A little math goes a long way
February 1, 2015 | By Lance MacNevin
What you need to know about the financial reality of operating snow and ice melt systems.
Hydronic snow and ice melting (SIM) systems have been around for decades, but some building owners are missing out on the benefits of these systems because they assume operating costs will be too high. Many are surprised to find that the annual operating costs for a SIM system can be less than for mechanical snow removal, using snowblowers or snowplows.
This article tackles the delicate topic of trying to estimate operating costs based on historical weather data, energy costs and some math. We will walk through the estimating process using a specific example. Note that we use “estimate,” not “predict.” Until weather forecasters guarantee the weather, operating costs cannot be guaranteed.
The process to estimate SIM system operating costs involves three steps:
1. Determine annual energy usage in Btus or kWh.
2. Calculate the cost of energy in $/Btu for the specific fuel available.
3. The annual operating cost is simply Annual energy usage x cost in $/Btu.
If this seems too simple, let’s look at an example. The location of this example is anonymous, but the parameters chosen correspond with many Canadian locations.
Step 1 Here is the method to determine annual energy usage with some design details left out, and all assumptions stated:
A. Operating load: Size the system output for the correct design load.
• The example uses 150 Btu/ft2 per hour (many operational hours will not even need this many Btus).
• This article is not focusing on sizing the system, but typical values across applications range from 75 Btu/ft2-hr to 225 Btu/ft2-hr.
B. Melting time: Research how many hours per year the system is expected to operate.
• Environment Canada has some of this data available; look for hours of snowfall.
• It could be 150, 200, or more, depending on the location.
C. Pick-up energy: Know the typical “cold-start” temperature for the heated area, and the specific heat of the thermal mass type. In many regions, the coldest days are not the snowy days.
• A common snowfall temperature is -10C (14F), sometimes warmer.
• For poured concrete 15 cm (6 in) thick, the concrete requires 15 Btu per ft2 per F to warm up, based on the “specific heat” of concrete of 0.23 Btu/lb-F.
D. Number of events: Estimate how many snowfalls per year will occur, as energy may be required to warm the heated area each time, depending on the controls scenario (cold-start versus idling versus always-on).
E. Calculate annual energy demand:
• First, for pick-up loads (i.e., warming the slab X times)
• Then, for operating (i.e., melting and evaporating)
• Add these together (energy for idling will be addressed later).
A commercial building has a 90 m2 (1 000 ft2) parking garage ramp, which is made of poured concrete 15 cm. (6 in.) thick embedded with 3/4 in. diameter PEX pipes at 20 cm. (8 in.) spacing.
1. The designer selects a system that requires an output of 150 Btu/ft2 per hour including reverse loss and edge losses to the cold ground.
• The total SIM operating load = 150 000 Btu/hr (simply 1 000 ft2 x 150 Btu/ft2-hr)
• Assume no standby losses due to well-insulated pipes carrying fluid to the manifold, and simple math
2. Location’s estimate for snow or ice is 200 hours per winter.
3. Typical temperature at start of snowfall is -10C (14F), though some days are warmer.
• Each time the concrete is warmed, it will take approximately 400 000 Btu of energy (math not shown)
4. This snowfall will occur over 25 events through the winter.
• System will turn on 25 times.
• System will run for an average of eight hours per activation (200 ÷ 25 = 8).
5. Now we add things up:
• Annual pick-up load is 25 x 400 000 Btu = 10 million Btu/year for ramp warming.
• Annual operational load is 150 000 Btu/hr x 200 hrs/year = 30 million Btu/year for melting
• Total annual load is 10 million + 30 million = 40 million Btu/year
This sounds like a lot, but how much does it actually cost to produce and deliver 40 million Btus? This brings us to Step 2.
Step 2 Here are the steps to calculate the cost of energy in $/Btu, with all assumptions stated:
1. Decide what type of fuel (natural gas, fuel oil, and so on) will be used and its cost. Research the energy content of that fuel (see Table 1).
2. Determine the efficiency of the heat source, if there is one (some SIM systems use waste heat).
3. Use the equation in Figure 1 to get the net cost per million Btu delivered.
1. Fuel: Select natural gas @ $0.30/m3 net fuel cost – be sure to know the local fuel cost
2. Energy: 1 m3 natural gas contains 36 000 Btu (this is fixed)
3. Heat source: We will use a condensing boiler running at 93 per cent combustion efficiency on average
4. Net cost per million Btu = $9.00/million Btu
Step 3 Calculate the annual operating cost by multiplying the annual load by the net cost per million Btu:
40 million Btu/year x $9.00/million Btu = $360/year
That is an annual operating cost of $360 and this is a fairly capable system, suitable for most residential and many light commercial applications. Obviously, each system must be designed and estimated individually. The annual operating cost will be higher for more expensive fuels, but this can be determined using the same process.
If the SIM system used an idling strategy to keep the slab warm in between snowfalls, be aware that the extra energy consumed between snowfalls could increase operating costs by a factor of four, five or six, depending on the location. However, this could still be less expensive than $2,500 for mechanical removal.
USING WASTE HEAT
Some facilities generate waste heat that needs to be rejected using chillers or geothermal heat pumps during portions of the year. Examples include office buildings, factories, hockey rinks and car dealerships with waste-oil boilers. In these applications, an always-on SIM system can be an efficient and effective solution for rejecting the excess heat, while providing all the benefits of a hydronic SIM system.
Like most things in the hydronics industry, using some math removes a lot of mystery. In this case, the math clearly shows that snow and ice melting syste
ms are economical and affordable uses of hydronics technology.
Beyond the math, there are many other reasons to consider using hydronic SIM in residential, commercial and institutional projects. Among these are: convenience of automatic snow and ice removal; increased safety for residents and visitors with reduced liability exposure; minimized environmental impact with no de-icing chemicals entering waterways; reduced maintenance costs on both outdoor and indoor surfaces; and the freeing up of building maintenance staff and budgets for more productive tasks.
When all of the factors are considered and presented effectively, many facility managers will appreciate that the benefits of hydronic SIM systems greatly outweigh the costs. <>
Lance MacNevin is the manager of REHAU Academy where he is responsible for training across North America. With over 20 years of hydronic experience, he is on the technical committee for CSA B214. He can be reached at email@example.com
Selling the system
Imagine you are talking with a client about hydronic snow and ice melt as a winter maintenance option. Compare $360 with an annual snow removal contract estimate of $100 per snow fall ($2,500 per year) for plowing with a truck-mounted scraper, which may damage outdoor surfaces. Afterward, someone must do the salting and sanding. And where do you put all the snow?
In this example, the hydronic SIM system is more than 80 per cent less expensive to operate over the winter than relying on mechanical snow removal. In other words, the SIM system costs roughly one-seventh as much each winter to operate while providing convenience, safety and protection of outdoor surfaces. And with an advanced control system, it is fully automatic and starts working at the first snowflake.