxide emissions in greenhouses.
There are several advantages to using heat pumps for greenhouse temperature management:
- Although heat pumps use electricity, peak demand for heating a greenhouse generally corresponds to off-peak electricity prices in most areas; providing a cost-effective energy option.
- Heat pumps can use the free heat stored in the ground, which vastly improves efficiency so that heat pumps can transfer three or four times more energy than they use.
- There are several installation configurations that provide different cost and efficiency profiles, so there is likely to be a feasible option for every greenhouse enterprise.
- Using either a closed water loop for transferring thermal energy or a water to air arrangement, these systems can be readily integrated with existing hydronic heating infrastructure.
What is a heat pump?
Heat energy will naturally move from a source of higher energy to a sink of lower energy—analogous to water flowing downhill. A heat pump is a device, which transfers energy against the temperature gradient, that is, ‘uphill’. A heat pump basically uses the same mechanism as found in refrigerators and air conditioners. When a fluid is compressed, it releases heat into the surrounding environment and when the pressure is released, it absorbs heat, cooling the surrounding environment. A heat pump can be optimised for transferring heat energy in either direction and so has the potential to be used for cooling and heating.
Sources and sinks
The efficiency of a heat pump is related to the differential between the source temperature and the sink temperature. The smaller the temperature gap, the more efficient the process.
The most common and available source of heat is the surrounding air. The typical domestic reverse cycle air conditioner is a good example of using the outside air to extract or dissipate heat energy. The main drawback to this is that when heat is needed, the outside air is generally cool to cold and so the heat pump must transfer heat energy up a relatively steep gradient (‘hill’). As a result, more energy is used, which reduces the overall efficiency.
Another source or sink for energy is the ground – geothermal. There are basically two options. The one most people commonly think of involves drawing heat from deep in the earth, such as from ‘hot rocks’. This is a relatively expensive option and depends on the specific geology of an area.
However, the ground directly under your feet is a significant source and sink for heat energy. The sheer mass of the earth means that the ground has a relatively stable temperature throughout the year, so when heat is needed, the ground is relatively warm compared to the air and when cooling is required, the ground is relatively cool compared with the surrounding air. The result of this stability is that the temperature gradient can be quite small, so the efficiency of transferring heat energy with a heat pump is relatively high.
The ground temperature on the NSW Central Coast is approximately 12-15°C year-round. During the peak of winter, minimum air temperatures are commonly below 10°C. If a greenhouse is to be heated to a minimum night temperature of 18°C, a ground source heat pump would only need to raise the temperature by 5 or 6°C not the full 8-12°C. The smaller the difference between the temperatures of the source and the sink, the more efficiently a heat pump works.
Co-efficiency of Performance (COP)
The measure of efficiency for a heat pump is the Co-efficiency of Performance (COP). The COP describes the return on investment of a heat pump. For example, if the COP is three, then for each kilowatt of energy used by the heat pump, it will transfer 3 kilowatts. It is this advantage that makes heat pumps worth considering. The COP increases as the temperature gradient between the source and sink decreases. This means if there is a small temperature difference, the heat pump may have a COP of four or even five.
Dollars and sense
A net present value analysis was undertaken to compare the potential financial situation for each of four heating sources (hydronic heating with a typical gas fired boiler using natural gas, LPG or a ground source heat pump system and direct heating with electricity). Calculations are based on a 10 span, 5000 square metres, gable-type greenhouse with a ridge height of 5m and a 4m gutter height. The example structure has a single-skin polyethylene cladding, no thermal screens and is located in Western Sydney. The analysis assumes a target minimum greenhouse air temperature of 18°C and 75% internal relative humidity.
Table 1 and Table 2 show (in red) the estimated net present value (cost calculated in today’s dollars) for the different heating options. The values in green include an additional cost of $23 for each tonne of carbon dioxide equivalents, which would be emitted. Table 1 is based on a collation of energy forecasts, which indicate the price of electricity will rise by 2% pa and gas will increase by 8.6% pa over the next decade (the latter from a lower cost base). Table 2 shows the same situation, however, the price of gas and electricity are assumed to increase at the same rate of 5% pa over the next 10 years. Graph 1 compares the costs and emissions from the four energy options and estimated greenhouse heat load over the course of a year.
An investment in a ground source heat pump system could offer significant opportunity for greenhouse operators to reduce farm energy costs. This technology is well proven and is increasingly being installed for domestic as well as commercial and large community situations such as schools.
The example illustrated above, considers a situation where there is currently no heating system. If another scenario is considered, in which there is an existing LPG-fired boiler, which is to be replaced with a ground source heat pump, the costs of the investment in the new heat pump system are offset by the savings in LPG (and also the resale value of the LPG boiler). In this situation, the net present value of the change-over, calculated over 10 years, is around $1.7m; but with savings of more than $150,000 pa of LPG that is not required, the payback of such a conversion is just over one year. If an additional $100,000 is assumed to be necessary to upgrade electricity supply, the payback is still less than two years and the return on the investment is estimated to be 116%. This means that for every dollar invested, $1.16 is returned.
The results of this project indicate that ground source heat and heat pumps are an important option to be evaluated when considering an investment in or an upgrade of a greenhouse heating system. It is essential that you closely evaluate the costs and potential savings for your specific situation before making any investment decision. (The numbers and results provided in this article are examples only.)
There are some additional elements, which also need to be considered when reviewing this technology as an option for your enterprise. If you are using GHP with a relatively low pipe temperature, you need to ensure that there is sufficient pipe surface area to enable adequate heat exchange within the greenhouse.
Access to three phase power is likely to be required because electricity is used as the energy source for a heat pump. The cost of upgrading power if necessary would need to be factored into any investment decision. Also, to draw large electricity loads, you may incur an additional peak supply charge from the utility company.
Electricity has relatively high carbon dioxide emission levels whereas some other energy options, such as biomass combustion are considered carbon neutral. However, the efficiency of heat pumps can substantially reduce the amount of electricity required and therefore can lower accountable emissions.
Finally, it is important to note that the example assumes the heating system meets the entire peak heat demand. This may not necessarily be the most economical arrangement in all situations. Depending on the fluctuations in heat demand in a location, a boiler may be sized to meet a proportion of the peak demand in order to improve overall boiler efficiency. In such a situation, a smaller, supplementary boiler might be used to meet the occasional peak.
The integration of heating options is an important deliberation. A ground source heat pump can be readily used in conjunction with a combustion boiler, either to provide the primary heat load or as an auxiliary system.
As part of this project looking at the merits of ground source heat pump systems, greenhouse heat load estimators have been produced and are available online. These calculators provide an easy and fast way of estimating the amount of energy required and the costs of different energy sources for your greenhouse.
[Note: Direct Energy (www.directenergy.com.au/) supplied the Ground Source Heat Pump, which underpins some of the data in this article.]
About the author
Jeremy Badgery-Parker is the principal of South Australian-based Primary Principles Pty Ltd, an innovative, independent agricultural, horticultural and rural development consultancy. The business fosters a SULT Approach – Solutions through Understanding, Learning and Technology. An integrated Results Based Management (RBM) focus is taken to rural industry and community development, small acreage agriculture, farmer and community capacity building, as well as professional project management. Email: firstname.lastname@example.org Ω
October 2013 / Issue 136